CBSE Secondary Syllabus 2027 Science Reading Material

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Reading Material
Class X (2026-27)
Science 086
Unit 1
Periodic Classification of Elements
In Class IX we have learnt that matter around us is present in the form of elements,
compounds and mixtures and the elements contain atoms of only one type. Do you
know how many elements are known till date? At present, 118 elements are known to
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us. All these have different properties. Out of these 118, only 94 are naturally occurring.
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As different elements were being discovered, scientists gathered more and more
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information about the properties of these elements. They found it difficult to organise all
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that was known about the elements. They started looking for some pattern in their
properties, on the basis of which they could study such a large number of elements with
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1.1 Making Order Out Of Chaos–Early Attempts
at the Classification of Elements
We have been learning how various things or living beings
can be classified on the basis of their properties. Even in other
situations, we come across instances of organisation based
on some properties. For example, in a shop, soaps are kept
together at one place while biscuits are kept together
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elsewhere. Even among soaps, bathing soaps are stacked
separately from washing soaps. Similarly, scientists made
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several attempts to classify elements according to their

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properties and obtain an orderly arrangement out of chaos.
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The earliest attempt to classify the elements resulted in
grouping the then known elements as metals and non-metals.
Later further classifications were tried out as our knowledge
of elements and their properties increased.
1.1.1 Döbereiner’s Triads
In the year 1817, Johann Wolfgang Döbereiner, a German chemist, tried to arrange the
elements with similar properties into groups. He identified some groups having three
elements each. So he called these groups ‘triads’. Döbereiner showed that when the
three elements in a triad were written in the order of increasing atomic masses; the atomic
mass of the middle element was roughly the average of the atomic masses of the other
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For example, take the triad consisting of lithium (Li), sodium (Na) and potassium (K)

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with the respective atomic masses 6.9, 23.0 and 39.0. What is the average of the atomic

s e masses of Li and K? How does this compare with the atomic mass of Na?
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arranged downwards in order of increasing atomic masses. Can you find out which of
a these groups form Döbereiner triads?
Table 1
Group A Atomic Group B Atomic Group C Atomic
elements Mass elements Mass elements Mass
N 14.0 Ca 40.1 Cl 35.5
P 31.0 Sr 87.6 Br 79.9
As 74.9 Ba 137.3 I 126.9

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You will find that groups B and C form Döbereiner triads. Döbereiner could identify only
three triads from the elements known at
that time (Table 2). Hence, this system of
classification into triads was not found to
be useful.
Table 2 Döbereiner triads
Li Ca Cl
Na Sr Br
K Ba I

1.1.2 Newlands’ Law of Octaves
The attempts of Döbereiner encouraged other chemists to correlate the properties of
elements with their atomic masses. In 1866, John Newlands, an English scientist,
arranged the then known elements in the order of increasing atomic masses. He started
with the element having the lowest atomic mass (hydrogen) and ended at thorium which
was the 56th element. He found that every eighth element had properties similar to that
of the first. He compared this to the octaves found in music. Therefore, he called it the
‘Law of Octaves’. It is known as ‘Newlands’ Law of Octaves’. In Newlands’ Octaves, the
properties of lithium and sodium were found to be the same. Sodium is the eighth
element after lithium. Similarly, beryllium and magnesium resemble each other. A part
of the original form of Newlands’ Octaves is given in Table 3.
Table 3: Newlands’ Octaves

o It was found that the Law of Octaves was applicable only upto calcium, as after
calcium every eighth element did not possess properties similar to that of the first.
o It was assumed by Newlands that only 56 elements existed in nature and no more
elements would be discovered in the future. But, later on, several new elements
were discovered, whose properties did not fit into the Law of Octaves.
o In order to fit elements into his Table, Newlands adjusted two elements in the same
slot, but also put some unlike elements under the same note. Can you find
examples of these from Table 3? Note that cobalt and nickel are in the same slot
and these are placed in the same column as fluorine, chlorine and bromine which
have very different properties than these elements. Iron, which resembles cobalt
and nickel in properties, has been placed far away from these elements.

Thus, Newlands’ Law of Octaves worked well with lighter elements only.

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Questions
1. Did Döbereiner’s triads also exist in the columns of Newlands’ Octaves?
Compare and find out.
2. What were the limitations of Döbereiner’s classification?
3. What were the limitations of Newlands’ Law of Octaves?

1.2 Making Order Out of Chaos – Mendeléev’s Periodic Table
Even after the rejection of Newlands’
Law of Octaves, many scientists
continued to search for a pattern that
correlated the properties of elements
with their atomic masses.
The main credit for classifying elements
goes to Dmitri Ivanovich Mendeléev, a
Russian chemist. He was the most
important contributor to the early
development of a Periodic Table of
elements wherein the elements were
arranged on the basis of their
fundamental property, the atomic mass,
and also on the similarity of chemical
properties.
When Mendeléev started his work, 63 elements were known. He examined the
relationship between the atomic masses of the elements and their physical and
chemical properties. Among chemical properties, Mendeléev concentrated on the
compounds formed by elements with oxygen and hydrogen. He selected hydrogen and
oxygen as they are very reactive and formed compounds with most elements. The
formulae of the hydrides and oxides formed by an element were treated as one of the
basic properties of an element for its classification. He then took 63 cards and on each
card he wrote down the properties of one element. He sorted out the elements with
similar properties and pinned the cards together on a wall. He observed that most of the
elements got a place in a Periodic Table and were arranged in the order of their
increasing atomic masses. It was also observed that there occurs a periodic recurrence
of elements with similar physical and chemical properties. On this basis, Mendeléev
formulated a Periodic Law, which states that ‘the properties of elements are the periodic
function of their atomic masses’.
Mendeléev’s Periodic Table contains vertical columns called ‘groups’ and horizontal
rows called ‘periods’ (Table 4).

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Mendeléev’s Periodic Table was published in a German journal in 1872. In the formula
for oxides and hydrides at the top of the columns, the letter ‘R’ is used to represent any
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of the elements in the group. Note the way formulae are written. For example, the

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hydride of carbon, CH4, is written as RH4 and the oxide CO2, as RO2.
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1.2.1 Achievements of Mendeléev’s
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While developing the Periodic Table, g were a few instances where Mendeléev had
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to place an element with a slightlyagreater atomic mass before an element with a slightly
lower atomic mass. The sequence was inverted so that elements with similar properties
could be grouped together. For example, cobalt (atomic mass 58.9) appeared before
nickel (atomic mass 58.7). Looking at Table 4, can you find out one more such anomaly?
Further, Mendeléev left some gaps in his Periodic Table. Instead of looking upon these
gaps as defects, Mendeléev boldly predicted the existence of some elements that had
not been discovered at that time. Mendeléev named them by prefixing a Sanskrit
numeral, Eka (one) to the name of preceding element in the same group. For instance,
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boron, Eka–aluminium and Eka–silicon, respectively. The properties of Eka–Aluminium

m predicted by Mendeléev and those of the element, gallium which was discovered later
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g la Table 5 : Properties of eka–aluminium and gallium a
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This provided convincing evidence for both the correctness
and usefulness of Mendeléev’s Periodic Table. Further, it was the extraordinary success
of Mendeléev’s prediction that led chemists not only to accept his Periodic Table but
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also recognise him, as the originator of the concept on which it is based. Noble gases
like helium (He), neon (Ne) and argon (Ar) have been mentioned in many a context
before this. These gases were discovered very late because they are very inert and
present in extremely low concentrations in our atmosphere. One of the strengths of
Mendeléev’s Periodic Table was that, when these gases were discovered, they could
be placed in a new group without disturbing the existing order.
1.2.2 Limitations of Mendeléev’s Classification
Electronic configuration of hydrogen resembles that of alkali metals. Like alkali metals,
hydrogen combines with halogens, oxygen and sulphur to form compounds having
similar formulae, as shown in the examples here.
On the other hand, just like halogens, hydrogen also exists as diatomic molecules and
it combines with metals and non-metals to form covalent compounds.
Activity 1
o Looking at its resemblance to alkali metals and the halogen family, try to assign
hydrogen a correct position in Mendeléev’s Periodic Table.
o To which group and period should hydrogen be assigned?

Certainly, no fixed position can be given to hydrogen in the Periodic Table. This was the
first limitation of Mendeléev’s Periodic Table. He could not assign a correct position to
hydrogen in his Table.
Isotopes were discovered long after Mendeléev had proposed his periodic classification
of elements. Let us recall that isotopes of an element have similar chemical properties,
but different atomic masses.
Activity 2
o Consider the isotopes of chlorine, Cl-35 and Cl-37.
o Would you place them in different slots because their atomic masses are
different?
o Or would you place them in the same position because their chemical properties
are the same?

Thus, isotopes of all elements posed a challenge to Mendeleev’s Periodic Law. Another
problem was that the atomic masses do not increase in a regular manner in going from
one element to the next. So it was not possible to predict how many elements could be
discovered between two elements — especially when we consider the heavier
elements.
Questions
1. Use Mendeléev’s Periodic Table to predict the formulae for the oxides of the
following elements:
K, C, AI, Si, Ba.
2. Besides gallium, which other elements have since been discovered that were left
by Mendeléev in his Periodic Table? (any two)
3. What were the criteria used by Mendeléev in creating his Periodic Table?
4. Why do you think the noble gases are placed in a separate group?

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1.3 Making Order Out of Chaos – The Modern Periodic Table
In 1913, Henry Moseley showed that the atomic number (Symbolished as Z) of an
element is a more fundamental property than its atomic mass as described below.
Accordingly, Mendeléev’s Periodic Law was modified and atomic number was adopted
as the basis of Modern Periodic Table and the Modern
Periodic Law can be stated as follows:
‘Properties of elements are a periodic function of their atomic number.’
Let us recall that the atomic number gives us the number of protons in the nucleus of
an atom and this number increases by one in going from one element to the next.
Elements, when arranged in order of increasing atomic number Z, lead us to the
classification known as the Modern Periodic Table (Table 6). Prediction of properties of
elements could be made with more precision when elements were arranged on the
basis of increasing atomic number.
Activity 3
o How were the positions of cobalt and nickel resolved in the Modern Periodic Table?
o How were the positions of isotopes of various elements decided in the Modern
Periodic Table?
o Is it possible to have an element with atomic number 1.5 placed between hydrogen
and helium?
o Where do you think should hydrogen be placed in the Modern Periodic Table?

Table 6: Modern Periodic Table

As we can see, the Modern Periodic Table takes care of three limitations of Mendléev’s
Periodic Table. The anomalous position of hydrogen can be discussed after we see
what are the bases on which the position of an element in the Modern Periodic Table
depends.

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1.3.1 Position of Elements in the Modern Periodic
The Modern Periodic Table has 18 vertical columns known as ‘groups’ and 7 horizontal
rows known as ‘periods’. Let us see what decides the placing of an element in a certain
group and period.
Activity 4
o Look at the group 1 of the Modern Periodic Table, and name the elements
present in it.
o Write down the electronic configuration of the first three elements of group 1.
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o What similarity do you find in their electronic configurations?
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o How many valence electrons are present in these three elements? . co
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You will find that all these elements contain the same number of valence electrons.
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Similarly, you will find that the elements present in any one group have the same
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number of valence electrons. For example, elements fluorine (F) and chlorine (Cl),
belong to group 17, how many electrons do fluorine and chlorine have in their outermost
shells? Hence, we can say that groups in the Periodic Table signify an identical
outershell electronic configuration. On the other hand, the number of shells increases
as we go down the group.
There is an anomaly when it comes to the position of hydrogen because it can be placed
either in group 1 or group 17 in the first period.
Can you say why?
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Activity 5
o If you look at the long form of the Periodic Table, you will find that the elements
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Li, Be, B, C, N, O, F, and Ne are present in the second period. Write down their
electronic configuration.
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o Do these elements also contain the same number of valence electrons?
o Do they contain the same number of shells?
You will find that these elements do not have the same number of valence electrons,
but they contain the same number of shells. You also observe that the number of
valence shell electrons increases by one unit, as the atomic number increases by one
unit on moving from left to right in a period.
Or we can say that atoms of different elements with the same number of occupied shells
are placed in the same period. Na, Mg, Al, Si, P, S, Cl and Ar belong to the third period
of the Modern Periodic Table, since the electrons in the atoms of these elements are
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c. o How many elements are there in the first, second, third and fourth periods? m
confirm the above statement. Each period marks a new electronic shell getting filled. .co
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the maximum number of electrons that can be accommodated in a shell depends on
the formula 2n2 where ‘n’ is the number of the given shell from the nucleus.
For example,
K Shell – 2 × (1) 2 = 2, hence the first period has 2 elements.
L Shell – 2 × (2) 2 = 8, hence the second period has 8 elements.
M Shell – 2 × (3) 2 = 18, but the outermost shell can have only
8 electrons, so the third period also has only 8 elements.

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The position of an element in the Periodic Table tells us about its chemical reactivity.
As you have learnt, the valence electrons determine the kind and number of bonds
formed by an element. Can you now say why Mendeléev’s choice of formulae of
compounds as the basis for deciding the position of an element in his Table was a good
one? How would this lead to elements with similar chemical properties being placed in
the same group?
1.3.2 Trends in the Modern Periodic Table
Valency: As you know, the valency of an element is determined by the number of
valence electrons present in the outermost shell of its atom.
Activity 6
o How do you calculate the valency of an element from its electronic configuration?
o What is the valency of magnesium with atomic number 12 and sulphur with
atomic number 16?
o Similarly find out the valencies of the first twenty elements.
o How does the valency vary in a period on going from left to right?
o How does the valency vary in going down a group?

Atomic size: The term atomic size refers to the radius of an atom. The atomic size may
be visualised as the distance between the centre of the nucleus and the outermost shell
of an isolated atom. The atomic radius of hydrogen atom is 37 pm (picometre, 1 pm =
10–12m).
Let us study the variation of atomic size in a group and in a period.
Activity 7
o Atomic radii of the elements of the second period are given below:
Period II elements: B Be O N Li C
Atomic radius (pm): 88 111 66 74 152 77
o Arrange them in decreasing order of their atomic radii.
o Are the elements now arranged in the pattern of a period in the Periodic Table?
o Which elements have the largest and the smallest atoms?
o How does the atomic radius change as you go from left to right in a period?

You will see that the atomic radius decreases in moving from left to right along a period.
This is due to an increase in nuclear charge which tends to pull the electrons closer to
the nucleus and reduces the size of the atom.
Activity 8
o Study the variation in the atomic radii of first group elements given below and
arrange them in an increasing order.
Group 1 Elements: Na Li Rb Cs K
Atomic Radius (pm): 186 152 244 262 231
o Name the elements which have the smallest and the largest atoms.
o How does the atomic size vary as you go down a group?
You will see that the atomic size increases down the group. This is because new shells
are being added as we go down the group. This increases the distance between the
outermost electrons and the nucleus so that the atomic size increases in spite of the
increase in nuclear charge.

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Metallic and Non-metallic Properties
Activity 9
o Examine elements of the third period and classify them as metals and non-
metals.
o On which side of the Periodic Table do you find the metals?
o On which side of the Periodic Table do you find the non-metals?

As we can see, the metals like Na and Mg are towards the left-hand side of the Periodic
Table while the non-metals like sulphur and chlorine are found on the right-hand side.
In the middle, we have silicon, which is classified as a semi-metal or metalloid because
it exhibits some properties of both metals and non-metals.
In the Modern Periodic Table, a zig-zag line separates metals from non-metals. The
borderline elements – boron, silicon, germanium, arsenic, antimony, tellurium and
polonium – are intermediate in properties and are called metalloids or semi-metals.
As you have seen in Metals and Non-Metals, metals tend to lose electrons while forming
bonds, that is, they are electropositive in nature.
Activity 10
o How do you think the tendency to lose electrons will change in a group?
o How will this tendency change in a period?

As the effective nuclear charge acting on the valence shell electrons increases across
a period, the tendency to lose electrons will decrease. Down the group, the effective
nuclear charge experienced by valence electrons is decreasing because the outermost
electrons are farther away from the nucleus. Therefore, these can be lost easily. Hence
metallic character decreases across a period and increases down a group.
Non-metals, on the other hand, are electronegative. They tend to form bonds by gaining
electrons. Let us learn about the variation of this property.
Activity 11
o How would the tendency to gain electrons change as you go from left to right
across a period?
o How would the tendency to gain electrons change as you go down a group?

As the trends in the electronegativity show, non-metals are found on the right-hand side
of the Periodic Table towards the top.
These trends also help us to predict the nature of oxides formed by the elements
because it is known to you that the oxides of metals are basic and that of non-metals
are acidic in general.
Questions
1. How could the Modern Periodic Table remove various anomalies of Mendeléev’s
Periodic Table?
2. Name two elements you would expect to show chemical reactions similar to
magnesium. What is the basis for your choice?
3. Name
(a) three elements that have a single electron in their outermost shells.

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(b) two elements that have two electrons in their
(c) three elements with filled outermost shells.
4. (a) Lithium, sodium, potassium are all metals that react with water to liberate
hydrogen gas. Is there any similarity in the atoms of these elements?
(b) Helium is an unreactive gas and neon is a gas of extremely low reactivity.
What, if anything, do their atoms have in common?
5. In the Modern Periodic Table, which are the metals among the first ten elements?
6. By considering their position in the Periodic Table, which one of the following
elements would you expect to have maximum metallic characteristic?
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What you have learnt
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o Elements
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classified on the basis of similarities in their properties.
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o Döbereiner the elements into triads and Newlands gave the Law of
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oa Mendeléev arranged the elements in increasing order of their atomic masses and
according to their chemical properties.
o Mendeléev even predicted the existence of some yet to be discovered elements
on the basis of gaps in his Periodic Table.
o Anomalies in arrangement of elements based on increasing atomic mass could
be removed when the elements were arranged in order of increasing atomic
number, a fundamental property of the element discovered by Moseley.
o Elements in the Modern Periodic Table are arranged in 18 vertical columns called
groups and 7 horizontal rows called periods.
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o Elements thus arranged show periodicity of properties including atomic size,
valency or combining capacity and metallic and non-metallic character.
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Exercise
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1. Which of the following statements is not a correct statement about the trends
when going from left to right across the periods of periodic Table.
(a) The elements become less metallic in nature.
(b) The number of valence electrons increases.
(c) The atoms lose their electrons more easily.
(d) The oxides become more acidic.
2. Element X forms a chloride with the formula XCl2, which is a solid with a high
melting point. X would most likely be in the same group of the Periodic Table as
(a) Na (b) Mg (c) AI (d) Si
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(a) two shells, both of which are completely filled with electrons?

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(c) a total of three shells, with four electrons in its valence shell? la
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(d) a total of two shells, with three electrons in its valence shell?
a (e) twice as many electrons in its second shell as in its first shell?
4.
(a) What property do all elements in the same column of the Periodic Table
as boron have in common?
(b) What property do all elements in the same column of the Periodic Table
as fluorine have in common?
5. An atom has electronic configuration 2, 8, 7.
(a) What is the atomic number of this element?
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(b) To which of the following elements would it be chemically similar?
(Atomic numbers are given in parentheses.)
N(7) F(9) P(15) Ar(18)
6. The position of three elements A, B and C in the Periodic Table are shown
below –
Group 16 Group 17
- -
- A
- -
B C
(a) State whether A is a metal or non-metal.
(b) State whether C is more reactive or less reactive than A.
(c) Will C be larger or smaller in size than B?
(d) Which type of ion, cation or anion, will be formed by element A?
7. Nitrogen (atomic number 7) and phosphorus (atomic number 15) belong to
group 15 of the Periodic Table. Write the electronic configuration of these two
elements. Which of these will be more electronegative? Why?
8. How does the electronic configuration of an atom relate to its position in the
Modern Periodic Table?
9. In the Modern Periodic Table, calcium (atomic number 20) is surrounded by
elements with atomic numbers 12, 19, 21 and 38. Which of these have physical
and chemical properties resembling calcium?
10. Compare and contrast the arrangement of elements in Mendeléev’s Periodic
Table and the Modern Periodic Table.

Group Activity
I. We have discussed the major attempts made for classifying elements. Find out
(from the internet or library) about other attempts to classify elements.
II. We have studied the long form of the Periodic Table. The Modern Periodic Law
has been used to arrange elements in other ways too. Find out what are these.

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Unit 2
Heredity and Evolution
2.1 Evolution
We have noted that there is an inbuilt tendency to variation during reproduction, both
because of errors in DNA copying, and as a result of sexual reproduction. Let us now
look at some consequences of this tendency.

2.1.1 An Illustration
Consider a group of twelve red beetles. They live, let us assume, in some bushes with
green leaves. Their population will grow by sexual reproduction, and therefore, can
generate variations. Let us imagine also that crows eat these beetles. The more beetles
the crows eat; the fewer beetles are available to reproduce. Now, let us think about
some different situations (Fig. 1) that can develop in this beetle population.

Fig. 1 Variation in a population – inherited and otherwise
In the first situation, a colour variation arises during reproduction, so that there is one
beetle that is green in colour instead of red. This beetle, moreover, can pass the colour
on to its progeny, so that all its progeny beetles are green. Crows cannot see green-
coloured beetles on the green leaves of the bushes, and therefore cannot eat them.
What happens then? The progeny of green beetles is not eaten, while the progeny of
red beetles continues to be eaten. As a result, there are more and more green beetles
than red ones in the beetle population.
In a second situation, again, a colour variation arises during reproduction, but now it
results in a beetle that is blue in colour instead of red. This beetle can also pass the
colour on to its progeny, so that all its progeny beetles are blue. Crows can see blue-
coloured beetles in the green leaves of the bushes as well as they can see red ones,
and therefore can eat them. What happens initially? In the population, as it expands,
there are a few blue beetles, but most are red. But at this point, an elephant comes by,
and stamps on the bushes where the beetles live. This kills most of the beetles. By
chance, the few beetles that have survived are mostly blue. The beetle population slowly
expands again, but now, the beetles in the population are mostly blue.
It is obvious that in both situations, what started out as a rare variation came to be a
common characteristic in the population. In other words, the frequency of an inherited
trait changed over generations. Since genes control traits, we can say that the frequency
of certain genes in a population changed over generations. This is the essence of the
idea of evolution.

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But there are interesting differences, too, in the two situations.
variation became common because it gave a survival advantage. In other words, it was
naturally selected. We can see that the natural selection is exerted by the crows. The
more crows there are, the more red beetles would be eaten, and the more the proportion
of green beetles in the population would be. Thus, natural selection is directing evolution
in the beetle population. It results in adaptations in the beetle population to fit their
environment better.
In the second situation, the colour change gave no survival advantage. Instead, it was
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simply a matter of accidental survival of beetles of one colour that changed the common
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characteristic of the resultant population. The elephant would not have caused such
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major havoc in the beetle population if the beetle population had been very large. So,
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accidents in small populations can change the frequency of some genes in a population,
even if they give no survival advantage. This is the notion of genetic drift, which provides ag
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diversity without any adaptations.
Now consider a third situation. In this, as the beetle population begins to expand, the
bushes start suffering from a plant disease. The amount of leaf material for the beetles
is reduced. The beetles are poorly nourished as a result. The average weight of adult
beetles decreases from what it used to be when leaves were plentiful, but there is no
genetic change occurring. After a few years and a few beetle generations of such
scarcity, the plant disease is eliminated. There is a lot of leaf food. At this time, what
would we expect the weight of the beetles to be?

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2.1.2 Acquired and Inherited Traits
c
m. of the
We discussed the idea that the germ cells of sexually
in specialised reproductive tissue. If the eweight
reproducing populations are made

starvation, that will not change the DNA
s
la of ofthea starving
beetle is reduced because of
germ cells. Therefore, low weight is not
a g
a trait that can be inherited by the progeny beetle. Therefore, even if some
generations of beetles are low in weight because of starvation, that is not an example
of evolution, since the change is not inherited over generations. Change in non-
reproductive tissues cannot be passed on to the DNA of the germ cells. Therefore, the
experiences of an individual during its lifetime cannot be passed on to its progeny, and
cannot direct evolution.
Consider another example of how an individual cannot pass on to its progeny the
experiences of its lifetime. If we breed a group of mice, all their progeny will have tails,
m
.co
as expected. Now, if the tails of these mice are removed by surgery in each generation,
m do these tailless mice have tailless progeny? The answer is no, and it makes sense

m .co because removal of the tail
s e m
s e cannot change the genes of the
g la
la
germ cells of the mice.
g a
a This is the reason why the ideas
of heredity and genetics that we
have discussed earlier are so
essential for understanding
evolution. Even Charles
Darwin, who came up with the
idea of evolution of species by
natural selection in the nineteenth century, could not work out the mechanism. It is ironic
that he could have done so if he had seen the significance of the experiments his
m . c
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s em
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Page 13 of 25

Austrian contemporary, Gregor Mendel, was doing. But then, Mendel too did not notice
Darwin’s work as relevant to his!

Do You Know? Origin of life on earth
Darwin’s theory of evolution tells us how life evolved from simple to more complex forms
and Mendel’s experiments give us the mechanism for the inheritance of traits from one
generation to the next. But neither tells us anything about how life began on earth in the
first place. J.B.S. Haldane, a British scientist (who became a citizen of India later),
suggested in 1929 that life must have developed from the simple inorganic molecules
which were present on earth soon after it was formed. He speculated that the conditions
on earth at that time, which were far from the conditions we see today, could have given
rise to more complex organic molecules that were necessary for life. The first primitive
organisms would arise from further chemical synthesis.
How did these organic molecules arise? An answer was suggested by the experiment
conducted by Stanley L. Miller and Harold C. Urey in 1953. They assembled an
atmosphere similar to that thought to exist on early earth (this had molecules like
ammonia, methane and hydrogen sulphide, but no oxygen) over water. This was
maintained at a temperature just below 100°C and sparks were passed through the
mixture of gases to simulate lightning. At the end of a week, 15% of the carbon (from
methane) had been converted to simple compounds of carbon including amino acids
which make up protein molecules. So, can life arise afresh on earth even now?
Question
1. What are the different ways in which individuals with a particular trait may
increase in a population?
2. Why are traits acquired during the life-time of an individual not inherited?
3. Why are the small numbers of surviving tigers a cause of worry from the point of
view of genetics?

2.2 Speciation
What we have seen so far is micro-evolution. That means that the changes are small,
even though they are significant. Also, they simply change the common characteristics
of a particular species. But this does not properly explain how new species come into
existence. That can be said to have happened only if this group of beetles we are
thinking about, splits into two populations that cannot reproduce with each other. When
this happens, they can be called two independent species. So, can we extend the
reasoning we have used above to explain such speciation?
Consider what would happen if the bushes the beetles feed on are spread widely over
a mountain range. The beetle population becomes very large as a result. But individual
beetles feed mostly on a few nearby bushes throughout their lifetime. They do not travel
far. So, in this huge population of beetles, there will be sub-populations in
neighbourhoods. Since male and female beetles have to meet for reproduction to
happen, most reproduction will be within these sub-populations. Of course, an
occasional adventurous beetle might go from one site to another. Or a beetle is picked
up by a crow from one site and dropped in the other site without being eaten. In either
case, the migrant beetle will reproduce with the local population. This will result in the
genes of the migrant beetle entering a new population. This kind of gene flow is bound
to happen between populations that are partly, but not completely separated. If,
however, between two such sub-populations a large river comes into existence, the two

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populations will be further isolated. The levels of gene flow between them will decrease
even further.
Over generations, genetic drift will accumulate different changes in each sub-
population. Also, natural selection may also operate differently in these different
geographic locations. Thus, for example, in the territory of one sub-population, crows
are eliminated by eagles. But this does not happen for the other sub-population, where
crow numbers are very high. As a result, the green variation will not be selected at the
first site, while it will be strongly selected at the second.
Together, the processes of genetic drift and natural selection will result in these two
isolated sub-populations of beetles becoming more and more different from each other.
Eventually, members of these two groups will be incapable of reproducing with each
other even if they happen to meet.
There can be a number of ways by which this can happen. If the DNA changes are
severe enough, such as a change in the number of chromosomes, eventually the germ
cells of the two groups cannot fuse with each other. Or a new variation emerges in which
green females will not mate with red males, but only with green males. This allows very
strong natural selection for greenness. Now, if such a green female beetle meets a red
male from the other group, her behaviour will ensure that there is no reproduction
between them. Effectively, new species of beetles are being generated.

Questions
1. What factors could lead to the rise of a new species?
2. Will geographical isolation be a major factor in the speciation of a self-
pollinating plant species? Why or why not?
3. Will geographical isolation be a major factor in the speciation of an organism
that reproduces asexually? Why or why not?

2.3 Evolution and Classification
Based on these principles, we can work out the evolutionary relationships of the species
we see around us. It is a sort of going backwards in time. We can do this by identifying
hierarchies of characteristics between
species. In order to understand this process, let us think back to our discussion on the
classification of organisms in Class IX.
Similarities among organisms will allow us to group them and then study the groups.
For this, which characteristics decide more fundamental differences among organisms,
and which ones decide less basic differences? What is meant by ‘characteristics’,
anyway? Characteristics are details of appearance or behaviour; in other words, a
particular form or a particular function. That we have four limbs is thus a characteristic.
That plants can do photosynthesis is also a characteristic.
Some basic characteristics will be shared by most organisms. The cell is the basic unit
of life in all organisms. The characteristics in the next level of classification would be
shared by most, but not all organisms. A basic characteristic of cell design that differs
among different organisms is whether the cell has a nucleus. Bacterial cells do not,
while the cells of most other organisms do. Among organisms with nucleated cells,
which ones are unicellular and which ones multi-cellular? That property marks a very
basic difference in body design, because of specialisation of cell types and tissues.
Among multi-cellular organisms, whether they can undertake photosynthesis or not will

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.
.co s e m

s em l a
a ag organisms that cannot
provide the next level of classification. Among the multi-cellular
do photosynthesis, whether the skeleton is inside the body or around the body will mark
another fundamental design difference. We can see that, even in these few questions
that we have asked, a hierarchy is developing that allows us to make classification
groups.
The more characteristics two species will have in common, the more closely they are
related. And the more closely they are related, the more recently they will have had a
common ancestor. An example will help. A brother and a sister are closely related. They
m
have common ancestors in the first generation before them, namely, their parents. A
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girl and her first cousin are also related, but less than the girl and her brother. This
. e m
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is because cousins have common ancestors, their grandparents, in the second
l as
generation s g
g a
l is in fact a reflection of their evolutionary relationship.
of species a
before them, not in the first one. We can now appreciate that classification

a
We can thus build up small groups of species with recent common ancestors, then
super-groups of these groups with more distant common ancestors, and so on. In
theory, we can keep going backwards like this
until we come to the notion of a single species at the very beginning of evolutionary
time. If that is the case, then at some point in the history of the earth, non-living material
must have given rise to life. There are many theories about how this might have
happened. It would be interesting to come up with theories of our own!

o m
2.3.1 Tracing Evolutionary Relationships c
. how do we
m
When we try to follow evolutionary relationships,
e characteristics
identify characteristics as common? These
a s
l because they are
inherited from a common ancestor.a g
in different organisms would be similar

As an example, consider the fact that mammals have four
limbs, as do birds, reptiles and amphibians (Fig. 2). The
basic structure of the limbs is similar though it has been
modified to perform different functions in various
vertebrates. Such a homologous characteristic helps to
identify an evolutionary relationship between apparently
different species.
m
m However, all similarities simply in organ shape are not
.co
m .co necessarily because of common ancestry. What would we
s e m
la
think about the wings of birds and bats, for
s e example (Fig. 3)? Birds and bats have
g
g la wings, but squirrels and lizards do not. So a
a are birds and bats more closely related to
each other than to squirrels or lizards?
Before we jump to this conclusion, let us
look at the wings of birds and bats more
closely. When we do that, we find that the wings of bats are skin folds stretched mainly
between elongated fingers. But the wings of birds are a feathery covering all along the
arm. The designs of the two wings, their structure and components, are thus very
different. They look similar because they have a common use for flying, but their origins
m . c
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are not common. This makes them analogous characteristics, rather than homologous
characteristics. It would now be interesting to think about whether bird arms and bat
arms should be considered homologous or analogous!
2.3.2 Fossils
Such studies of organ structure can be done not only on current species, but also on
species that are no longer alive. How do we know that these extinct species ever
existed? We know this from finding fossils (Fig. 4). What are fossils? Usually, when
organisms die, their bodies will decompose and be lost. But every once in a while, the
body or at least some parts may be in an environment that does not let it decompose
completely. If a dead insect gets caught in hot mud, for example, it will not decompose
quickly, and the mud will eventually harden and retain the impression of the body parts
of the insect. All such preserved traces of living organisms are called fossils.

How do we know how old the fossils are? There are two components to this estimation.
One is relative. If we dig into the earth and start finding fossils, it is reasonable to
suppose that the fossils we find closer to the surface are more recent than the fossils
we find in deeper layers. The second way of dating fossils is by detecting the ratios of
different isotopes of the same element in the fossil material. It would be interesting to
find out exactly how this method works!

How do fossils form layer by layer?
Let us start 100 million years ago. Some invertebrates on the
sea-bed die, and are buried in the sand. More sand
accumulates, and sandstone forms under pressure.
Millions of years later, dinosaurs living in the area die, and their bodies, too, are buried
in mud. This mud is also compressed into rock, above the rock containing the earlier
invertebrate fossils.

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Again, millions of years later, the bodies of horse-like creatures dying in the area are
fossilised in rocks above these earlier rocks.
Much later, by erosion or water flow wears away some
of the rock and exposes the horse-like fossils. As we
dig deeper, we will find older and older fossils.

2.3.3 Evolution by Stages
A question that arises here is – if complicated organs,
such as the eye, are selected for the advantage they
provide, how can they be generated by a single DNA
change? Surely such complex organs will be created
bit-by-bit over generations? But how can each
intermediate change be selected for? There are a
number of possible explanations. Even an
intermediate stage (Fig. 5), such as a rudimentary
eye, can be useful to some extent. This might be
enough to give a fitness advantage. In fact, the eye –
like the wing – seems to be a very popular adaptation.
Insects have them, so does an octopus, and so do
vertebrates. And the structure of the eye in each of
these organisms is different – enough for them to have
separate evolutionary origins.
Also, a change that is useful for one property to start
with can become useful later for quite a different
function. Feathers, for example, can start out as
providing insulation in cold weather (Fig. 6). But later,
they might become useful for flight. In fact, some
dinosaurs had feathers, although they could not fly
using the feathers. Birds seem to have later adapted
the feathers to flight. This, of course, means that birds
are very closely related to reptiles, since dinosaurs
were reptiles!
It is all very well to say that very dissimilar looking
structures evolve from a common ancestral design. It
is true that analysis of the organ structure in fossils
allows us to make estimates of how far back
evolutionary relationships go. But those are guesses
about what happened in history. Are there any current examples of such a process?

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.
.co s e m

s em l a
a agover more than two thousand
The wild cabbage plant is a good example. Humans have,
years, cultivated wild cabbage as a food plant, and generated different vegetables from
it by selection (see Fig. 7). This is, of course, artificial selection rather than natural
selection. So some farmers have wanted to select for very short distances between
leaves, and have bred the cabbage we eat. Some have wanted to select for arrested
flower development, and have bred
broccoli, or for sterile flowers, and
have made the cauliflower. Some
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have selected for swollen parts, and

. c om
come up with kohlrabi. Some have
simply looked for slightly larger
e m .
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leaves, and come up with a leafy
l as
as
vegetable called kale. Would we have
l ag
ag
thought that all these structures are
descended from the same ancestor if
we had not done it ourselves?
Another way of tracing evolutionary relationships depends on the original idea that we
started with. That idea was that changes in DNA during reproduction are the basic
events in evolution. If that is the case, then comparing the DNA of different species
should give us a direct estimate of how much the DNA has changed during the formation
of these species. This method is now extensively used to define evolutionary
relationships.
m
Molecular phylogeny
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We have been discussing how changes in the DNA during cell division would lead to
s
a
changes in the proteins that are made from this new DNA. Another point that has been
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made is that these changes would accumulate from one generation to the next. Could
this be used to trace the changes in DNA backwards in time and find out where each
change diverged from the other? Molecular phylogeny does exactly this. This approach
is based on the idea that organisms which are more distantly related will accumulate a
greater number of differences in their DNA. Such studies trace the evolutionary
relationships and it has been highly gratifying to find that the relationships among
different organisms shown by molecular phylogeny match the classification scheme that
we learnt in Class IX.
Questions
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1. Give an example of characteristics being used to determine how close two
species are in evolutionary terms.
e m
e m 2. Can the wing of a butterfly and the wing of a bat be considered homologous
las
las organs? Why or why not?
ag
ag 3. What are fossils? What do they tell us about the process of evolution?

2.4 Evolution should not be equated with ‘Progress’
In an exercise of tracing the family trees of species, we need to remember certain things.
Firstly, there are multiple branches possible at each and every stage of this process. So
it is not as if one species is eliminated to give rise to a new one. A new species has
emerged. But that does not necessarily mean, like the beetle example we have been
thinking about, that the old species will disappear. It will all depend on the environment.
Also, it is not as if the newly generated species are in any way ‘better’ than the older
m . c
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one. It is just that natural selection and genetic drift have together led to the formation
of a population that cannot reproduce with the original one. So, for example, it is not
true that human beings have evolved from chimpanzees. Rather, both human beings
and chimpanzees have a common ancestor a long time ago. That common ancestor is
likely to have been neither human or chimpanzee. Also, the first step of separation from
that ancestor is unlikely to have resulted in modern chimpanzees and human beings.
Instead, the two resultant species have probably evolved in their own separate ways to
give rise to the current forms.
In fact, there is no real ‘progress’ in the idea of evolution. Evolution is simply the
generation of diversity and the shaping of the diversity by environmental selection. The
only progressive trend in evolution seems to be that more and more complex body
designs have emerged over time. However, again, it is not as if the older designs are
inefficient! So many of the older and simpler designs still survive. In fact, one of the
simplest life forms – bacteria – inhabit the most inhospitable habitats like hot springs,
deep-sea thermal vents and the ice in Antarctica. In other words, human beings are not
the pinnacle of evolution, but simply yet another species in the teeming spectrum of
evolving life.
2.4.1 Human Evolution
The same tools for tracing evolutionary relationships – excavating, time-dating and
studying fossils, as well as determining DNA sequences – have been used for studying
human evolution. There is a great diversity of human forms and features across the
planet. So much so that, for a long time, people used to talk about human ‘races’. Skin
colour used to be the commonest way of identifying these so called races. Some were
called yellow, some black, white or brown. A major question debated for a long time
was, have these apparent groups evolved differently? Over recent years, the evidence
has become very clear. The answer is that there is no biological basis to the notion of
human races. All humans are a single species.
Not only that, regardless of where we have lived for the past few thousand years, we all
come from Africa. The earliest members of the human species, Homo sapiens, can be
traced there. Our genetic footprints can be traced back to our African roots. A couple of
hundred thousand years ago, some of our ancestors left Africa while others stayed on.
While the residents spread across Africa, the migrants slowly spread across the planet
– from Africa to West Asia, then to Central Asia, Eurasia, South Asia, East Asia. They
travelled down the islands of Indonesia and the Philippines to Australia, and they
crossed the Bering land bridge to the Americas. They did not go in a single line, so they
were not travelling for the sake of travelling, obviously. They went forwards and
backwards, with groups sometimes separating from each other, sometimes coming
back to mix with each other, even moving in and out of Africa. Like all other species on
the planet, they had come into being as an accident of evolution, and were trying to live
their lives the best they could.

Questions

1. Why are human beings who look so different from each other in terms of size,
colour and looks said to belong to the same species?
2. In evolutionary terms, can we say which among bacteria, spiders, fish and
chimpanzees have a ‘better’ body design? Why or why not?

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Unit 3
Magnetic Effects of Electric Current

3.1 Electric Motor
An electric motor is a rotating device that
converts electrical energy to mechanical
energy. Electric motor is used as an important
component in electric fans, refrigerators,
mixers, washing machines, computers, MP3
players etc. Do you know how an electric
motor works?
An electric motor, as shown in Fig. 1, consists
of a rectangular coil ABCD of insulated copper
wire. The coil is placed between the two poles
of a magnetic field such that the arm AB and
CD are perpendicular to the direction of the magnetic field. The ends of the coil are
connected to the two halves P and Q of a split ring. The inner sides of these halves are
insulated and attached to an axle. The external conducting edges of P and Q touch two
conducting stationary brushes X and Y, respectively, as shown in the Fig. 1.
Current in the coil ABCD enters from the source battery through conducting brush X
and flows back to the battery through brush Y. Notice that the current in arm AB of the
coil flows from A to B. In arm CD it flows from C to D, that is, opposite to the direction
of current through arm AB. On applying Fleming’s left hand rule for the direction of force
on a current-carrying conductor in a magnetic field (see Fig. 13.13). We find that the
force acting on arm AB pushes it downwards while the force acting on arm CD pushes
it upwards. Thus the coil and the axle O, mounted free to turn about an axis, rotate anti-
clockwise. At half rotation, Q makes contact with the brush X and P with brush Y.
Therefore, the current in the coil gets reversed and flows along the path DCBA. A device
that reverses the direction of flow of current through a circuit is called a commutator. In
electric motors, the split ring acts as a commutator. The reversal of current also reverses
the direction of force acting on the two arms AB and CD. Thus the arm AB of the coil
that was earlier pushed down is now pushed up and the arm CD previously pushed up
is now pushed down. Therefore, the coil and the axle rotate half a turn more in the same
direction. The reversing of the current is repeated at each half rotation, giving rise to a
continuous rotation of the coil and to the axle.
The commercial motors use (i) an electromagnet in place of permanent magnet; (ii)
large number of turns of the conducting wire in the current carrying coil; and (iii) a soft
iron core on which the coil is wound. The soft iron core, on which the coil is wound, plus
the coils, is called an armature. This enhances the power of the motor.

Questions
1. State Fleming’s left-hand rule.
2. What is the principle of an electric motor?
3. What is the role of the split ring in an electric motor?

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a 3.2 Electromagnetic Induction ag
We have studied that when a current-carrying conductor is placed in a magnetic field
such that the direction of current is perpendicular to the magnetic field, it experiences a
force. This force causes the conductor to move. Now let us imagine a situation in which
a conductor is moving inside a magnetic field or a magnetic field is changing around a
fixed conductor. What will happen? This was first studied by English physicist Michael
Faraday. In 1831, Faraday made an important breakthrough by discovering how a
moving magnet can be used to generate electric currents. To observe this effect, let us
m
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perform the following activity.
Activity
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 Take a coil of wire AB having a large number of turns.
l as
l as
 Connect the ends of the coil to a
ag
ag galvanometer as shown in Fig. 13.16.
 Take a strong bar magnet and move its
north pole towards the end B of the coil. Do
you find any change in the galvanometer
needle?
 There is a momentary deflection in the
needle of the galvanometer, say to the
right. This indicates the presence of a
current in the coil AB. The deflection becomes zero the moment the motion of
the magnet stops.
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 Now withdraw the north pole of the magnet away from the coil. Now the
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galvanometer is deflected toward the left, showing that the current is now set up
s
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in the direction opposite to the first.
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ag
 Place the magnet stationary at a point near to the coil, keeping its north pole
towards the end B of the coil. We see that the galvanometer needle deflects
toward the right when the coil is moved towards the north pole of the magnet.
Similarly, the needle moves toward left when the coil is moved away.
 When the coil is kept stationary with respect to the magnet, the deflection of the
galvanometer drops to zero. What do you conclude from this activity?
A galvanometer is an instrument that can detect the presence of a
current in a circuit. The pointer remains at zero (the centre of the scale)
for zero current flowing through it. It can deflect either to the left or to
m
the right of the zero mark depending on the direction of current.
m can also check that if you had moved south pole of the magnet m
c. o You .co
m s ejust be opposite
s e towards the end B of the coil, the deflections in the galvanometer would
to the previous case. When the coil and the magnet are bothgstationary,l a
la athat motion ofthere is no

ag
deflection in the galvanometer. It is, thus, clear from this activity a magnet
with respect to the coil produces an induced potential difference, which sets up an
induced electric current in the circuit.
Michael Faraday was an experimental physicist. He had no formal
education. He worked in a book-binding shop during his early years. He
used to read books that came for binding. This way Faraday developed
his interest in science. He got an opportunity to listen to some public
lectures by Humphrey Davy of Royal Institute. He made careful notes of

m
Davy’s lectures and sent them to Davy. Soon he was made an assistant
. c
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in Davy’s laboratory at the Royal Institute. Faraday made several path-breaking
discoveries that include electromagnetic induction and the laws of electrolysis. Several
universities conferred on him the honorary degrees but he turned down such honours.
Faraday loved his science work more than any honour.
Let us now perform a variation of Activity 13.8 in which the moving magnet is replaced
by a current-carrying coil and the current in the coil can be varied.
Activity
 Take two different coils of copper wire
having large number of turns (say 50
and 100 turns respectively). Insert them
over a non-conducting cylindrical roll, as
shown in Fig. 3. (You may use a thick
paper roll for this purpose.)
 Connect the coil-1, having larger
number of turns, in series with a battery
and a plug key. Also connect the other
coil-2 with a galvanometer as shown.
 Plug in the key. Observe the galvanometer. Is there a deflection in its needle?
You will observe that the needle of the galvanometer instantly jumps to one side
and just as quickly returns to zero, indicating a momentary current in coil-2.
 Disconnect coil-1 from the battery. You will observe that the needle momentarily
moves, but to the opposite side. It means that now the current flows in the
opposite direction in coil-2.
In this activity we observe that as soon as the current in coil-1 reaches either a steady
value or zero, the galvanometer in coil-2 shows no deflection.
From these observations, we conclude that a potential difference is induced in the coil-
2 whenever the electric current through the coil–1 is changing (starting or stopping).
Coil-1 is called the primary coil and coil-2 is called the secondary coil. As the current in
the first coil changes, the magnetic field associated with it also changes. Thus the
magnetic field lines around the secondary coil also change. Hence the change in
magnetic field lines associated with the secondary coil is the cause of induced electric
current in it. This process, by which a changing magnetic field in a conductor induces a
current in another conductor, is called
electromagnetic induction. In practice we can
induce current in a coil either by moving it in a
magnetic field or by changing the magnetic field
around it. It is convenient in most situations to
move the coil in a magnetic field.
The induced current is found to be the highest
when the direction of motion of the coil is at right
angles to the magnetic field. In this situation, we
can use a simple rule to know the direction of the
induced current. Stretch the thumb, forefinger and middle finger of right hand so that
they are perpendicular to each other, as shown in Fig. 4. If the forefinger indicates the
direction of the magnetic field and the thumb shows the direction of motion of conductor,

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then the middle finger will show the direction of induced current. This simple rule is
called Fleming’s right-hand rule.
Question
1. Explain different ways to induce current in a coil.

3.3 Electric Generator
Based on the phenomenon of electromagnetic induction, the experiments studied above
generate induced current, which is usually very small. This principle is also employed
to produce large currents for use in homes and industry. In an electric generator,
mechanical energy is used to rotate a conductor in a magnetic field to produce
electricity.
An electric generator, as shown in Fig. 4, consists of a rotating rectangular coil ABCD
placed between the two poles of a permanent magnet. The two ends of this coil are
connected to the two rings R1 and R2. The inner side of these rings are made insulated.
The two conducting stationary brushes B1 and B2 are kept pressed separately on the
rings R1 and R2, respectively. The two rings R1 and R2 are internally attached to an
axle. The axle may be mechanically rotated from outside to rotate the coil inside the
magnetic field. Outer ends of the two brushes are connected to the galvanometer to
show the flow of current in the given external circuit.
When the axle attached to the two rings is
rotated such that the arm AB moves up (and
the arm CD moves down) in the magnetic field
produced by the permanent magnet. Let us say
the coil ABCD is rotated clockwise in the
arrangement shown in Fig. 4. By applying
Fleming’s right-hand rule, the induced currents
are set up in these arms along the directions
AB and CD. Thus an induced current flows in
the direction ABCD. If there are larger numbers
of turns in the coil, the current generated in
each turn adds up to give a large current
through the coil. This means that the current in the external circuit flows from B2 to B1.
After half a rotation, arm CD starts moving up and AB moving down. As a result, the
directions of the induced currents in both the arms change, giving rise to the net induced
current in the direction DCBA. The current in the external circuit now flows from B1 to
B2. Thus after every half rotation the polarity of the current in the respective arms
changes. Such a current, which changes direction after equal intervals of time, is called
an alternating current (abbreviated as AC). This device is called an AC generator.
To get a direct current (DC, which does not change its direction with time), a split-ring
type commutator must be used. With this arrangement, one brush is at all times in
contact with the arm moving up in the field, while the other is in contact with the arm
moving down. We have seen the working of a split ring commutator in the case of an
electric motor (see Fig. 4). Thus a unidirectional current is produced. The generator is
thus called a DC generator.
The difference between the direct and alternating currents is that the direct current
always flows in one direction, whereas the alternating current reverses its direction

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periodically. Most power stations constructed these days
changes direction after every 1/100 second, that is, the frequency of AC is 50 Hz. An
important advantage of AC over DC is that electric power can be transmitted over long
distances without much loss of energy.

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