Page No 55: - Chapter 4 Structure Of Atom class 9 ncert solutions Science - SaraNextGen [2024-2025]

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On April 24, 2024, 11:35 AM

Question 8:

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

Answer:

(i) Atomic number

The atomic number of an element is the total number of protons present in the atom of that element. For example, nitrogen has 7 protons in its atom. Thus, the atomic number of nitrogen is 7.

(ii) Mass number

The mass number of an element is the sum of the number of protons and neutrons present in the atom of that element. For example, the atom of boron has 5 protons and 6 neutrons. So, the mass number of boron is 5 + 6 = 11.

(iii) Isotopes

Isotopes are atoms of the same element having the same atomic number, but different mass numbers. For example, hydrogen has three isotopes. They are protium , deuterium, and tritium .

(iv) Isobars

Isobars are atoms having the same mass number, but different atomic numbers i.e., isobars are atoms of different elements having the same mass number. For example,  and are isobars.

Two uses of isotopes are:

(i) One isotope of uranium is used as a fuel in nuclear reactors.

(ii) One isotope of cobalt is used in the treatment of cancer.

Question 9:

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

Answer:

An atom of Na has a total of 11 electrons. Its electronic configuration is 2, 8, 1. But, Na⁺ ion has one electron less than Na atom i.e., it has 10 electrons. Therefore, 2 electrons go to K-shell and 8 electrons go to L-shell, thereby completely filling K and L shells.

Question 10:

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.

Answer:

It is given that two isotopes of bromine are  (49.7%) and  (50.3%). Then, the average atomic mass of bromine atom is given by:

Question 11:

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

Answer:

It is given that the average atomic mass of the sample of element X is 16.2 u.

Let the percentage of isotope  be y%. Thus, the percentage of isotope  will be (100 − y) %.

Therefore,

Therefore, the percentage of isotope is 10%.

And, the percentage of isotope is (100 − 10) % = 90%.

Question 12:

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

Answer:

By Z = 3, we mean that the atomic number of the element is 3. Its electronic configuration is 2, 1. Hence, the valency of the element is 1 (since the outermost shell has only one electron).

Therefore, the element with Z = 3 is lithium.

Question 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 = Number of protons + Number of neutrons

= 6 + 6

= 12

Mass number of Y = Number of protons + Number of neutrons

= 6 + 8

= 14

These two atomic species X and Y have the same atomic number, but different mass numbers. Hence, they are isotopes.

Question 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.

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

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

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

Answer:

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

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

(c) The mass of an electron is about  times that of proton. (T)

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

Question 15:

Put tick ( ) against correct choice and cross ( ) against wrong choice in the following Question:

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

(a) Atomic nucleus

(b) Electron

(c) Proton

(d) Neutron

Answer:

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

Question 16:

Put tick ( ) against correct choice and cross ( ) against wrong choice in the following Question:

Isotopes of an element have

(a) the same physical properties

(b) different chemical properties

(c) different number of neutrons

(d) different atomic numbers

Answer:

Isotopes of an element have

Question 17:

Put tick ( ) against correct choice and cross ( ) against wrong choice in the following Question:

Number of valence electrons in Cl⁻ ion are:

(a) 16

(b) 8

(c) 17

(d) 18

Answer:

Number of valence electrons in Cl⁻ ion are: