9th Standard Chapter 4 Structure of the Atom

Class 9 Science: Structure of the Atom - Exercises with Answers

Exercises and Answers

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

Electrons: Negatively charged, mass 1/1836 of proton, revolve around nucleus.
Protons: Positively charged, mass 1 unit, present in the nucleus.
Neutrons: Neutral, mass equal to proton, present in the nucleus.

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

It could not explain the results of scattering experiments and the stability of the atom, and why electrons do not radiate energy while revolving.

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

It could not explain the stability of the atom as revolving electrons should lose energy and fall into the nucleus.

4. Describe Bohr's model of the atom.

Electrons revolve in fixed circular orbits called energy levels without radiating energy. Energy is absorbed or emitted when an electron jumps between levels.

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

Thomson: Sphere with electrons embedded.
Rutherford: Nucleus at center with electrons revolving around it.
Bohr: Electrons revolve in fixed orbits with quantized energy.

6. Summarise the rules for writing of distribution of electrons in various shells for the first eighteen elements.

The maximum number of electrons in a shell = 2n². The outer shell can have a maximum of 8 electrons. Shells are filled in sequence according to energy levels.

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

Valency is the combining capacity of an element. Silicon has a valency of 4, and oxygen has a valency of 2 as it requires 2 electrons to complete its octet.

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

Atomic number: Number of protons, e.g., Carbon (6).
Mass number: Sum of protons and neutrons, e.g., Carbon-12.
Isotopes: Same atomic number, different mass number, e.g., ¹H, ²H.
Isobars: Same mass number, different atomic number, e.g., ⁴⁰Ar and ⁴⁰Ca.
Uses: Isotopes are used in medical diagnosis and radiotherapy.

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

Sodium loses one electron to form Na⁺, thus having 10 electrons distributed as 2,8, filling K and L shells completely.

10. If bromine atom is available in two isotopes ⁷⁹Br (49.7%) and ⁸¹Br (50.3%), calculate the average atomic mass of bromine atom.

Average mass = (79 × 49.7/100) + (81 × 50.3/100) = 39.26 + 40.74 = 80 u.

11. The average atomic mass of a sample of element X is 16.2 u. What are the percentages of isotopes ¹⁶X and ¹⁸X in the sample?

Let the percentage of ¹⁶X be x and ¹⁸X be (100 - x).
16x + 18(100 - x) = 1620
16x + 1800 - 18x = 1620
-2x = -180
x = 90% (¹⁶X) and 10% (¹⁸X).

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

Z = 3 means the element is Lithium (Li). Electronic configuration: 2,1. It will lose 1 electron to complete the shell, so valency = 1.

13. Composition of the nuclei of two atomic species X and Y: X: 6p, 6n; Y: 6p, 8n. What is the relation between them?

Same atomic number, different mass numbers, so they are isotopes of Carbon: ¹²C and ¹⁴C.

14. State True (T) or False (F):

(a) F
(b) T
(c) T
(d) T

15. Rutherford’s alpha-particle scattering experiment discovered:

(a) Atomic Nucleus

16. Isotopes of an element have:

(b) Different chemical properties

17. Number of valence electrons in Cl⁻ ion are:

(c) 17

18. Correct electronic configuration of sodium:

(b) 2,8,1

19. Complete the following table:

Atomic Number	Mass Number	Number of Neutrons	Number of Protons	Number of Electrons	Name
9	19	10	9	9	Fluorine
16	32	16	16	16	Sulphur
12	24	12	12	12	Magnesium
2	4	2	2	2	Helium
1	2	1	1	1	Deuterium