CBSE- Structure of Atom-NCERT Solutions

Class IX Science
NCERT Solution for Structure of the Atom

QUESTIONS FROM NCERT TEXTBOOK

Q1. Compare the properties of electrons, protons and neutrons.

Ans.

Q2. What are the limitations of J.J. Thomson’s model of the atom?

Ans. According to J.J. Thomson’s model of an atom, the electrons are embedded all over in the positively charged spheres. But experiments done by other scientists showed that protons are present only in the centre of the atom and electrons are distributed around it.

Q3. What are the limitations of Rutherford’s model of the atom?

Ans. According to Rutherford’s model of an atom the electrons are revolving in a circular orbit around the nucleus. Any such particle that revolves would undergo acceleration and radiate energy. The revolving electron would lose its energy and finally fall into the nucleus, the atom would be highly unstable.

But we know that atoms are quite stable.

Q4. Describe Bohr’s model of the atom.

Ans. Bohr’s model of the atom

(1) Atom has nucleus in the centre.

(2) Electrons revolve around the nucleus.

(3) Certain special orbits known as discrete orbits of electrons, are allowed inside the atom.

(4) While revolving in discrete orbits the electrons do not radiate energy.

(5) These orbits or shells are called energy levels.

(6) These orbits or shells are represented by the letters K, L, M, N or the numbers n = 1, 2, 3, 4

Q5. Compare all the proposed Bohr’s models of an atom given in this chapter.

Ans.

Q6. Summarise the rules for writing of distribution of electrons in various shells for the first eighteen elements.

Ans. The rules for writing of distribution of electrons in various shells for the first eighteen elements are:

(i) The maximum number of electrons present in a shell is formula—2n²

n = orbit number i.e., 1, 2, 3

Maximum number of electrons in different shells are:

K shell n = 1 2n² ⇒ 2(1)² = 2

L shell n = 2 2n² ⇒ 2(2)² = 8

M shell n = 3 2n² ⇒ 2(3)² = 18

N shell n = 4 2n² ⇒ 2(4)² = 32

(ii) The maximum number of electrons that can be accommodated in the outermost orbit is 8.

(iii) Electrons are not accommodated in a given shell unless the inner shells are filled. (Shells are filled step-wise).

Q7. Define valency by taking examples of silicon and oxygen.

Ans. Valency is the combining capacity of an atom.

Atomic number of oxygen = 8

Atomic number of silicon = 14

K L M

Electronic configuration of oxygen = 2 6 –

Electronic configuration of silicon = 2 8 4

In the atoms of oxygen the valence electrons are 6 (i.e., electrons in the outermost shell). To fill the orbit, 2 electrons are required. In the atom of silicon, the valence electrons are 4. To fill this orbit 4 electrons are required.

Hence, the combining capacity of oxygen is 2 and of silicon is 4.

i.e., Valency of oxygen = 2

Valency of silicon = 4

Q8. Explain with examples:

(i) Atomic number (ii) Mass number,

(iii) Isotopes and (iv) Isobars.

Give any two uses of isotopes.

Ans. (i) Atomic number: The atomic number of an element is equal to the number of protons in the nucleus of its atom.

e.g., Oxygen has 6 protons hence atomic no. = 6.

(ii) Maas number: The mass number of an atom is equal to the number of protons and neutrons in its nucleus.

Nucleons = number of protons + number of neutrons

Example: Protons + Neutrons = Nucleus = Mass number

6 + 6 = 12

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

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

Both calcium and argon have same mass number but different storm number.

Two uses of isotopes are:

(i) An isotope of iodine is used in the treatment of goitre.

(ii) An isotope of uranium is used as a fuel in nuclear reactors.

Q9. Na+ has completely filled K and L shells. Explain.

Ans. Sodium atom (Na), has atomic number = 11

Number of protons = 11

Number of electrons = 11

Electronic configuration of Na = K L M

2 8 1

Sodium atom (Na) looses 1 electron to become stable and form Na+ ion. Hen it has completely filled K and L shells.

Q10. If bromine atom is available in the form of say, two isotopes

(49.7%) and (50.3%), calculate the average atomic mass of bromine atom.

Ans. The average atomic mass of bromine atom

Q11. The average atomic mass of a sample of an element X is 16.2 u. What are percentages of isotopes

Q12. If Z = 3, what would be the valency of the element? Also, name the element.

Ans. Z = 3, (i.e., atomic number → z)

Electronic configuration = 2, 1

Valency = 1

Name of the element is lithium.

Q13. Composition of the nuclei of two atomic species X and Y are given as under

Give the mass number of X and Y. What is the relation between the two species?

Ans. Mass number of X = Protons + Neutrons

= 6 + 6

= 12

Mass number of Y = Protons + Neutrons

= 6 + 8 = 14

As the atomic number is same i.e., = 6

[atomic number = number of protons].

Both X and Y are isotopes of same element.

Q14. 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.

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

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

Ans (a) False (b) False (c) True (d) False

Put tick

against correct choice and cross (x) against wrong choice in questions 15, and 17.

Q15. Rutherford’s alpha particle scattering experiment was responsible for the disco venl

(a) Atomic nucleuse (b) Electrone (c) Protone (d) Neutron

Ans (a) Atomic nucleus

Q16. Isotopes of an element have

(a) the same physical properties (b) different chemical properties

Ans (c) different number of neutrons

Q17. Number of valence electrons in Cr ion are:

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

Ans (b) 8

Q18. 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

Ans (d) 2, 8, 1

Q19. Complete the following table.

Ans.