NCERT Solutions for Class 10 Science Periodic Classification of Elements [Old Book]

NCERT
SOLUTIONS
CLASS - 10th

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Class : 10th
Subject : Science
Chapter : 5
Chapter Name : Periodic Classi cation Of Elements

Q1 Did Döbereiner’s triads also exist in the columns of Newlands’ Octaves? Compare and nd out.

Answer. Only one triad of Dobereiner's triads exists in the columns of Newland’s octaves. The triad
formed by the elements Li, Na, and K of Dobereiner's triads also occurred in the columns of
Newlands' octaves. Dobereiner's triads
Li Ca Cl
Na Sr Br
K Bal
Newlands' octaves

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Q2 What were the limitations of Döbereiner’s classi cation?

Answer. Limitation of Dobereiner's classi cation: All known elements could not be classi ed into
groups of triads on the basis of their properties.

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Q3 What were the limitations of Newlands’ Law of Octaves?

Answer. Limitations of Newlands' law of octaves:
(i) It was not applicable throughout the arrangements. It was applicable up to calcium only. The
properties of the elements listed after calcium showed no resemblance to the properties of the

elements above them.
(ii) Those elements that were discovered after Newlands' octaves did not follow the law of octaves.
(iii) The position of cobalt and nickel in the group of the elements (F, Cl) of different properties
could not be explained.
(iv) Placing of iron far away from cobalt and nickel, which have similar properties as iron, could
also not be explained.

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Q1 Use Mendeléev’s Periodic Table to predict the formulae for the oxides of the following
elements: K, C, AI, Si, Ba.

Answer. K is in group 1. Therefore, the oxide will be K O. 2

C is in group 4. Therefore, the oxide will be CO .
2

A1 is in group 3. Therefore, the oxide will be AL O .
2 3

Si is in group 4. Therefore, the oxide will be SiO .
2

Ba is in group 2. Therefore, the oxide will be BaO.

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Q2 Besides gallium, which other elements have since been discovered that were left by Mendeléev
in his Periodic Table? (any two)

Answer. Scandium and germanium

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Q3 What were the criteria used by Mendeléev in creating his Periodic Table?

Answer.Mendeleev's periodic table was based on the observation that the properties of elements
are a periodic function of their atomic masses. This means that if elements are arranged in the
increasing order of their atomic masses, then their properties get repeated after regular intervals .

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Q4 Why do you think the noble gases are placed in a separate group?

Answer. Noble gases are inert elements. Their properties are different from the all other elements.
Therefore, the noble gases are placed in a separate group.

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Q1 How could the Modern Periodic Table remove various anomalies of Mendeléev’s Periodic
Table?

Answer. Mendeleev was unable to give xed position to hydrogen and isotopes in the periodic
table. In Mendeleev's periodic table, the increasing manner of atomic mass of the elements is not

always regular from one to its next. It was believed that a more fundamental property than atomic
mass could explain periodic properties in a better manner. It was Henry Moseley who
demonstrated that atomic number of an element could explain periodic properties in a better way
than atomic mass of an element and arranged the elements in increasing order of their atomic
numbers. Then it was found that the various anomalies of Mendeleev's periodic table were
removed by the modern periodic table.

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Q2 Name two elements you would expect to show chemical reactions similar to magnesium. What
is the basis for your choice?

Answer. Calcium (Ca) and strontium (Sr) are expected to show chemical reactions similar to
magnesium (Mg). This is because the number of valence electrons (2) is same in all these three
elements. And since chemical properties are due to valence electrons, they show same chemical
reactions.

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Q3 Name
(a) three elements that have a single electron in their outermost shells.
(b) two elements that have two electrons in their outermost shells.
(c) three elements with lled outermost shells.

Answer. (a) Lithium (Li), sodium (Na), and potassium (K) have a single electron in their outermost
shells.
(b) Magnesium (Mg) and calcium (Ca) have two electrons in their outermost shells.
(c) Neon (Ne), argon (Ar), and xenon (Xe) have lled outermost shells.

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Q4 (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?

Answer. (a) Yes. The atoms of all the three elements lithium, sodium, and potassium have one
electron in their outermost shells.
(b) Both helium (He) and neon (Ne) have lled outermost shells. Helium has a duplet in its K shell,
while neon has an octet in its L shell.

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Q5 In the Modern Periodic Table, which are the metals among the rst ten elements?

Answer. Among the rst ten elements, lithium (Li) and beryllium (Be) are metals.

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Q6 By considering their position in the Periodic Table, which one of the following elements would
you expect to have maximum metallic characteristic?
Ga Ge As Se Be

Answer. Since Be lies to the extreme left hand side of the periodic table, Be is the most metallic
among the given elements.

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Q1 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.

Answer. (c) The atoms lose their electrons more easily.
(On moving from left to right across the periods of the periodic table, the non-metallic character
increases. Hence, the tendency to lose electrons decreases.)

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Q2 Element X forms a chloride with the formula XCL , which is a solid with a high melting
2

point. X would most likely be in the same group of the Periodic Table as
(a) Na (b) Mg (c) AI (d) Si

Answer. (b) X would most likely be in the same group of the Periodic Table as magnesium (Mg).

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Q3 Which element has
(a) two shells, both of which are completely lled with electrons?
(b) the electronic con guration 2, 8, 2?
(c) a total of three shells, with four electrons in its valence shell?
(d) a total of two shells, with three electrons in its valence shell?
(e) twice as many electrons in its second shell as in its rst shell?

Answer. (a) Neon has two shells, both of which are completely lled with electrons (2 electrons in
K shell and 8 electrons in L shell).
(b) Magnesium has the electronic con guration 2, 8, 2.
(c) Silicon has a total of three shells, with four electrons in its valence shell (2 electrons in K shell,
8 electrons in L shell and 4 electrons in M shell).
(d) Boron has a total of two shells, with three electrons in its valence shell (2 electrons in K shell
and 3 electrons in L shell).
(e) Carbon has twice as many electrons in its second shell as in its rst shell (2 electrons in K shell

and 4 electrons in L shell).

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Q4 (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 uorine have in
common?

Answer. (a) All the elements in the same column as boron have the same number of valence
electrons (3).
Hence, they all have valency equal to 3.
(b) All the elements in the same column as uorine have the same number of valence electrons (7).

Hence, they all have valency equal to 1.

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Q5 An atom has electronic con guration 2, 8, 7.
(a) What is the atomic number of this element?
(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)

Answer. (a) The atomic number of this element is 17.
(b) It would be chemically similar to F(9) with con guration as 2, 7.

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Q6 The position of three elements A, B and C in the Periodic Table are shown below –

(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?

Answer. (a) A is a non-metal.
(b) C is less reactive than A, as reactivity decreases down the group in halogens.
(c) C will be smaller in size than B as moving across a period, the nuclear charge increases and
therefore, electrons come closer to the nucleus.
(d) A will form an anion as it accepts an electron to complete its octet.

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Q7 Nitrogen (atomic number 7) and phosphorus (atomic number 15) belong to group 15 of the
Periodic Table. Write the electronic con guration of these two elements. Which of these will be
more electronegative? Why?

Answer. Element...K ... L...M
Nitrogen ...2...5
Phosphorus. 2...8....5
Nitrogen is more electronegative than phosphorus. On moving down a group, the number of shell
increases.
Therefore, the valence electrons move away from the nucleus and the effective nuclear charge
decreases.
This causes the decrease in the tendency to attract electron and hence electronegativity decreases.

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Q8 How does the electronic con guration of an atom relate to its position in the Modern Periodic
Table?

Answer. In the modern periodic table, atoms with similar electronic con gurations are placed in
the same column. In a group, the number of valence electrons remains the same. Elements across
a period show an increase in the number of valence electrons.

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Q9 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?

Answer. The element with atomic number 12 has same chemical properties as that of calcium.
This is because both of them have same number of valence electrons (2).

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Q10 Compare and contrast the arrangement of elements in Mendeléev’s Periodic Table and the
Modern Periodic Table.

Answer.

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