NCERT Solutions Class 9 Science Chapter 8 Journey Inside the Atom

F R E E S T U D Y M AT E R I A L F O R E V E R Y S T U D E N T

CLASS 9 · SCIENCE

NCERT Solutions

Chapter 8: Journey Inside the
Atom

NCERT Textbook — Exploration

BOOK PAGES SECTIONS QUESTIONS MEDIUM

140 – 161 24 63 English

Solutions, notes, sample papers & more at 64 pages

Class 9 Science Chapter 8 Journey Inside the Atom AglaSem · NCERT Solutions

CLASS 9 · SCIENCE · EXPLORATION

NCERT Solutions — Chapter 8: Journey Inside the Atom
Two thousand years after Acharya Kanada imagined the parmanu, three experiments — cathode rays, a gold
foil and a puzzle about helium's mass — forced scientists to open the atom up. This chapter follows that trail
through Thomson, Rutherford, Bohr and Chadwick, and ends with the tools you will use for the rest of
chemistry: atomic number, mass number, electronic configuration, valency, isotopes and isobars.

TEXTBOOK BOOK PAGES

Exploration (Class 9) 140 – 161

SECTIONS QUESTIONS

24 63

MEDIUM

English

Think It Over — Page 140
Chapter opener

THINK IT OVER

Q1 Are atoms the smallest indivisible particles?

No. An atom can be broken into smaller particles, so it is not indivisible.
The idea that it was indivisible came from philosophy, not experiment — Acharya Kanada's
parmanu and the Greek atomos (which literally means “indivisible”). Dalton kept the same
assumption in 1808. Experiments then broke it:

Radioactivity — certain elements were found to emit invisible particles and energy on their
own. Something was coming out of the atom.
Cathode rays (Thomson, 1897) — a stream of negatively charged particles, much lighter
than the atom, came out of the cathode whatever metal the cathode was made of and
whatever gas filled the tube. These were electrons, present in every atom.

Today we know an atom contains three subatomic particles — electrons, protons and
neutrons — and that protons and neutrons themselves have inner structure.

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Class 9 Science Chapter 8 Journey Inside the Atom AglaSem · NCERT Solutions

Why it happens: the electron's independence of the cathode material is the key
clue. If a particle identical in every way comes out of copper, aluminium or iron, it
cannot belong to any one element — it must be a building block common to all of
them.

Q2 Why do electrons not fall into the nucleus even though they are attracted to
protons in it?

Because an electron in a stationary state does not lose energy — that is Bohr's postulate, and it
is exactly the point at which his model beat Rutherford's.
Follow the reasoning in two steps.

What classical physics predicts. An electron moving in a circle is continuously changing
direction, so it is accelerating (Chapter 4). An accelerating charge should radiate energy.
Losing energy, it would spiral inward and crash into the nucleus in a tiny fraction of a second.
Every atom would collapse — and matter would not exist.
What actually happens. Atoms are stable; a piece of iron stays iron. So the classical
prediction is wrong for an atom. Bohr proposed that electrons are allowed only in certain
fixed shells (K, L, M, N …), and that while an electron stays in its shell its energy stays
constant, even though it is moving. Energy is exchanged only when the electron jumps from
one shell to another, and then only in a fixed amount equal to the energy difference between
the two levels.

So the electrostatic attraction of the protons does hold the electron in — it is what keeps the
electron bound to the atom at all — but the electron settles into an allowed energy level instead
of falling all the way in.

Did you know? Bohr had no derivation for stationary states; he simply postulated
them because they matched experiment. That is a legitimate scientific move — a
postulate stands until a deeper theory (here, quantum mechanics) explains it.

Q3 Why did scientists keep modifying atomic models?

Because each new experiment produced a result the old model could not explain, and a model
that cannot explain the evidence has to be changed.

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Class 9 Science Chapter 8 Journey Inside the Atom AglaSem · NCERT Solutions

MODEL WHAT IT EXPLAINED WHAT BROKE IT

Dalton (1808) Atoms as indivisible building blocks of Radioactivity and cathode rays — atoms
matter emit smaller particles

Thomson How positive and negative charge balance Gold foil experiment — a few α-particles
to make a neutral atom bounced straight back

Rutherford Empty space, a dense central nucleus, Could not explain why atoms are stable
(1911) orbiting electrons

Bohr (1913) Stability, through fixed energy levels K, L, Later experiments needed electron clouds,
M, N not sharp orbits

Quantum Electrons as clouds — regions of probability Still being refined
mechanical

Why it happens: this is how science works, not a sign of failure. A model is a tool for
prediction. When a prediction fails, the model is replaced by one that keeps
everything the old model got right and explains the new result too. Notice that each
new model kept the useful part of the last one — Bohr kept Rutherford's nucleus.

In-text Questions — Page 140

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Class 9 Science Chapter 8 Journey Inside the Atom
a g l AglaSem · NCERT Solutions

Chapter opener
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Class 9 Science Chapter 8 Journey Inside the Atom AglaSem · NCERT Solutions

Why it happens: what makes a cell alive is not a different type of atom but the
organisation — the way carbon, hydrogen, oxygen and nitrogen are joined into large
molecules that can copy themselves and run reactions. Break the arrangement and
you are left with ordinary atoms.

Q2 You may be wondering — Is an atom truly the smallest unit of matter, or can it be
divided even further?

It can be divided further. An atom is made of three subatomic particles:

PARTICLE SYMBOL RELATIVE CHARGE WHERE IT SITS

Electron e– –1 outside the nucleus, in shells

Proton p+ +1 in the nucleus

Neutron n0 0 in the nucleus

The word “atom” still means “indivisible”, but the name is now only historical. What is true is that
an atom is the smallest particle that still behaves like that element: split a carbon atom and you
no longer have carbon.

Did you know? Protons and neutrons are themselves made of still smaller particles
called quarks — you will meet them in higher classes. The electron, so far, has shown
no inner structure.

In-text Questions — Page 141
8.1 Rediscovering the Roots of Atomic Theory

Q1 What are atoms made up of?

Atoms are made of electrons, protons and neutrons, arranged in two distinct regions.

The nucleus at the centre holds the protons and neutrons (together called nucleons). It
carries all the positive charge and almost all the mass. Its diameter is about 10–15 m.

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Class 9 Science Chapter 8 Journey Inside the Atom AglaSem · NCERT Solutions

The electrons move around the nucleus in shells. They occupy nearly the whole volume of
the atom (diameter ≈ 10–10 m) but contribute almost no mass.

Diameter of atom ÷ diameter of nucleus

= (10–10 m) ÷ (10–15 m)

= 105, i.e. the atom is one lakh times wider than its nucleus

Why it happens: that ratio is why the atom is described as “mostly empty space”. If
an atom were the size of a cricket ground (about 100 m across), the nucleus would
be a black pepper grain a few millimetres wide at the centre.

Q2 What would atoms look like if we could see them?

Not like the diagrams in this chapter — there are no coloured spheres and no drawn orbits.
We can actually get pictures of atoms today. A Scanning Tunnelling Microscope (STM) maps a
surface atom by atom; a Transmission Electron Microscope (TEM) shows how atoms are
arranged inside a very thin sample. In an STM image (Fig. 8.15) atoms appear as a regular array
of rounded bumps — no colour, no sharp edges, no visible internal parts.
What such an image really shows is the electron cloud — the region where electrons are most
likely to be found. The nucleus, being 105 times smaller, is far too tiny to appear at all.

Tip: the textbook's own Note says it plainly — atoms have no colour. The colours in
the diagrams are only to tell electrons, protons and neutrons apart.

Q3 What makes the atoms of one element different from the atoms of another
element?

The number of protons in the nucleus — the atomic number, Z.
Z is the element's identity card. Change Z and you have a different element, no matter what else
stays the same:

Hydrogen: 1 proton, Z = 1.
Helium: 2 protons, Z = 2.
Sodium: 11 protons, Z = 11.

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Class 9 Science Chapter 8 Journey Inside the Atom AglaSem · NCERT Solutions

Neutrons do not change the identity — 126C, 136C and 146C all have 6 protons, so all three are
carbon.

Why it happens: a neutral atom has as many electrons as protons, so Z also fixes
the electron count, and therefore the electronic configuration and the number of
valence electrons. Chemistry is decided by valence electrons, so Z decides how an
element behaves.

Pause and Ponder — Page 143
8.2.1 Thomson's model of an atom

PAUSE AND PONDER

Q1 Suppose you made up your own ‘atom’, as Thomson described, using clay for the
positive charge and small beads for the electrons spread through it. What will
happen if: (i) the positive charge on the clay is lesser than the total negative charge
of the beads? (ii) by mistake, the clay itself carries a bit of negative charge? Would
your model still represent a neutral atom?

(i) The model would carry a net negative charge — it would represent a negative ion (an
anion), not a neutral atom.

Net charge = (positive charge on clay) + (total negative charge on beads)

If positive < negative in size, the sum is negative
e.g. clay +8, beads 10 × (–1) = –10 → net = –2

A real example is the oxide ion O2–: 8 protons, 10 electrons, net charge –2.
(ii) No — it would not represent a neutral atom, and worse, it would no longer represent an
atom at all.

Arithmetically the charges could never cancel: negative clay + negative beads can only add
up to something negative.
Conceptually the whole point of Thomson's model is that the atom's positive charge lives in
the sphere and its negative charge lives in the embedded particles. If the sphere itself is
negative, there is nothing left to balance the electrons.

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Class 9 Science Chapter 8 Journey Inside the Atom AglaSem · NCERT Solutions

Why it happens: neutrality is an exact bookkeeping condition — number of protons
= number of electrons, so total positive charge = total negative charge in size and
they cancel. Any model of a neutral atom must satisfy it.

Q2 Could an orange or a lemon, which also contain seeds inside soft pulp, be a good
comparison? In what ways does it match Thomson’s idea and where does it fall
short?

Yes, it works as far as the watermelon does — and it fails in exactly the same places.
Where it matches

Seeds (electrons) are embedded inside a bulk material (the positively charged pulp), not
stuck on the outside.
The seeds are spread through the fruit rather than gathered at one point — Thomson's
electrons are distributed throughout the sphere.
The fruit is roughly spherical, like the model's sphere of positive charge.

Where it falls short

An orange is divided into segments with membranes, and its seeds sit in a few segments —
Thomson's positive charge is perfectly uniform, with no compartments.
The seeds are fixed; electrons are in motion.
An orange has a thick peel — a distinct outer boundary; the atom has no skin.
Scale is completely wrong. A seed is a good fraction of the size of the fruit. An electron is
vanishingly small compared with the atom, and its mass is negligible compared with the
atom's mass.
Most importantly, the model itself is wrong: there is no spread-out positive pulp in a real
atom. All the positive charge is squeezed into a nucleus 105 times smaller than the atom.

Tip: every analogy in science has a boundary. State what the analogy is supposed to
show (here: negative particles embedded in positive matter) and stop using it
beyond that.

Q3 Why did Thomson conclude that electrons are present in all atoms?

Because the cathode rays came out identical no matter what the tube was made of or filled
with.

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Class 9 Science Chapter 8 Journey Inside the Atom
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Thomson repeated the discharge-tube experiment while changing two things:
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About 106 atoms — one million atoms stacked one on top of another.
Formula: number of atoms across the thickness = thickness of sheet ÷ diameter of one atom.

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Class 9 Science Chapter 8 Journey Inside the Atom AglaSem · NCERT Solutions

Thickness of sheet = 0.1 mm

1 mm = 10–3 m, so 0.1 mm = 0.1 × 10–3 m = 10–4 m

Diameter of one atom ≈ 10–10 m

Number of atoms ≈ (10–4 m) ÷ (10–10 m)

= 10(–4) – (–10)

= 106

= 106 atoms, i.e. about one million (10 lakh) atoms

Notice that the metres cancel, so the answer is a pure number — as it must be, since it is a
count.

Why it happens: the calculation works because the answer is a ratio of two lengths.
This is also a fair estimate rather than an exact count: atoms in paper are packed in
fibres, not in a neat vertical column, and 10–10 m is a typical atomic diameter rather
than the value for any one element.

Try This: a human hair is about 0.07 mm = 7 × 10–5 m thick. Number of atoms across
it ≈ (7 × 10–5 m) ÷ (10–10 m) = 7 × 105, i.e. about 7 lakh atoms.

Think as a Scientist — Page 144

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Class 9 Science Chapter 8 Journey Inside the Atom AglaSem · NCERT Solutions

8.2.2 A. Rutherford's model of an atom

THINK AS A SCIENTIST

Q1 Observe Fig. 8.4 of the gold foil experiment. Predict the observations you would
expect if the gold foil in the experiment were made thicker. Also, draw a simple
diagram to show the observations you expect. Hint: Compare thin foil vs thick foil.
How does the thickness affect the chances of hitting a nucleus?

Thin gold foil

Beam of
α-particles

Fig. 8.4: Schematic view of the gold foil experiment.

Prediction: more deflection and less transmission. With a thicker foil you would see

fewer α-particles passing straight through undeflected;
more α-particles deflected, and more of them deflected through large angles;
a larger fraction bouncing back towards the source;
many particles arriving at odd angles because they were deflected more than once on the
way through.

Why. A thicker foil means more layers of gold atoms along the α-particle's path. Each extra layer
is one more chance of passing close to a nucleus. If the chance of a significant deflection in one
layer is small, the chance of escaping all the layers untouched falls as the number of layers
rises.

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Class 9 Science Chapter 8 Journey Inside the Atom AglaSem · NCERT Solutions

Thin foil (as used) Thicker foil (predicted)

α α

passes straight through deflected bounced back gold nucleus

Thin foil vs thicker foil. With more layers of gold atoms in the path, fewer α-particles get through
untouched and more are deflected — some of them more than once.

Why it happens: this is exactly why Geiger and Marsden used an extremely thin
foil — only a few hundred atoms thick. In a thin foil, an α-particle that is deflected
has almost certainly been deflected once, by a single nucleus. That lets you work
backwards from the angle to the size and charge of the scatterer. In a thick foil the
multiple deflections pile up and the information is lost.

Check it yourself: the conclusion “the atom is mostly empty space” comes from the
fact that most particles went straight through. If a thick foil stopped most of them,
would that disprove the nucleus? No — it would only mean the beam met more
nuclei. The physics of one collision has not changed.

Pause and Ponder — Page 144
8.2.2 A. Rutherford's model of an atom

PAUSE AND PONDER

Q4 What do you think would happen if α-particles were replaced with negatively
charged particles in Rutherford’s gold foil experiment?

The particles would still be deflected, but they would be attracted to the nucleus instead of
repelled — so the deflections would bend the other way, and nothing would ever bounce
straight back off a nucleus.

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Class 9 Science Chapter 8 Journey Inside the Atom AglaSem · NCERT Solutions

Α-PARTICLES (CHARGE +2) NEGATIVE PARTICLES (E.G.
ELECTRONS)

Force from nucleus Repulsion Attraction

Path near a nucleus Pushed away, curves outward Pulled in, curves around the nucleus

Large-angle backscatter Yes, a few bounce back No head-on rebound; particles swing past

Effect of the atom's Almost none (α is ~7300 times Strong — same mass, so each collision changes
electrons heavier) the path a lot

Two extra problems make the experiment much worse with negative particles:

An electron has the same mass as the atom's own electrons, so it is scattered badly by them.
The α-particle, being about 7300 times heavier than an electron, ploughs past them and only
the nucleus can turn it. That is what makes the α-particle such a clean probe.
An attractive force pulls the particle towards the target rather than pushing it away, so it can
be captured instead of scattered.

Why it happens: both charges obey the same law — like charges repel, unlike
charges attract, and the force grows as the particle gets closer. The existence of a tiny
concentrated nucleus would still show up as strong deflections. But Rutherford's
specific, unmistakable clue — a particle returning almost along its own path —
needs repulsion, and so needs a positive probe.

Q5 Rutherford found that a few α-particles bounced back sharply. How does this single
surprising result completely rule out Thomson’s ʻplum pudding modelʼ of the atom?

Because in Thomson's atom there is nothing hard enough to bounce off.
Work through what each model predicts.

Thomson's model: the positive charge is spread evenly through the whole atom, and the
electrons are far too light to matter. An α-particle passing through meets a weak, thinly
spread positive charge on every side, and the pushes from different directions largely cancel.
The most it can suffer is a small nudge — a deflection of a fraction of a degree. A reversal is
impossible.
What was observed: a few α-particles came almost straight back, deflected through more
than 90°.

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Class 9 Science Chapter 8 Journey Inside the Atom
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To reverse a fast, heavy, positively charged particle you need a very large repulsive force, and
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to get a very large force you need a very large positive charge concentrated in a very small
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Class 9 Science Chapter 8 Journey Inside the Atom AglaSem · NCERT Solutions

Tip: a weak question asks for a fact you can look up. A strong question aims at the
limit of what the evidence can support.

Pause and Ponder — Page 145
8.2.2 C. Discovery of the proton

PAUSE AND PONDER

Q7 Assertion (A): Rutherford concluded that most of the mass of an atom is
concentrated in a small region at the centre called the nucleus. Reason (R):
According to Thomson’s model, electrons are embedded in a uniformly distributed
positive charge sphere. Choose the correct option: (i) Both A and R are true, and R is
the correct explanation of A. (ii) Both A and R are true, but R is not the correct
explanation of A. (iii) A is true, but R is false. (iv) A is false, but R is true.

The correct option is (ii) Both A and R are true, but R is not the correct explanation of A.
A is true. From the gold foil experiment Rutherford concluded that all the positive charge and
most of the mass of an atom sit in a tiny central region, the nucleus. The evidence: only a heavy,
concentrated target can turn a fast α-particle through a large angle.
R is true. That is a correct statement of Thomson's plum pudding model — electrons embedded
in a uniform sphere of positive charge.
But R does not explain A. R only tells you what the earlier model said. Thomson's model does
not lead to the nucleus — it contradicts it. What actually explains A is the observation: most α-
particles passed straight through (so the atom is mostly empty) while a few were deflected
through large angles or bounced back (so the mass and charge are concentrated).

Why it happens: in assertion–reason questions, check three things in order — is A
true, is R true, and does R cause or justify A? Two true statements about the same
topic are not automatically an explanation. Here R is the model that the evidence
overturned, not the reason the evidence points to a nucleus.

In-text Questions — Page 146

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Class 9 Science Chapter 8 Journey Inside the Atom AglaSem · NCERT Solutions

8.2.3 Bohr's model of the atom

Q1 You may be wondering, how does Bohr’s model explain stability?

By a postulate: the allowed circular paths are stationary states, and in a stationary state an
electron's energy stays constant even though it is moving.
Set the two models side by side.

RUTHERFORD BOHR

Electron path any circular orbit only certain fixed shells K, L, M, N (n = 1, 2,
3, 4)

Energy while continuously radiated away constant — no loss
orbiting

Predicted fate spirals into the nucleus; atom stays in its shell; atom is stable
collapses

Energy change continuous only in jumps, equal to the gap between two
levels

The energy of the levels rises as you go outward: K (n = 1) is closest to the nucleus and lowest in
energy; an electron in L (n = 2) has more energy than one in K. An electron changes shell only by
absorbing or releasing exactly the energy difference between the two levels — nothing in
between is allowed, so there is no smooth spiral inward.

Why it happens: Bohr did not derive stationary states; he assumed them because
they made the model match experiment, especially the sharp lines seen in atomic
spectra. That is what a postulate is — an assumption judged by the results it
produces. Quantum mechanics later explained why only certain energies are
allowed.

Threads of Curiosity — Page 146

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Class 9 Science Chapter 8 Journey Inside the Atom AglaSem · NCERT Solutions

8.2.3 Bohr's model of the atom

THREADS OF CURIOSITY

Q1 Why are Bohr’s shells called K, L, M, N... and not, A, B, C, D?

Because the letters were borrowed from X-ray spectroscopy, where the naming had already
started at K.

The physicist Charles Barkla, studying X-rays given out by elements, found a series of X-ray
lines and called the first one he identified the K series.
He deliberately did not begin at A. He left room in case a series with even higher energy —
earlier in the alphabet — was discovered later.
No such series was ever found, so the letters below K were never used.
Bohr simply adopted Barkla's existing notation for the atomic shells: K, L, M, N, …

Why it happens: the two namings match physically, not just by accident. The K X-ray
line is produced when an electron drops into the innermost shell, so “K” already
meant “innermost level” before Bohr used it. Keeping the letter avoided two
competing names for the same thing.

Did you know? The shells also carry the number n = 1, 2, 3, 4 …, and it is n that goes
into the formula for the maximum number of electrons a shell can hold, 2n². The
letters are the older name; the numbers are the useful one.

What if … — Page 147
8.3 What Components Contribute to the Mass of an Atom?

WHAT IF …

Q1 an atom had no empty space? How would this have affected the size of various
objects?

Everything would shrink enormously — while keeping exactly the same mass.

How much? The nucleus is about 105 times smaller in diameter than the atom. If all the empty
space were removed, every length would shrink by that factor, so every volume would shrink by
its cube.

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Class 9 Science Chapter 8 Journey Inside the Atom AglaSem · NCERT Solutions

Length scale factor = 105

Volume scale factor = (105)3 = 1015

A cricket ball of diameter 7 cm would become

7 cm ÷ 105 = 7 × 10–5 cm = 0.7 µm — smaller than a speck of dust

Your body, about 1.6 m tall, would become

1.6 m ÷ 105 = 1.6 × 10–5 m = 16 µm — about the width of a single hair

The mass, however, would be unchanged, so the density would go up by 1015. A cricket ball of
160 g compressed into 0.7 µm would still weigh 160 g. A teaspoon of such matter would weigh
millions of tonnes.

Why it happens: almost all the mass of an atom is in the nucleus, and almost all the
volume is in the electron region. Remove the electron region and you lose the
volume but not the mass — that is the whole reason nuclear matter is so
fantastically dense.

Did you know? This is not only a thought experiment. In a neutron star, gravity
really does crush atoms until the nuclei touch. A neutron star packs more mass than
the Sun into a ball roughly the size of a city.

Threads of Curiosity — Page 147
8.3.1 Discovery of the Neutron

THREADS OF CURIOSITY

Q1 You may wonder that since all the protons with like charges are squished together
in a nucleus why do they not push each other away?

They do repel one another — but a much stronger attraction, the nuclear force, holds the
nucleus together, and the neutrons are what make that possible.
Neutrons help in two ways:

Page 18 of 64

as e
Class 9 Science Chapter 8 Journey Inside the Atom
a g l AglaSem · NCERT Solutions

Being uncharged, they sit between the protons and increase the distance between them,
co m
which weakens the electrostatic repulsion.
se m.
o m l a
g them tightly.
.c proton–neutron and neutron–neutron alike) and binds
They add to the nuclear force — the short-range attraction that acts between all nucleons
m a
se
(proton–proton,
l a
ag is why the neutron-to-proton ratio rises as atoms get heavier. Count them:
This

m
.co ag
ELEMENT PROTONS NEUTRONS N:P

a sem
agl
Carbon 6 6 1.00

Oxygen 8 8 1.00

m
co1.15
m .
e
Iron 26 30

m l as
Uranium
m .co 92 146
a g 1.59

l a se
a g Why it happens: the two forces behave differently with distance. Electrostatic

m a s
agl
repulsion is long-range — every proton pushes on every other proton right across

m .co
se
the nucleus. The nuclear force is short-range — it acts only between nucleons that

g l a
are practically touching. So as a nucleus grows, the total repulsion grows faster than
a
the binding attraction, and extra neutrons are needed to keep it stable. Beyond a

m
point no ratio works, which is why the heaviest nuclei are radioactive.

. co
e m
m l as
.co a g
sem Scientific Contributions — Page 148
la Discovery of the Neutron
India's
ag8.3.1
se m
com l a
INDIA'S SCIENTIFIC CONTRIBUTIONS

. a g
e m
g l as about Dhruva!
a
Q1 Research and explore more

. c om
m a s em Centre (BARC),
Dhruva is India's largest research reactor, at the Bhabha Atomic Research

. co Mumbai. It is not a power reactor — it does not generate
Trombay,
a gl electricity for the grid. Its
as emjob is to be an intense, steady source of neutrons for experiments.
agl What a research reactor is for
c
m .
m a s e
gl
Neutron scattering. A beam of neutrons is fired at a sample and the scattered neutrons are

. co a
m
recorded. Because neutrons are uncharged, they pass through the electron cloud and
e map where the atoms sit inside a solid — something X-rays
interact with the nuclei, sosthey
a
l
ag such as hydrogen.
do poorly for light atoms

co m
m .
m ase
.co


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Class 9 Science Chapter 8 Journey Inside the Atom AglaSem · NCERT Solutions

Making radioisotopes for hospitals — for example the 6027Co used in cancer radiotherapy
and the 13153I used for thyroid disorders.
Testing materials under neutron irradiation before they are used in power reactors.

What Indian scientists have learnt using it — the internal structure of superconductors, how
lithium ions move inside battery electrodes, the shapes of drug molecules, and the behaviour of
industrial alloys. These results feed directly into better medicines, better energy storage and
stronger alloys made in India.

Try This: make a one-page profile of Dhruva. Find out (i) the year it reached full
power, (ii) its thermal power rating in MW, (iii) what it uses as fuel, moderator and
coolant, and (iv) the name of the smaller BARC reactor commissioned in 1956 that
came before it. Use the BARC and Department of Atomic Energy websites, which are
the reliable sources here.

Why it matters: Homi Jehangir Bhabha, the father of India's nuclear programme,
argued that a country must own its research tools, not rent them. A reactor like
Dhruva means Indian scientists can run neutron experiments at home instead of
waiting for beam time abroad.

Pause and Ponder — Page 149
8.4 Symbols of Elements

PAUSE AND PONDER

Q8 Imagine you are a scientist who has discovered a new element. Name this element
after yourself and justify that the symbol you have chosen follows the IUPAC rules.

This one is yours to invent, so here is a worked model you can copy the reasoning from.
Sample answer: I name the element Anandium, after myself, with the symbol An.
Justification against the IUPAC norms given in the chapter:

Page 20 of 64

Class 9 Science Chapter 8 Journey Inside the Atom AglaSem · NCERT Solutions

RULE DOES “AN” OBEY IT?

The symbol is the first letter, or the first two letters, of Yes — Anandium
the name

First letter capital, second letter (if any) small Yes — A capital, n lowercase. Not AN, not an

The symbol must not already be taken Checked — An is not the symbol of any of the 118 known
elements

Names of elements named after a person end in -ium Yes — Anand + ium = Anandium, like Curium and
Einsteinium

If “An” had already been in use, the third IUPAC norm allows a letter from later in the name
instead of the second — for example Ad (from Andium) or Am. That is exactly how chlorine got
Cl and zinc got Zn.

Tip: IUPAC — the International Union of Pure and Applied Chemistry — is the body
that actually approves names and symbols. Real elements are named after places
(Polonium, Nihonium), scientists (Bohrium, Meitnerium) or properties, and no
element may be named after a living person's own request.

Q9 What problems could arise if every scientist used different symbols for the same
element?

Chemistry would stop being a shared language, and the errors would start at the very first step
of every calculation.

Formulae would become ambiguous. If one chemist wrote Na for sodium and another
wrote S for sodium, then “SO” could mean sodium oxide or sulfur monoxide. There is no way
to tell from the paper alone.
Equations could not be balanced or checked by anyone else, so results could not be
reproduced — and reproducibility is what makes a claim scientific.
Numerical work would go wrong. Mistake sulfur (mass ≈ 32 u) for sodium (≈ 23 u) and
every mass calculation in the experiment is wrong.
Language barriers would return. Iron is loha in Hindi, fer in French, Eisen in German — but
Fe everywhere. A single symbol lets a chemist in Chennai read a paper from Japan.
Safety would suffer. A chemical drum, a medicine label or a factory sign must be readable
by any trained person, immediately and without doubt.

Page 21 of 64

Class 9 Science Chapter 8 Journey Inside the Atom AglaSem · NCERT Solutions

Why it happens: a symbol is not a decoration; it is a piece of data that gets used in a
calculation. Standardising it is exactly like standardising SI units. That is why the
naming of elements was handed to one international body, IUPAC, instead of being
left to individual scientists.

Did you know? Dalton's 1803 pictorial symbols — a circle with a dot for hydrogen, a
filled circle for carbon — were a real attempt at a standard, but they were hard to
print and hard to remember. Berzelius replaced them in 1813 with letters taken from
Latin names, which is the system we still use.

In-text Questions — Page 150
8.5 Atomic Number

Q1 Can you now say that elements with different atomic numbers are distinct from
each other, and the atomic number uniquely identifies an element?

Yes. The atomic number Z uniquely identifies an element, and two atoms with different Z are
always different elements.

Z = 1 → hydrogen; Z = 2 → helium; Z = 11 → sodium. There is no second element with Z = 11.
Nothing else works this way. The mass number does not identify an element — 40Ar, 40K
and 40Ca all have A = 40 but are three different elements (isobars).
The neutron number does not identify it either — 126C, 136C and 146C have 6, 7 and 8
neutrons and are all carbon (isotopes).

Why it happens: in a neutral atom the number of electrons equals the number of
protons, so Z fixes the electron count. The electron count fixes the electronic
configuration, which fixes the number of valence electrons — and valence electrons
decide chemistry. Change Z and you change the chemistry; change only the
neutrons and the chemistry stays the same.

Page 22 of 64

Class 9 Science Chapter 8 Journey Inside the Atom AglaSem · NCERT Solutions

Q2 Observe Fig. 8.10. How many neutrons and protons are present in a lithium atom,
and what is its atomic number?

A piece of
lithium metal

Lithium atoms

Lithium
Nucleus
atom

Electron

Neutrons
and protons

Fig. 8.10: Neutrons and protons in the nucleus.

Reading the enlarged nucleus in Fig. 8.10, the lithium nucleus contains 3 protons (the red
spheres) and 4 neutrons (the green spheres) — 7 nucleons in all. Three electrons are shown
moving around it.

Page 23 of 64

as e
Class 9 Science Chapter 8 Journey Inside the Atom
a g l AglaSem · NCERT Solutions

co m
e m.
Number of protons = 3
m l as
.co
Number of neutrons = 4
m a g
l a se
g
Atomic number, Z = number of protons = 3
aMass number, A = p+ + n0 = 3 + 4 = 7

co m
Symbol: 73Li
e m . ag
g l as
a
This agrees with Table 8.3, which lists lithium as 3 protons, 4 neutrons, mass number 7. Being

m
neutral, the atom also has 3 electrons, arranged as K = 2, L = 1.

co
se m.
o m l a
Why it happens: notice how Fig. 8.10 zooms in three times — from a piece of

m .cmetal, to the packed lithium atoms, to one atom, to itsagnucleus. Each step is
se 105 times smaller than the last but one. That is the picture you should carry:
lithium

g l a
about
a solid metal is atoms packed together, and each atom is mostly the space its 3

m a s
agl
electrons occupy.

m .co
l a se
a g
Pause and Ponder — Page 150
co m
.
8.6 Mass Number

e m
m l as
.co g
PAUSE AND PONDER

em a
a s
agl Q10 An atom with an atomic number of 26 has 56 nucleons. Find out its number of
m
electrons, protons and neutrons.

a se
. com a g l
m
ase

agl
Electrons 26, protons 26, neutrons 30. The element is iron, 5626Fe.

Formulae: Z = number of protons = number of electrons (neutral atom); nucleons = A = p+ + n0,

co m
so n0 = A – Z.
m .
m as e
.co a g l
se m
g l a
a c
m .
m a s e
em . co agl
g l as
a

co m
m .
m ase
.co


a g l Page 24 of 64

Class 9 Science Chapter 8 Journey Inside the Atom AglaSem · NCERT Solutions

Given: Z = 26, A = 56 nucleons

Number of protons = Z = 26

Number of electrons = Z = 26 (the atom is neutral)

Number of neutrons = A – Z

= 56 – 26

= 30

Why it happens: “nucleons” means protons and neutrons together, so 56 is the mass
number, not the neutron count. Once you know Z you know both the protons and
the electrons; subtracting Z from A leaves the neutrons.

Check it yourself: 26 + 30 = 56 ✓, and this is the commonest isotope of iron — the
metal in every railway line and every haemoglobin molecule in your blood.

Q11 The nucleus of an atom contains 20 protons. If its mass number is 41, find the
number of neutrons in it.

21 neutrons.

Formula: Mass number A = number of protons + number of neutrons, so n0 = A – p+.

Given: p+ = 20, A = 41

n0 = A – p+

= 41 – 20

= 21 neutrons

With Z = 20 the element is calcium, so this atom is 4120Ca. It also has 20 electrons, arranged 2,
8, 8, 2.

Page 25 of 64

Class 9 Science Chapter 8 Journey Inside the Atom AglaSem · NCERT Solutions

Did you know? Ordinary calcium is 4020Ca with 20 neutrons. 4120Ca is one of its
isotopes — same Z, one extra neutron, so the same chemistry but a slightly greater
mass.

Q12 An atom has 18 neutrons and an atomic number of 17. What is its mass number?

Mass number = 35.
Formula: A = number of protons + number of neutrons, and number of protons = Z.

Given: Z = 17, so p+ = 17; n0 = 18

A = p+ + n 0

= 17 + 18

= 35

Z = 17 makes the element chlorine, so the atom is 3517Cl — the more abundant of chlorine's
two natural isotopes, making up about 75% of all chlorine atoms.

Why it happens: the mass number counts nucleons only. The 17 electrons are left
out because an electron's mass is roughly 1/1836 of a proton's — negligible in this
bookkeeping.

Q13 An atom ²³A has 11 electrons. Find the number of neutrons in it.

12 neutrons.

Reading the symbol: in AZX the upper number is the mass number. Here the upper number is
23, so A = 23. The atom is neutral, so the number of protons equals the number of electrons.

Page 26 of 64

Class 9 Science Chapter 8 Journey Inside the Atom AglaSem · NCERT Solutions

Number of electrons = 11

⇒ number of protons, Z = 11 (neutral atom)

Mass number, A = 23

Number of neutrons = A – Z

= 23 – 11

= 12 neutrons

Z = 11 identifies the element as sodium, so ²³A is 2311Na. Its electronic configuration is 2, 8, 1 —
one valence electron, which is why sodium is such a reactive metal.

Tip: “A” here is just a stand-in letter for an unknown element, not the symbol of a
real one. The question is testing whether you can get Z from the electron count.

In-text Questions — Page 151
8.7 How Are Electrons Distributed in Different Energy Levels?

Q1 Helium contains two protons in its nucleus and two electrons. In which way will the
two electrons be arranged in its atomic shell?

Both electrons go into the K-shell: the configuration of helium is simply 2.
Rule: a shell can hold at most 2n² electrons, and shells are filled from the innermost outward.

For the K-shell, n = 1

Maximum electrons = 2n²

= 2 × 1²

=2

Helium has 2 electrons, and the K-shell can take exactly 2

⇒ configuration of He = 2, with the L-shell empty

Page 27 of 64

Class 9 Science Chapter 8 Journey Inside the Atom AglaSem · NCERT Solutions

So helium's only shell is completely full. That is the whole reason helium is chemically inert — it
has nothing to gain, lose or share. For the first shell, 2 electrons (a duplet) plays the same role
that an octet plays for later shells.

Why it happens: electrons fill the lowest available energy level first, and the K-shell
is the closest to the nucleus and lowest in energy. The second electron has no reason
to go to the L-shell while there is still room in K.

Check it yourself: lithium has 3 electrons. The K-shell takes 2 and is then full, so the
third must start the L-shell — giving 2, 1. That single outer electron makes lithium a
reactive metal, while helium, one place before it, is a gas that reacts with nothing.

Pause and Ponder — Page 152
8.7.1 Building up atoms

PAUSE AND PONDER

Q14 Identify the number of electrons in the outermost shell of the following elements:
(i) ¹²₆C (ii) ¹⁹₉F (iii) ²⁸₁₄Si

(i) 4 (ii) 7 (iii) 4.

Method: the lower number in AZX is the atomic number Z, which equals the number of
electrons in a neutral atom. Fill the shells K, L, M in turn, using the maximum 2n² and the rule
that the outermost shell may not hold more than 8.

ATOM Z = ELECTRONS K (MAX 2) L (MAX 8) M OUTERMOST SHELL

(i) 126C 6 2 4 – L, with 4 electrons

(ii) 199F 9 2 7 – L, with 7 electrons

(iii) 2814Si 14 2 8 4 M, with 4 electrons

For silicon, check the order: K takes 2, then L must be filled completely (8) before M begins,
leaving 14 – 2 – 8 = 4 for the M-shell.

Page 28 of 64

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Class 9 Science Chapter 8 Journey Inside the Atom
a g l AglaSem · NCERT Solutions

co m
m.
Why it happens: the mass number on top plays no part here. Electrons are counted

as e
comisotopes have identical chemical properties. l
by Z alone, so 12C and 14C have exactly the same outermost shell — which is
precisely .why a g
a s em
agl
Tip: carbon and silicon both end with 4 valence electrons, and both form four bonds.

co m
. ag
That family resemblance is what puts them in the same group of the periodic table.

e m
g l as
a
Write the electronic configuration of the elements having atomic numbers 12, 16
m
Q15

co
m.
and 18.

m as e
ANSWER .co
a g l
s
Z =a12
em
a g l → 2, 8, 2 | Z = 16 → 2, 8, 6 | Z = 18 → 2, 8, 8.
Rules used: maximum electrons in a shell = 2n²; the outermost shell holds at most 8; shells fill in

m a s
agl
order K → L → M.
.co
a s em
Capacities: K (n = 1) = 2 × 1² = 2
a gl (n = 2) = 2 × 2² = 8 M (n = 3) = 2 × 3² = 18
L

co m
Z = 12 (magnesium): 12 – 2 = 10 left after K; 10 – 8 = 2 left after L → K 2, L 8, M 2 = 2, 8,
m .
m as e
2
.co a g l
a s eZm= 16 (sulfur): 16 – 2 = 14; 14 – 8 = 6 → K 2, L 8, M 6 = 2, 8, 6
agl Z = 18 (argon): 18 – 2 = 16; 16 – 8 = 8 → K 2, L 8, M 8 = 2, 8, 8
se m
com g l a
m . a
e
Note argon carefully. The M-shell can hold 18 electrons, but here it is the outermost shell, and
asSo filling stops at 8 — giving argon a complete octet, which is
an outermost shell is limited tol8.
g
a
why it is an unreactive noble gas.

. comVALENCY
m
Z ELEMENT CONFIGURATION VALENCE ELECTRONS

m as e
.co Magnesium
12 2, 8, 2 2
a g l 2 (loses 2)

a s em 16
agl Sulfur 2, 8, 6 6 2 (gains 2)

.c
Argon 2, 8, 8 8
s e m
m a
18 0 (octet complete)

em . co agl
g l as
a

co m
m .
m ase
.co


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Class 9 Science Chapter 8 Journey Inside the Atom AglaSem · NCERT Solutions

Q16 Solve this riddle: I am an atom with a mass number of 23 and 11 protons. I am a
soft metal and react vigorously with water. Who am I and how many neutrons do I
have? You can also create one such riddle.

I am sodium, 2311Na, and I have 12 neutrons.

Number of protons = 11 ⇒ atomic number Z = 11

Z = 11 identifies the element as sodium (Na)

Number of neutrons = A – Z

= 23 – 11

= 12 neutrons

Do the clues fit? Sodium's configuration is 2, 8, 1 — a single valence electron, loosely held. It
gives that electron away very easily, so sodium is a soft metal that reacts vigorously with water,
releasing hydrogen gas. Both clues match.
Sample riddle of your own:

Try This: “My atomic number is 17 and I have 18 neutrons. I am a greenish-yellow
gas, I need just one electron to complete my octet, and a little of me is added to
drinking water to make it safe. Who am I, and what is my mass number?”

Answer: chlorine. A = 17 + 18 = 35, so 3517Cl; configuration 2, 8, 7; valency 1.

In-text Questions — Page 153
8.8 Combining Capacity of an Atom: Valency

Q1 In NH₃ (ammonia) and MgCl₂ (magnesium chloride), what will be the combining
capacities of nitrogen and magnesium respectively?

Nitrogen: 3. Magnesium: 2.

Page 30 of 64

Class 9 Science Chapter 8 Journey Inside the Atom AglaSem · NCERT Solutions

Rule: the combining capacity of an element is the number of hydrogen or chlorine atoms that
combine with one of its atoms — because hydrogen and chlorine each have a combining
capacity of one.

In NH3: one N atom combines with 3 H atoms

⇒ combining capacity of nitrogen = 3

In MgCl2: one Mg atom combines with 2 Cl atoms

⇒ combining capacity of magnesium = 2

Cross-check with the electronic configurations — the two methods must agree:

ELEMENT CONFIGURATION VALENCE TO COMPLETE THE VALENCY
ELECTRONS OCTET

Nitrogen (Z = 7) 2, 5 5 needs 3 more 3 ✓
Magnesium (Z = 2, 8, 2 2 loses its 2 outer 2 ✓
12) electrons

Why it happens: the formula of a compound is not arbitrary — it is fixed by how
many electrons each atom must gain, lose or share to reach a stable octet. Nitrogen
is 3 electrons short of 8, so it shares with three hydrogens. Magnesium has 2
electrons more than a stable shell, so it hands one each to two chlorine atoms, each
of which was 1 electron short.

Q2 Can you predict what happens to the atoms that already have eight electrons in
their outermost shell (except the elements with one shell only, where only two
electrons are possible)? Will they still try to lose or gain electrons?

No. They stay as they are. Such atoms are already stable, so they are almost completely
unreactive.

A complete octet is the arrangement every other atom is trying to reach by losing, gaining
or sharing electrons. An atom that already has it has no reason to change.
Elements with a full outermost shell are the noble gases — neon (2, 8), argon (2, 8, 8) and so
on. Helium is the exception noted in the question: it has only one shell, and 2 electrons fill it,
so a duplet is complete for helium.

Page 31 of 64

Class 9 Science Chapter 8 Journey Inside the Atom AglaSem · NCERT Solutions

Their valency is 0. They do not normally form compounds, and they exist in nature as single,
separate atoms rather than as molecules.

Why it happens: ask what the atom would gain by changing. Losing an electron
from a full shell means breaking into a stable arrangement — it costs a great deal of
energy. Gaining an electron means starting a brand new shell much further from the
nucleus, where the attraction is weak. Either way the atom ends up less stable than
it began, so neither happens.

Did you know? “Almost” unreactive, not absolutely. Under extreme conditions
chemists have made compounds of xenon, such as XeF₄. The octet rule is a very
good guide at your level, and the chapter notes that some compounds appear to
break it — you will meet those in higher classes.

Page 32 of 64

Class 9 Science Chapter 8 Journey Inside the Atom AglaSem · NCERT Solutions

Q3 Examine Table 8.4. Add one more column to it, and write down the common valency of

NAME SYMBOL ATOMIC NUMBER NUMBER NUMBER DIST
OF THE NUMBER OF OF OF
ELEMENT PROTONS NEUTRONS ELECTRONS K

Hydrogen H 1 1 - 1 1

Helium He 2 2 2 2 2

Lithium Li 3 3 4 3 2

Beryllium Be 4 4 5 4 2

Boron B 5 5 6 5 2

Carbon C 6 6 6 6 2

Nitrogen N 7 7 7 7 2

Oxygen O 8 8 8 8 2

Fluorine F 9 9 10 9 2

Neon Ne 10 10 10 10 2

Sodium Na 11 11 12 11 2

Magnesium Mg 12 12 12 12 2

Aluminium Al 13 13 14 13 2

Silicon Si 14 14 14 14 2

Phosphorus P 15 15 16 15 2

Sulfur S 16 16 16 16 2

Chlorine Cl 17 17 18 17 2

Argon Ar 18 18 22 18 2

Table 8.4: Symbols, atomic numbers, number of protons, number of neutrons, number of elec
electronic distribution of atoms of the first eighteen elements (page 152).

Page 33 of 64

as e
Class 9 Science Chapter 8 Journey Inside the Atom
a g l AglaSem · NCERT Solutions

Here is Table 8.4 with the extra column. Rule: if the valence shell has fewer than 4 electrons the
co m
atom loses them; if more than 4 it gains the electrons needed to reach 8; with exactly 4 it shares.
e m.
m l as
.co g
Valency is that number of electrons lost, gained or shared.

m a
l a se
a g
ELEMENT SYMBOL Z CONFIGURATION VALENCE
ELECTRONS
LOSE /
GAIN /
VALENCY

com
SHARE

e m . ag
s
H 1 1 1 shares or
a
Hydrogen 1

agl
loses 1

Helium He 2 2 2 duplet 0

co m
m.
complete

m l a se 1
.co ag
Lithium Li 3 2, 1 1 loses 1

se m
g l a
Beryllium Be 4 2, 2 2 loses 2 2

a Boron B 5 2, 3 3 loses / 3

m a s
.co agl
shares 3

se m
l a
Carbon C 6 2, 4 4 shares 4 4

Nitrogen N 7
g
a 2, 5 5 gains 3 3

co m
O 8 2, 6 6
.
gains 2
m
Oxygen 2

m l a segains 1
.co ag
Fluorine F 9 2, 7 7 1

s m
eNeon
gl a Ne 10 2, 8 8 octet
a
0
complete

se m
com g l a
.
Na 11 2, 8, 1 1 loses 1
a
Sodium 1

m
ase
a13gl 2, 8, 3
Magnesium Mg 12 2, 8, 2 2 loses 2 2

Aluminium Al 3 loses 3 3

co m
m .
e
Silicon Si 14 2, 8, 4 4 shares 4 4

m l as
m .co
Phosphorus P 15 2, 8, 5 5
a g gains / 3

l a se shares 3

ag
.c
m
Sulfur S 16 2, 8, 6 6 gains 2 2

m a s e
. co agl
Chlorine Cl 17 2, 8, 7 7 gains 1 1
m
se 2, 8, 8
l a
ag
Argon Ar 18 8 octet 0
complete

com
m .
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.co


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Class 9 Science Chapter 8 Journey Inside the Atom AglaSem · NCERT Solutions

Why it happens: read the valency column downwards and a pattern appears — 1, 0,
1, 2, 3, 4, 3, 2, 1, 0, then 1, 2, 3, 4, 3, 2, 1, 0 again. It rises to 4 and falls back, and it
repeats after neon. That repetition is the seed of the periodic table: elements with
the same valency (Li and Na; Be and Mg; F and Cl) behave alike because they have
the same number of valence electrons.

In-text Questions — Page 154
8.9.1 Isotopes

Q1 Can you guess how many electrons each of these isotopes have?

One electron each. Protium, deuterium and tritium all have exactly 1 electron.

ISOTOPE SYMBOL PROTONS NEUTRONS MASS NUMBER ELECTRONS
A

Protium (~99.98%) 1 H 1 0 1 1
1

Deuterium 2 H 1 1 2 1
1
(~0.015%)

Tritium (traces) 3 H 1 2 3 1
1

Reasoning: all three are hydrogen, so Z = 1 for each. A neutral atom has as many electrons as
protons, so each has 1 electron, sitting in the K-shell.

Why it happens: this is precisely why isotopes have the same chemical properties.
Chemistry is decided by the valence electrons, and all three have the identical
configuration “1”. They differ only in neutron count, which changes the mass — so
their physical properties such as boiling and melting points do differ slightly.

Did you know? Water made with deuterium instead of protium is called heavy water,
D₂O. It is chemically water, but it is about 10% denser and it freezes at 3.8 °C instead
of 0 °C. Indian nuclear reactors such as those at Kakrapar and Kaiga use heavy water
as a moderator.

Page 35 of 64

Class 9 Science Chapter 8 Journey Inside the Atom AglaSem · NCERT Solutions

Pause and Ponder — Page 156
8.9.1 A. Average atomic mass

PAUSE AND PONDER

Q17 Two different atoms have 11 protons each, but one has 12 neutrons, and the other
has 13 neutrons. How do their atomic numbers and mass numbers compare? Are
they the same element or different elements?

Same atomic number, different mass numbers — they are the same element (sodium), and
they are isotopes of each other.

Atomic number Z = number of protons

First atom: Z = 11 Second atom: Z = 11 → equal

Mass number A = p+ + n0

First atom: A = 11 + 12 = 23 → 2311Na

Second atom: A = 11 + 13 = 24 → 2411Na

→ different, by 1

Z = 11 in both cases, and Z is what fixes the identity of an element, so both are sodium. Same Z,
different A is the definition of isotopes.

Why it happens: both atoms are neutral, so both have 11 electrons, arranged 2, 8,
1. Identical valence shell ⇒ identical chemistry. Both are soft metals that react
vigorously with water. The extra neutron only makes the second atom about 4%
heavier, which shows up in physical properties, not chemical ones.

Check it yourself: would the answer change if one atom had 11 protons and the
other 12? Yes, completely — 11 protons is sodium and 12 is magnesium. Different
elements, and they would not be isotopes at all.

Page 36 of 64

Class 9 Science Chapter 8 Journey Inside the Atom AglaSem · NCERT Solutions

Q18 If a bromine atom is available in the form of, say two isotopes, ⁷⁹₃₅Br (49.7%) and
⁸¹₃₅Br (50.3%), calculate the average atomic mass of the bromine atom.

Average atomic mass of bromine ≈ 80.01 u, i.e. about 80 u.
Formula (weighted average):

Average atomic mass = (mass1 × % abundance1/100) + (mass2 × % abundance2/100)

Substituting:

= (79 u × 49.7/100) + (81 u × 50.3/100)

= (79 × 49.7 + 81 × 50.3) ÷ 100 u

79 × 49.7 = 3926.3

81 × 50.3 = 4074.3

Sum = 3926.3 + 4074.3 = 8000.6

Average atomic mass = 8000.6 ÷ 100

= 80.006 u ≈ 80.01 u

Rounded sensibly for the two-significant-figure data, ≈ 80 u.

Why it happens: the two isotopes are almost equally abundant (49.7% and 50.3%),
so the weighted average sits almost exactly halfway between 79 u and 81 u — and
the halfway point is 80 u. The tiny excess above 80 comes from the heavier isotope
being slightly the more common one. Contrast chlorine, where the abundances are
75% and 25%, so the average 35.5 u sits much closer to 35 u than to 37 u.

Tip: no single bromine atom weighs 80.01 u. Every bromine atom is either 79 u or 81
u. The average describes a large collection — in 1000 bromine atoms you would
expect about 497 of 79Br and 503 of 81Br.

Page 37 of 64

Class 9 Science Chapter 8 Journey Inside the Atom AglaSem · NCERT Solutions

Revise, Reflect, Refine — Pages 158 – 160
End-of-chapter exercise

REVISE, REFLECT, REFINE

Q1 Choose the correct options and explain the reason for the correct and incorrect
options in the context of Ernest Rutherford’s gold foil experiment: (i) The
experiment clearly showed the existence of neutrons in the nucleus. (ii) The results
disproved the plum pudding model and led to the idea of a nucleus at the centre of
the atom. (iii) The large deflection of a few alpha particles indicated that most of
the mass of the atom and positive charge are packed into a tiny centre. (iv) The way
alpha particles were deflected showed that electrons move around the nucleus.

Correct options: (ii) and (iii). Incorrect: (i) and (iv).

OPTION VERDICT REASON

(i) showed neutrons Incorrect Neutrons are uncharged, so they exert no electrostatic force on an
exist α-particle and cannot show up in a scattering experiment. The
neutron was discovered 21 years later, in 1932, by James
Chadwick.

(ii) disproved plum Correct Thomson's uniformly spread positive charge could produce only
pudding, led to the tiny deflections. Large-angle deflection and backscattering are
nucleus impossible in that model, so the model had to go — and a small,
dense, positive centre took its place.

(iii) mass and Correct Only a target that is both highly charged and much heavier than
positive charge the α-particle can turn it through a large angle. Since most particles
packed into a tiny passed straight through, that target must occupy a minute fraction
centre of the atom's volume.

(iv) showed how Incorrect The experiment says nothing about electrons. An α-particle is about
electrons move 7300 times heavier than an electron, so passing electrons barely
alter its path. Electron arrangement came later, from Bohr's work
on spectra.

Why it happens: an experiment can only tell you about what it interacts with. α-
particles are charged and heavy, so they probe concentrated charge and
concentrated mass — nothing else.

Page 38 of 64

as e
Class 9 Science Chapter 8 Journey Inside the Atom
a g l AglaSem · NCERT Solutions

co m
m.
Which of the following statements are correct or incorrect according to the Bohr’s
se
Q2

o m l a
atomic model? Give a reason for each statement. (i) Electrons lose energy while
g can exist
m
moving.c in fixed orbits and slowly fall into the nucleus. (ii) Electrons
a
l a se
anywhere around the nucleus with no fixed energy. (iii) Electrons revolve around
a g the nucleus in orbits of fixed energy without losing energy. (iv) Electrons can be
found between energy levels as they move around the nucleus.

co m
m . ag
l a se
g incorrect.
Only (iii) is correct. (i), (ii) and (iv)aare

co m
m.
STATEMENT VERDICT REASON

a se
. com l
ag and it is the failure Bohr
(i) lose energy and fall into Incorrect That is the classical prediction that Bohr rejected. It is what

em
would happen in Rutherford's model,
s
the nucleus

l a
ag (ii) anywhere, with no fixed
set out to fix. In a stationary state the energy stays constant.

Incorrect Bohr's central idea is the opposite — electrons occupy only

m a s
.co agl
energy certain allowed shells, each with a definite energy. Energy is

em
quantised, not arbitrary.

Correctgl a s
(iii) revolve in orbits of
fixed energy without losing
a This is exactly the postulate of stationary states. The shells K,
L, M, N (n = 1, 2, 3, 4) each hold a definite energy, and an

com
energy electron moving in one does not radiate.

The space between two shells is not e
.
m An electron
a s
com agl
(iv) found between energy Incorrect allowed.

m .
levels changes shell by absorbing or releasing exactly the energy

ase
gap between them — it is in one level or the other, never in

agl
between.

se m
com g l a
. a
Why it happens: the four statements are a checklist of the two ideas Bohr added —
m
ase
that only certain energies are allowed, and that an electron in an allowed state does

agl
not radiate. Statement (i) denies the second; statements (ii) and (iv) deny the first.

co m
m .
Y,eand Z are given as
The composition of the nuclei of three atomic species X, s
o m l a
ag of neutrons — X: 19, Y: 18,
.c follows. Number of protons — X: 18, Y: 17, Z: 17. Number
Q3

se m
g l a Z: 20. Explain the relation between the following: (i) Y and Z (ii) Z and X
a c
m .
m a s e
m .
(i) Y and Z are isotopes. (ii) Z and X are isobars.
e
co agl
g l as
First work out Z (atomic number) and A (mass number) for each species, using A = p+ + n0.
a

co m
m .
m as e
.co


a g l Page 39 of 64

Class 9 Science Chapter 8 Journey Inside the Atom AglaSem · NCERT Solutions

SPECIES PROTONS = Z NEUTRONS MASS NUMBER A = P + N ELEMENT

X 18 19 18 + 19 = 37 37
18Ar (argon)

Y 17 18 17 + 18 = 35 35
17Cl (chlorine)

Z 17 20 17 + 20 = 37 37
17Cl (chlorine)

(i) Y and Z — isotopes. Both have 17 protons, so both are chlorine, but their mass numbers are
35 and 37. Same atomic number, different mass number = isotopes. They have the same
electronic configuration (2, 8, 7), so identical chemical properties; only their masses differ. These
are in fact the two natural isotopes of chlorine, present in the ratio 3 : 1.
(ii) Z and X — isobars. Both have mass number 37, so both nuclei contain 37 nucleons, but Z
has 17 protons and X has 18. Same mass number, different atomic number = isobars. They are
different elements — chlorine and argon — with completely different chemistry: chlorine (2, 8,
7) needs one electron and is highly reactive, argon (2, 8, 8) has a full octet and is inert.

Why it happens: the two ideas are easy to keep apart if you ask which number is
being held fixed. Iso-topes hold the proton number fixed, so they are the same
element. Iso-bars hold the total nucleon number fixed, so they weigh the same but
are different elements.

Q4 What conclusion did Rutherford draw about the position and characteristics of the
atom’s positively charged part based on the few alpha particles that bounced back
or were deflected at large angles in the gold foil experiment?

Rutherford concluded that the positive charge is not spread through the atom but is
concentrated in an extremely small region at the centre, which he called the nucleus.
Position. At the centre of the atom, occupying only a minute fraction of its volume. The atom's
diameter is about 10–10 m; the nucleus is about 10–15 m across — some 105 times smaller.
Everything outside is essentially empty space through which the electrons move.
Characteristics.

It carries all the positive charge of the atom.
It contains almost all the mass — the electrons outside are so light that their mass can be
ignored.
It is therefore extremely dense.

How each observation forced each conclusion:

Page 40 of 64

Class 9 Science Chapter 8 Journey Inside the Atom AglaSem · NCERT Solutions

OBSERVATION CONCLUSION

Most α-particles passed straight through, The atom is mostly empty space
undeflected

A few were deflected through large angles There is a concentrated positive charge that repels them strongly

A very few bounced almost straight back That charge sits on something far heavier than the α-particle, and
occupies a very small volume

Why it happens: the rarity of the large deflections is as important as the deflections
themselves. If backscattering had been common, the target would have to be large.
Because it was very rare, the target must be tiny — and to still turn the particle right
around, everything heavy and positive must be packed into it.

Q5 Explain and arrange the following statements in the correct chronological order to
show how atomic models have evolved over time. (i) Bohr’s model proposed that
electrons move in fixed orbits around the nucleus, each with a definite energy. (ii)
Thomson’s model depicted the atom as a ʻplum puddingʼ with electrons embedded
in a sphere of positive charge. (iii) Rutherford’s model proposed that atoms have a
dense central nucleus. (iv) Dalton’s model described atoms as indivisible particles.

Correct chronological order: (iv) → (ii) → (iii) → (i), that is Dalton → Thomson → Rutherford →
Bohr.

Page 41 of 64

Class 9 Science Chapter 8 Journey Inside the Atom AglaSem · NCERT Solutions

ORDER MODEL YEAR WHAT IT PROPOSED WHY THE NEXT ONE WAS
NEEDED

1st (iv) Dalton 1808 All matter is made of Radioactivity and cathode rays
indivisible atoms — the first showed atoms give out smaller
scientific description of particles
matter

2nd (ii) after Electrons embedded in a Gold foil experiment: a few α-
Thomson 1897 sphere of positive charge — particles bounced back
the plum pudding

3rd (iii) 1911 Dense central nucleus, mostly Could not explain why atoms do
Rutherford empty space, electrons not collapse
revolving around it

4th (i) Bohr 1913 Electrons in fixed shells, each Later work showed electrons are
of definite energy; no energy clouds, not sharp orbits — the
loss while in a shell quantum mechanical model

The logic of the order. Each model had to wait for the discovery that made it possible.
Thomson's model could not come before the electron was found in 1897. Rutherford's could not
come before the gold foil experiment. Bohr's could not come before there was a nucleus to orbit
— his model is Rutherford's with one new rule added.

Why it happens: notice that no model was thrown away entirely. Dalton's atom
survives as the unit of an element; Thomson's electron survives; Rutherford's
nucleus survives inside Bohr's model and inside the modern one. Science advances
by keeping what works and repairing what fails.

Q6 Electrons move around the nucleus in orbits. Why do they not fly away from the
atom? Explain what keeps them attracted to the nucleus.

They do not fly away because of the electrostatic force of attraction between the negatively
charged electrons and the positively charged nucleus.

The nucleus contains protons, each carrying charge +1. Each electron carries –1. Unlike
charges attract.
This attraction is directed continuously towards the centre, so it keeps pulling the electron
back in as the electron moves. It is the force that bends the electron's path into a closed shell
instead of a straight line.

Page 42 of 64

Class 9 Science Chapter 8 Journey Inside the Atom AglaSem · NCERT Solutions

The role it plays is the same as that of gravity for a planet going round the Sun: an inward
force that keeps a moving body in a closed path.

The other half of the answer is why the electron does not fall in, which is the opposite risk.
Bohr's postulate settles that: while an electron stays in an allowed shell its energy is constant, so
it neither radiates away its energy nor spirals inward. It escapes only if it is given enough energy
from outside to reach beyond the outermost level — which is exactly what happens when an
atom is ionised.

Why it happens: the strength of the attraction also explains why electrons in
different shells behave so differently. An electron in the K-shell is close to the
nucleus, strongly held, and hard to remove. A valence electron in an outer shell is far
away and weakly held — which is why sodium (2, 8, 1) gives up its single outer
electron so readily and reacts vigorously with water.

Q7 Assertion (A): The discovery of subatomic particles helped in understanding the
atomic structure. Reason (R): The number of electrons is equal to the number of
protons in an atom. Choose the correct option: (i) Both A and R are true, and R is the
correct explanation of A. (ii) Both A and R are true, but R is not the correct
explanation of A. (iii) A is true, but R is false. (iv) A is false, but R is true.

The correct option is (ii) Both A and R are true, but R is not the correct explanation of A.
A is true. Every step forward in atomic structure came from finding a subatomic particle. The
electron (Thomson, 1897) proved the atom was divisible; the proton gave the nucleus its charge;
the neutron (Chadwick, 1932) finally explained atomic masses. Without these discoveries there
would be no model of the atom at all.
R is true. In a neutral atom the number of electrons does equal the number of protons — that
is why the atom carries no net charge, and it is what makes the atomic number Z serve as both
the proton count and the electron count.
But R does not explain A. R is one particular fact that we learnt because the particles were
discovered — it is a consequence, not a cause. It says nothing about why discovering particles
advanced our understanding. A is a statement about the whole history of the subject; R is a
single detail inside it.

Page 43 of 64

as e
Class 9 Science Chapter 8 Journey Inside the Atom
a g l AglaSem · NCERT Solutions

co m
m.
Why it happens: test the direction of the arrow. Ask “does R lead to A?” Knowing that

m l a se
electrons = protons does not by itself tell you anything about how atomic structure
o
m
came to be.c understood. Now ask “does A lead to R?” Yes — it wasagthe discovery of
l a se and protons that let us state the equality at all. The arrow runs the wrong
g so R cannot be the explanation of A.
electrons
away,

co m
e m . ag
g l as
a
Q8 Magnesium is essential for many biological processes, including muscle
contraction. For an atom of magnesium with a mass number of 24 and atomic

m
number 12, determine the number of (i) protons, (ii) neutrons, (iii) electrons, and
co
m.
also illustrate the arrangement of electrons in a magnesium atom.

o m l a se
ANSWER .c a g
se m
l a
ag
(i) 12 protons (ii) 12 neutrons (iii) 12 electrons. The electronic configuration is 2, 8, 2.

Formulae: Z = number of protons = number of electrons in a neutral atom; n0 = A – Z.
m a s
m .co agl
Given: A = 24, Z = 12
l a se
a g
(i) Number of protons = Z = 12
com
m .
as e
. com of neutrons = A – Z a g l
m
ase
(ii) Number

agl = 24 – 12

se m
com a
= 12

. a g l
m
ase
agl
(iii) Number of electrons = Z = 12 (the atom is electrically neutral)

co m
.
Arrangement of the 12 electrons. Fill K first (maximum 2 × 1² = 2), then L (maximum 2 × 2² = 8),

em
as
then the rest into M.
m l
.co
m K-shell: 2 electrons → 12 – 2 = 10 remain a g
l a se
ag
.c
m
L-shell: 8 electrons → 10 – 8 = 2 remain

m a s e
. co agl
M-shell: 2 electrons
em
g l as
Electronic configuration of Mg = 2, 8, 2
a

co m
m .
m ase
.co


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Class 9 Science Chapter 8 Journey Inside the Atom AglaSem · NCERT Solutions

Mg

K=2 L=8 M=2
12 protons and 12 neutrons in the nucleus
Electron arrangement in a magnesium atom (Z = 12): 12 electrons fill K → L → M as 2, 8, 2. The two
outermost electrons are the valence electrons.

Why it happens: the two electrons in the outermost M-shell are the valence
electrons. Magnesium loses them to reach the stable configuration 2, 8, which is why
its valency is 2 and why it forms Mg²⁺ ions — the form in which magnesium works in
your muscles and in chlorophyll.

Page 45 of 64

Class 9 Science Chapter 8 Journey Inside the Atom AglaSem · NCERT Solutions

Q9 Find the following information for the elements shown in Fig. 8.17: (i) Name of the
element (ii) Symbol (iii) Total number of electrons (iv) Number of valence electrons
(v) Valency of the element (vi) Number of protons (vii) Atomic number

(a) (b) (c) (d)

Fig. 8.17

Counting the electrons drawn on each shell in Fig. 8.17 gives the configurations 2,1 · 2,5 · 2,8,3 ·
2,7, which identify the four elements.

(A) (B) (C) (D)

Electrons drawn (K, L, M) 2, 1 2, 5 2, 8, 3 2, 7

(i) Name of the element Lithium Nitrogen Aluminium Fluorine

(ii) Symbol Li N Al F

(iii) Total number of electrons 3 7 13 9

(iv) Number of valence electrons 1 5 3 7

(v) Valency 1 (loses 1) 3 (gains 3) 3 (loses 3) 1 (gains 1)

(vi) Number of protons 3 7 13 9

(vii) Atomic number Z 3 7 13 9

Page 46 of 64

Class 9 Science Chapter 8 Journey Inside the Atom AglaSem · NCERT Solutions

Li N Al F

(a) Li (b) N (c) Al (d) F
2, 1 2, 5 2, 8, 3 2, 7

The four atoms of Fig. 8.17, redrawn with the electrons counted off the printed figure: lithium (2,1),
nitrogen (2,5), aluminium (2,8,3) and fluorine (2,7).

How each column was obtained. Add up the electrons on all the shells — that gives the total
number of electrons. The atom is neutral, so the number of protons and the atomic number are
the same number. The electrons on the outermost shell are the valence electrons. For valency,
apply the rule: fewer than 4 valence electrons → lose them; more than 4 → gain enough to
reach 8.

(a) 2 + 1 = 3 electrons → Z = 3 → Li; 1 valence electron, so it loses 1 → valency 1

(b) 2 + 5 = 7 electrons → Z = 7 → N; 5 valence electrons, needs 8 – 5 = 3 → valency 3

(c) 2 + 8 + 3 = 13 electrons → Z = 13 → Al; 3 valence electrons, so it loses 3 → valency 3
(d) 2 + 7 = 9 electrons → Z = 9 → F; 7 valence electrons, needs 8 – 7 = 1 → valency 1

Check it yourself: (b) and (c) both come out with valency 3, but for opposite reasons
— nitrogen gains three electrons, aluminium loses three. That is why they combine
with each other in a 1 : 1 ratio, as AlN.

Q10 Both Rutherford’s and Bohr’s models have electrons orbiting the nucleus. Why did
Rutherford’s model fail to explain atomic stability, while Bohr’s model succeeded?

Because Rutherford allowed the electron any orbit and let classical physics take its course, while
Bohr allowed only certain fixed orbits and postulated that an electron in one of them does not
radiate energy.
Why Rutherford's model fails.

An electron moving in a circle is constantly changing direction, so it is accelerating.

Page 47 of 64

Class 9 Science Chapter 8 Journey Inside the Atom AglaSem · NCERT Solutions

Classical physics says an accelerating charge must radiate energy.
Losing energy, the electron's orbit would shrink; it would spiral inward and fall into the
nucleus.
The atom would collapse in a tiny fraction of a second — but matter around us is stable. So
the model, as it stood, was incomplete.

Why Bohr's model succeeds.

Electrons may occupy only certain allowed shells — stationary states K, L, M, N (n = 1, 2, 3,
4) — and nothing in between.
In a stationary state the electron's energy remains constant even though it is moving, so no
energy is radiated.
Energy is exchanged only in jumps between two levels, equal to the difference in their
energies. Since there are no allowed states between shells, and none below the K-shell, there
is no continuous route inward.

RUTHERFORD (1911) BOHR (1913)

Allowed orbits any radius only fixed shells

Energy while orbiting radiated continuously constant

Prediction for the atom collapses stable

Why it happens: Bohr did not change the picture of the atom — he kept
Rutherford's nucleus and orbits exactly. What he changed was the rule governing
them, by adding a restriction that classical physics does not contain. That is why the
models look alike in a drawing but predict opposite fates. Bohr's rule was an
assumption, justified by the fact that it worked — it also explained the sharp lines in
atomic spectra, which continuous radiation could never do.

Q11 An atom ⁷⁰X has 31 electrons. How many neutrons are there in its nucleus?

39 neutrons.

Reading the symbol: the superscript in 70X is the mass number, so A = 70. The atom is neutral,
so protons = electrons.

Page 48 of 64

as e
Class 9 Science Chapter 8 Journey Inside the Atom
a g l AglaSem · NCERT Solutions

co m
e m.
Number of electrons = 31
m l as
.co
⇒ Number of protons, Z = 31
m a g
l a se
g
Mass number, A = 70
a

co m
. ag
Number of neutrons = A – Z
e m
= 70 – 31
g l as
= 39 neutrons a

co m
e m.
as
Z = 31 identifies the element as gallium, so 70X is 70 31Ga.
m l
.co a g
a s em it yourself: 31 protons + 39 neutrons = 70 nucleons ✓. Notice that neutrons
gl
Check
a already outnumber protons here (ratio 39 : 31 ≈ 1.26) — the trend the chapter

s
describes, that heavier nuclei need proportionally more neutrons to stay bound.
m a
m .co agl
l a se
a g
An atom has 79 protons and a mass number of 197. Calculate (i) the number of
Q12

m
neutrons, and (ii) the number of electrons.

. co
e m
m l as
.co g

m a
l a se
(i) 118 neutrons (ii) 79 electrons. The element is gold, 19779Au.

ag
Given: number of protons = 79, mass number A = 197
se m
com g l a
m . a
ase
agl
(i) Number of neutrons = A – number of protons

= 197 – 79

co m
m .
e
= 118
m l as
.co
m (ii) The atom is electrically neutral, so a g
l a se
ag
.c
m
Number of electrons = number of protons

m a s e
. co agl
= 79

se m
l a
g Geiger and Marsden beat into foil for the scattering experiment.
Z = 79 is gold — the veryametal

co m
m .
m ase
.co


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Class 9 Science Chapter 8 Journey Inside the Atom AglaSem · NCERT Solutions

Why it happens: with 79 protons crammed together, the electrostatic repulsion
inside a gold nucleus is enormous. It takes 118 neutrons — a neutron-to-proton ratio
of about 1.49 — to add enough short-range nuclear force, and enough spacing
between protons, to hold it together. Compare carbon, which manages with 6 and 6.

Q13 Complete the Table 8.5:

ATOMIC MASS NUMBER NUMBER NUMBER NAME OF
NUMBER NUMBER OF OF OF THE
NEUTRONS PROTONS ELECTRONS ELEMENTS

5 – 6 – – –

– 14 – – 7 Nitrogen

– 24 – 12 – –

15 – 16 – – –

– 1 0 – – –

Table 8.5, page 160 — reproduced exactly as printed, with the blanks shown as “–”.

Every blank follows from two relations: Z = number of protons = number of electrons (neutral
atom) and A = number of protons + number of neutrons.

ATOMIC MASS NUMBER OF NUMBER NUMBER OF NAME OF
NUMBER NUMBER NEUTRONS OF ELECTRONS THE
PROTONS ELEMENT

5 11 6 5 5 Boron

7 14 7 7 7 Nitrogen

12 24 12 12 12 Magnesium

15 31 16 15 15 Phosphorus

1 1 0 1 1 Hydrogen

Page 50 of 64

Class 9 Science Chapter 8 Journey Inside the Atom AglaSem · NCERT Solutions

Row by row:

Row 1: Z = 5 ⇒ protons = 5, electrons = 5. A = 5 + 6 = 11. Z = 5 → boron, 115B

Row 2: electrons = 7 ⇒ Z = 7, protons = 7. Neutrons = 14 – 7 = 7. Given: nitrogen, 147N ✓

Row 3: protons = 12 ⇒ Z = 12, electrons = 12. Neutrons = 24 – 12 = 12. Z = 12 →

magnesium, 2412Mg

Row 4: Z = 15 ⇒ protons = 15, electrons = 15. A = 15 + 16 = 31. Z = 15 → phosphorus,
31 P
15

Row 5: A = 1 and neutrons = 0 ⇒ protons = 1 – 0 = 1, so Z = 1 and electrons = 1. Z = 1 →

hydrogen, 11H

Did you know? Row 5 is protium, the only atom in the whole periodic table whose
nucleus contains no neutron at all — just a lone proton. It makes up about 99.98% of
all natural hydrogen.

Q14 Aman was discussing the structure of atom with his classmates. During the
discussion, he learnt that an element X has a mass number of 35 and contains 18
neutrons. Based on this information, answer the following questions: (i) How many
electrons and protons does element X have? (ii) What is its atomic number? (iii)
Identify the element X. (iv) Write its electronic configuration. (v) How many
valence electrons does it have? (vi) What will be the mass number if two neutrons
are added to its nucleus? (vii) What will be the relation of X with the new atom?

Start from A = 35 and n0 = 18.
(i) 17 protons and 17 electrons.

Page 51 of 64

Class 9 Science Chapter 8 Journey Inside the Atom AglaSem · NCERT Solutions

Number of protons = A – number of neutrons

= 35 – 18

= 17
The atom is neutral, so number of electrons = number of protons = 17

(ii) Atomic number Z = 17, since Z is defined as the number of protons.

(iii) The element is chlorine (Cl), written 3517Cl.

(iv) Electronic configuration = 2, 8, 7.

K-shell (max 2 × 1² = 2): 2 electrons → 17 – 2 = 15 remain

L-shell (max 2 × 2² = 8): 8 electrons → 15 – 8 = 7 remain

M-shell: 7 electrons

Configuration = 2, 8, 7

(v) 7 valence electrons — the electrons in the outermost (M) shell. Chlorine needs 8 – 7 = 1
more to complete its octet, so its valency is 1 and it is highly reactive.
(vi) New mass number = 37.

New number of neutrons = 18 + 2 = 20
Protons are unchanged at 17

New mass number A' = 17 + 20

= 37, i.e. 3717Cl

(vii) X and the new atom are isotopes of each other. Both have Z = 17, so both are chlorine,
but their mass numbers are 35 and 37 — same atomic number, different mass number.

Why it happens: adding neutrons cannot change the element, because the element
is defined by the proton count alone. Both atoms still have 17 electrons in the
arrangement 2, 8, 7, so both behave identically in every chemical reaction. Only the
mass differs. These two are in fact the real isotopes of chlorine, mixed in nature in
the ratio 3 : 1, which is why chlorine's average atomic mass is 35.5 u.

Page 52 of 64

Class 9 Science Chapter 8 Journey Inside the Atom AglaSem · NCERT Solutions

Q15 In an atom, there are 12 protons and 12 neutrons in the nucleus. Now, imagine
that all the electrons are replaced with some hypothetical particles that have the
same charge as electrons but are 500 times heavier. What effect will this
replacement have on the atom’s: (i) Atomic number (ii) Atomic mass (iii) Mass
number (iv) Overall charge

The atom is magnesium: 12 protons, 12 neutrons, and therefore 12 electrons.
(i) Atomic number — no change. It stays 12. The atomic number is defined as the number of
protons, and the nucleus has not been touched.
(ii) Atomic mass — it increases, by about 3.3 u. This is the only quantity that changes.

Mass of one electron ≈ 1/1836 of a proton's mass ≈ 5.5 × 10–4 u

Mass of one heavy particle = 500 × 5.5 × 10–4 u

= 0.275 u

Total for 12 such particles = 12 × 0.275 u

= ≈ 3.3 u

Original atomic mass ≈ 24 u (12 protons + 12 neutrons; electrons negligible)

New atomic mass ≈ 24 u + 3.3 u = ≈ 27.3 u

The point is not just the number but what it means: ordinarily the 12 electrons contribute only
12 × 5.5 × 10–4 u ≈ 0.0066 u, which is why we ignore them. At 500 times the mass they add about
14% to the atom, and they can no longer be ignored.
(iii) Mass number — no change. It stays 24.

Mass number A = number of protons + number of neutrons

= 12 + 12

= 24 (electrons never enter this count, whatever their mass)

(iv) Overall charge — no change. The atom remains neutral. The replacement particles have
the same charge as electrons, and there are still 12 of them.

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Class 9 Science Chapter 8 Journey Inside the Atom
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Total positive charge = 12 × (+1) = +12
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Total negative charge = 12 × (–1) = –12
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Net charge = +12 – 12 = 0
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Why it happens: this question separates three ideas that students often merge.
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Charge depends only on how many positive

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their mass. Mass number is a count
affected by anything outside the nucleus. Atomic mass is an actual mass, so every

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particle contributes; we drop the electrons only because their contribution is far too
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small to matter, and that approximation breaks the moment they become
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The Journey Beyond — Page 161
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Project work

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THE JOURNEY

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Create an ‘Atomic Prediction Board’ game based on atomic number, mass number,
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How to build it. Make 18 cards, one for each of the first eighteen elements in Table 8.4. On the
back of each card write four clues that get easier as you go down.
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Class 9 Science Chapter 8 Journey Inside the Atom AglaSem · NCERT Solutions

CLUE POINTS IF GUESSED EXAMPLE CARD
HERE

Clue 1 4 “My valency is 2 and I have 3 shells.”
(hardest)

Clue 2 3 “My mass number is 32 and I have 16 neutrons.”

Clue 3 2 “My electronic configuration is 2, 8, 6.”

Clue 4 (easiest) 1 “My atomic number is 16 and my symbol has one
letter.”

Rules. The reader turns over one card and reads Clue 1. Any player may answer. A wrong
answer means that player sits out the rest of that card. Keep reading clues until someone gets
it. Whoever answers scores the points for the clue that was on the table.

Make it harder by adding “relationship” cards — “I am an isobar of 4020Ca and my atomic
number is 18: who am I?” (argon) or “I am an isotope of the element with 6 protons and I am
used for dating fossils” (146C).

Tip: write the answer key on a separate sheet, not on the card, so a card can be
reused. Check every card against Table 8.4 before you play — a wrong clue teaches a
wrong fact.

Q2 Prepare a report on how the properties of atoms impact us in everyday life across
fields, such as healthcare, energy, agriculture and technology.

What a good report must contain: for each field, name the atomic property involved, give a
concrete Indian example, and explain the link between them. Do not just list applications —
show which property makes each one possible.

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Class 9 Science Chapter 8 Journey Inside the Atom AglaSem · NCERT Solutions

FIELD ATOMIC PROPERTY USED APPLICATION

Healthcare Radioactive isotopes emit penetrating 60 Co in cancer radiotherapy; 131 I to treat
27 53
radiation; chemically identical to the goitre and thyroid cancer, because the thyroid
stable isotope, so the body handles absorbs iodine
them the same way

Energy Heavy nuclei can be split, releasing 235
92U as fuel in nuclear power plants such as
energy; a neutron, being uncharged, Tarapur and Kudankulam
can enter a nucleus

Agriculture Isotopes act as tracers — same Tracking how much fertiliser a crop actually
chemistry, detectable mass or radiation takes up; irradiating seeds to develop
improved varieties; irradiating produce to
make it last longer

Technology Valence electrons decide electrical Silicon chips in every phone and computer;
behaviour; silicon (2, 8, 4) has exactly alloys designed by studying atomic
four arrangement using neutron scattering at
BARC

Archaeology A radioactive isotope decays at a fixed 14 C dating of fossils, wood and artefacts
6
and geology rate

Structure to follow: (1) a short introduction stating which atomic properties you will use —
isotopes, valence electrons, nuclear energy; (2) one section per field with the table row
expanded into a paragraph; (3) one paragraph on risks and safe handling of radioactive
materials; (4) your sources.

Tip: use the Department of Atomic Energy and BARC websites for Indian examples,
and give the exact reactor or hospital name where you can. A specific example is
worth ten general statements.

Q3 Create a role-play, stage play or story about the ‘Journey Inside the Atom’, and the
scientists who discovered and contributed to the identification of atomic structure.

The play works best if each scientist is put on stage to defend a claim and is then
challenged by the next. Here is a scene plan you can build on.

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Class 9 Science Chapter 8 Journey Inside the Atom AglaSem · NCERT Solutions

SCENE CHARACTER THE LINE THAT CARRIES THE SCIENCE

1 Acharya Kanada “Divide matter again and again, and you reach the parmanu — but I
reason this out, I do not measure it.”

2 John Dalton (1808) “I say the same, but now from experiment: atoms are the indivisible
building blocks of matter.”

3 J. J. Thomson (1897) “Something negative and very light comes out of my cathode —
whatever metal I use. Your atom is not indivisible.”

4 Geiger and “Sir, a few alpha particles have come back.” Rutherford: “As incredible
Marsden (1911) as a shell bouncing off tissue paper.”

5 Niels Bohr (1913) “Your nucleus is right, but your electron should have fallen in. Let me
fix the orbits — only certain shells, and no energy lost inside one.”

6 James Chadwick “Helium has two protons but four times hydrogen's mass. The
(1932) missing mass is a neutral particle — the neutron.”

7 Homi Jehangir “India will build its own reactors and its own institutions — TIFR and
Bhabha BARC — so we can ask these questions ourselves.”

8 Narrator “Even Bohr was not the last word. Today the electron is a cloud, and
the journey is not over.”

Staging ideas. Use a large hoop for a shell and let students holding blue cards walk round a
“nucleus” group holding red (protons) and green (neutrons) cards. To show a large-angle
deflection, have an “alpha particle” walk in a straight line, then swerve sharply on meeting the
nucleus. To show Bohr's jump, an electron steps from the inner hoop to the outer one only
when handed an “energy” card.

Check it yourself: make sure every scene ends with the evidence that forced the next
change. The story of the atom is a story of experiments, not of opinions.

Q4 Use selected software or digital tools and try to create animations or simulations of
various atomic models, and share them in the class.

Start with the two ready-made PhET simulations the book itself links to — they are free,
they run in a browser, and they let you change the variables that matter.

Rutherford Scattering (phet.colorado.edu/en/simulations/rutherford-scattering) — switch
between the plum pudding atom and the nuclear atom and watch the difference in the alpha
particle tracks. Increase the number of protons and see the deflections grow.

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Class 9 Science Chapter 8 Journey Inside the Atom AglaSem · NCERT Solutions

Isotopes and Atomic Mass (phet.colorado.edu/en/simulations/isotopes-and-atomic-mass)
— build isotopes by adding neutrons and watch the average atomic mass change as you
adjust the abundances. Try to reproduce chlorine's 35.5 u from 75% and 25%.

What to build yourself. If you use a block-based tool such as Scratch, make each electron a
sprite that moves on a circular path, and add a button that makes an electron jump from the K-
hoop to the L-hoop only when a fixed “energy quantum” is supplied — that single feature is the
whole of Bohr's model.
What to say when you present. For each simulation, state (i) which model it shows, (ii) what
you changed, (iii) what happened, and (iv) what that tells you about the real atom. A simulation
without that fourth point is only an animation.

Tip: a simulation is a model of a model. Always say what it leaves out — for example,
none of these show the electron as a cloud, which is what modern physics says it
really is.

Q5 Watch a film or documentary about the structure of the atom and write a report
answering the following questions: Which film or documentary did you watch, and
what was its main idea or topic? What did the film or documentary teach you about
the structure of the atom and the atomic model(s)? Which scientists were
mentioned in the film or documentary, and what were their contributions? What
part of the film or documentary did you find most interesting, and what question
do you still have?

This report is yours to write from what you actually watch, so here is a model answer showing
the depth expected.
Sample answer.

What I watched. An episode on the discovery of the atomic nucleus. Its main idea was that
our picture of the atom was rebuilt three times in thirty years, each time because one
experiment produced a result nobody expected.
What it taught me about the structure. That the atom is overwhelmingly empty space —
the film showed the scale as a pea in the middle of a stadium, which matches the book's
figure of 10–15 m against 10–10 m. It also made clear that Bohr's neat orbits are a stepping
stone, not the truth: the modern picture is a cloud of probability.
Scientists mentioned. J. J. Thomson (discovered the electron, 1897, and proposed the plum
pudding model); Ernest Rutherford (interpreted the gold foil results and proposed the
nuclear model, 1911); Niels Bohr (stationary states and energy levels, 1913); James Chadwick
(the neutron, 1932).

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Class 9 Science Chapter 8 Journey Inside the Atom
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Most interesting part. Learning that Geiger and Marsden were told to look for
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Take sodium (2, 8, 1), silicon (2, 8,g4)

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