# NCERT Class 9 Science Solutions: Chapter 4 – Structure of the Atom Part 4 (For CBSE, ICSE, IAS, NET, NRA 2022)

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Question 11:

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

A. It is given that the average atomic mass of the sample of element X is 16.2 u. Let the percentage of isotope Thus, the percentage of isotope

Therefore,

Therefore, the percentage of isotope

And, the percentage of isotope

Question 12:

Q. If , what would be the valency of the element? Also, name the element.

A. By , 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 is lithium.

Question 13:

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

X Y

Neutrons = 6 8

Give the mass numbers of X and Y. what is the relation between the two species?

A. Mass number of

Mass number of

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

Question 14:

Q. 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.

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.

A. 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.

A. 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.

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

Question 15:

Q. 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

A. Atomic nucleus

(b) Electron

A. Electron

(c) Proton

A. Proton

(d) Neutron

A. Neutron

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

A. the same physical properties

(b) Different chemical properties

A. different chemical properties

(c) Different number of neutrons

A. different number of neutrons

(d) Different atomic numbers

A. different atomic numbers

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