Page 58 - Understanding NCERT Science 09

8. Explain with examples (i) Atomic number, (ii) Mass number,
(iii) Isotopes and iv) Isobars. Give any two uses of isotopes.
+
9. Na has completely filled K and L shells. Explain.
10. If bromine atom is available in the form of, say, two isotopes

79 Br (49.7%) and 81 Br (50.3%), calculate the average atomic
35 35
mass of bromine atom.
11. The average atomic mass of a sample of an element X is 16.2 u.

What are the percentages of isotopes 16 X and 18 X in the
8
8
sample?
12. If Z = 3, what would be the valency of the element? Also, name
the element.
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?
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.
1
(c) The mass of an electron is about times that of proton.
2000
(d) An isotope of iodine is used for making tincture iodine,
which is used as a medicine.
Put tick (ü) 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
16. Isotopes of an element have
(a) the same physical properties
(b) different chemical properties
(c) different number of neutrons
(d) different atomic numbers.
17. Number of valence electrons in Cl ion are:
–
(a) 16 (b) 8 (c) 17 (d) 18

STRUCTURE OF THE ATOM 47

Rationalised 2023-24