Class 9th Science Chapter-4 Exercises

Structure of Atom

Exercise – Solutions

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Question 1. Compare the properties of electrons, protons and neutrons.

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Question 2. What are the limitations of J.J. Thomson’s model of the atom?

Answer:

• It could not explain the results of Rutherford’s α-particle scattering experiment.

• It failed to explain how positive charge is distributed in the atom.

• It did not explain the stability of the atom.

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Question 3. What are the limitations of Rutherford’s model of the atom?

Answer:

• According to classical physics, revolving electrons should lose energy and fall into the nucleus.

• Hence, the atom should be unstable, but atoms are stable in reality.

• It failed to explain the arrangement of electrons in shells.

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Question 4. Describe Bohr’s model of the atom.

Answer:

Bohr proposed the following postulates:

1. Electrons revolve around the nucleus in fixed circular orbits called energy levels or shells.

2. Each orbit has a definite energy.

3. Electrons do not radiate energy while revolving in these shells.

4. Energy is emitted or absorbed only when an electron jumps from one shell to another.

This model successfully explained the stability of the atom.

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Question 5. Compare all the proposed models of an atom given in this chapter.

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Question 6. Summarise the rules for distribution of electrons in shells (Bohr–Bury rules).

Answer:

1. Maximum electrons in a shell = 2n², where n is shell number.

2. Maximum electrons in the outermost shell = 8.

3. Electrons fill shells step by step, inner shells first.

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Question 7. Define valency by taking examples of silicon and oxygen.

Answer:

Valency is the combining capacity of an atom.

• Silicon (Atomic no. 14):
  Electronic configuration = 2, 8, 4
  Valency = 4 (needs 4 electrons to complete octet)

• Oxygen (Atomic no. 8):
  Electronic configuration = 2, 6
  Valency = 2 (needs 2 electrons to complete octet)

Question 8

Explain with examples:
(i) Atomic number
(ii) Mass number
(iii) Isotopes
(iv) Isobars
Give any two uses of isotopes.

Answer:

(i) Atomic number (Z):
Atomic number is the number of protons present in the nucleus of an atom.

Example:
Carbon has 6 protons → Atomic number = 6

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(ii) Mass number (A):
Mass number is the sum of protons and neutrons present in the nucleus.

Example:
Carbon has 6 protons and 6 neutrons → Mass number = 12

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(iii) Isotopes:
Isotopes are atoms of the same element having the same atomic number but different mass numbers.

Example:
¹₁H, ²₁H, ³₁H (isotopes of hydrogen)

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(iv) Isobars:
Isobars are atoms of different elements having the same mass number but different atomic numbers.

Example:
⁴⁰₂₀Ca and ⁴⁰₁₈Ar

Two uses of isotopes:

1. Cobalt-60 is used in the treatment of cancer.

2. Uranium-235 is used as fuel in nuclear reactors.

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Question 9

Na⁺ has completely filled K and L shells. Explain.

Answer:

Atomic number of sodium = 11
Electronic configuration of Na = 2, 8, 1

Sodium loses one electron to form Na⁺ ion.

Electronic configuration of Na⁺ = 2, 8

Hence, K and L shells are completely filled.

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Question 10

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

Answer:

$$\text{Average atomic mass}=\frac{(79\times 49.7)+(81\times 50.3)}{100}$$
$$=\frac{3926.3+4074.3}{100}=\frac{8000.6}{100}=\boxed{{\displaystyle 80.0u}}$$

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Question 11

The average atomic mass of an element X is 16.2 u. What are the percentages of isotopes ¹⁶₈X and ¹⁸₈X in the sample?

Answer:

Let percentage of ¹⁶₈X = x
Then percentage of ¹⁸₈X = 100 − x

$$\frac{16x+18(100-x)}{100}=16.2$$
$$16x+1800-18x=1620$$
$$-2x=-180\Rightarrow x=90$$

Percentages:

• ¹⁶₈X = 90%

• ¹⁸₈X = 10%

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Question 12

If Z = 3, what would be the valency of the element? Also name the element.

Answer:

Atomic number Z = 3 → Element is Lithium (Li)
Electronic configuration = 2, 1

Valency = 1

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Question 13

Composition of the nuclei of two atomic species X and Y is given below:

Give the mass numbers of X and Y. What is the relation between them?

Answer:

• Mass number of X = 6 + 6 = 12

• Mass number of Y = 6 + 8 = 14

Relation:
X and Y are isotopes of the same element.

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Question 14

For the following statements, write T for True and F for False:

(a) J.J. Thomson proposed that the nucleus contains only nucleons.
→ F

(b) A neutron is formed by an electron and a proton combining together.
→ F

(c) The mass of an electron is about 1/2000 times that of proton.
→ T

(d) An isotope of iodine is used for making tincture iodine.
→ F

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Question 15

Rutherford’s alpha-particle scattering experiment was responsible for the discovery of:

(a) Atomic nucleus
(b) Electron
(c) Proton
(d) Neutron

Answer:

✔ (a) Atomic nucleus

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Question 16

Isotopes of an element have:

(a) the same physical properties
(b) different chemical properties
(c) different number of neutrons
(d) different atomic numbers

Answer:

✔ (c) different number of neutrons

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Question 17

Number of valence electrons in Cl⁻ ion are:

(a) 16
(b) 8
(c) 17
(d) 18

Answer:

✔ (b) 8

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Question 18

Which one of the following is a correct electronic configuration of sodium?

(a) 2,8
(b) 8,2,1
(c) 2,1,8
(d) 2,8,1

Answer:

✔ (d) 2,8,1

Explanation (for understanding):
Sodium has atomic number 11, so it has 11 electrons.
These are distributed as:

• K shell = 2

• L shell = 8

• M shell = 1

Hence, the correct electronic configuration is 2,8,1.

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Question 19

Complete the following table:

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Answer (Completed Table):

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Key rules used (for exam clarity):

• Atomic number = number of protons

• Mass number = protons + neutrons

• For neutral atoms: electrons = protons

• If electrons = 0: it is a positive ion