NCERT Solutions for Class 9 Science Chapter 4 Structure of Atoms

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

Answer:

Electrons	Protons	Neutrons
Electrons are negatively charged	Protons are positively charged	No charges present
Located outside nucleus	Located inside the nucleus	Located inside the nucleus
Mass of electron is negligible	Mass of proton is 1 amu	Mass of neutron is 1 amu
Electrons are attracted towards positive charges	Protons are attracted towards negative charges	Do not get attracted towards any charges

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

Answer: Thomson’s model shows that atom is positively charged sphere and electrons are fixed in it. However, there was no experimental evidence to support his theory. Moreover, this theory does not satisfy scattering of light experiment done by rutherford.

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

Answer: In an atom electron revolve nucleus. While revolving electron emits energy and energy must be constantly decreasing. Due to loss of energy, radius of revolution will decreases and electron will fall in the nucleus. This will makes atom highly unstable. But in reality atoms are stable.

4. Describe Bohr’s model of the atom.

Answer:

• Electrons revolve in circular path, in discrete orbit with stable energy.
• Orbits are also called energy shells or energy levels.
• Energy of electron may change if electron jumps from one orbit to another.
• Orbits or shells are represented by alphabets K,L,M,N,… or by numbers n= 1,2,3,…

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

Answer:

Thomson’s Model	Rutherford’s Model	Bohr’s Model
Atom is positively charged sphere	Has positively charged nucleus in the centre where total mass of atom is concentrated.	Positively charged nucleus is present at the centre of the atom
Negatively charged electrons are equally spread all over the atom	Negatively charged electron revolves around nucleus in a defined path	Negatively charged electron revolves around the nucleus and electron does not radiate energy
Number of positive charge and negative charge is equal an atom	Size of atom is large as compared to its nucleus.	Orbits in atoms is labelled as K,L, M,N,… or n = 1,2,3,4,…

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

Answer: Electrons in one orbit = 2n²; n = shell number or energy level

• Electrons fill from inner shell to outer shell
• Duplet: Orbit allows 2 electrons in it
• Octet: Orbit allows 8 electrons in it

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

Answer: Valency of an atom is its combining capacity with other atoms during bond formation. Valency is measured by finding number of electrons required to complete the shell in which electrons are present or number of electron left when shell is completely filled.

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

Answer:

• Atomic number – Number of protons present in the nucleus of atom is called as atomic number. Atomic number of hydrogen is 1 as it contains one proton.
• Mass number – Sum of protons and neutrons in the nucleus is called mass number. Mass number of helium is 4.
• Isotopes – Atoms of same element having same atomic number but different mass number is called isotopes. Isotopes of hydrogen are protium, deuterium and tritium.
• Isobars – Element with same mass number and different atomic number is called isobar. Examples of isobars are – ¹⁶₈O, ¹⁶₇N.

Uses of isotopes :

1. Iodine isotopes is used in treatment of goitre
2. Uranium isotope is used as fuel in nuclear reactor
3. Cobalt isotope is used in treatment of cancer

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

Answer: Sodium (Na) has 11 electrons and its electronic configuration is K=2, L=8, M=1. Sodium loses one electron in M shell and acquires positive charge. K shell has capacity to hold 2 electron and L shell has capacity to hold 8 electron, thus, Na+ has completely filled K and L shells. As these shells as completely filled, thus, it is difficult to remove more electrons. This, makes Na+ more stable.

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

Answer: Given – Atomic masses of two isotopes of ⁷⁹₃₅Br (49.7%) and ⁸¹₃₅Br (50.3%).

∴ total atomic mass = (79 x 49.7/100) + (81 x 50.3 / 100)

= 39.26 + 40.74

= 80 u

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

Answer: Given : atomic mass of element X = 16.2 u

Let, percentage of ¹⁶₈X = p and

percentage of ¹⁸₈X = 100- p

∴ ( 16 x p/100 ) + (18 x (100 -p )/ 100) = 16.2

∴ ( 16p / 100 ) + ( (1800 – 18p) /100) = 16.2

∴ (16p+1800- 18p) 100 = 16.2

∴ 1800 – 2p = 1620

∴ 1800 – 1620 = 2p

∴ 180 = 2p

∴ p = 90

So, percentage of ¹⁶₈X = 90%

∴ Percentage of ¹⁸₈X = 100 – p = 100 – 90 = 10%

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

Answer: The atomic number of given element is 3.

Thus, electronic configuration will be K= 2, L= 1.

Therefore, valency is 1.

Element with atomic number 3 is Lithium.

13. Composition of the nuclei of two atomic species X and Y are given as under X Y Protons = 6 6, Neutrons = 6 8. Give the mass numbers of X and Y. What is the relation between the two species?

Answer: Mass number of X = 6+6 = 12 and mass number of Y = 6+8 = 14. It can be observed that atomic number of both the species is same, therefore, it is a same element. As number of neutron is different in both the species, their mass number is different. Thus, both species are isotopes.

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

(a) J.J. Thomson proposed that the nucleus of an atom contains only nucleons.

Answer: False. [ He proposed that nucleus of an atom contains electrons. ]

(b) A neutron is formed by an electron and a proton combining together. Therefore, it is neutral.

Answer: False. [ Neutron is not formed by combination of electron and proton. ]

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

Answer: True

(d) An isotope of iodine is used for making tincture iodine, which is used as a medicine.

Answer: False.

Put tick (✓) against correct choice and cross (×) against wrong choice in questions 15, 16 and 17

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

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

17. Number of valence electrons in Cl–ion are: (a) 16 (b) 8 (c) 17 (d) 18

Answer: (b) 8

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.

19. Complete the following table.

Atomic Number	Mass Number	Number of Neutrons	Number of Protons	Number of Electrons	Name of Atomic Species
9	–	10	–	–	–
16	32	–	–	–	Sulphur
–	24	–	12	–	–
–	2	–	1	–	–
–	1	0	1	0	–

Answer:

Atomic Number	Mass Number	Number of Neutrons	Number of Protons	Number of Electrons	Name of Atomic Species
9	19	10	9	9	Fluorine
16	32	16	16	16	Sulphur
12	24	12	12	12	Magnesium
1	2	1	1	1	Hydrogen deuterium
1	1	0	1	0	Hydrogen

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