a. Two postulates of Bohr’s model of an atom are-

1. Electrons revolve rapidly around the nucleus in fixed circular paths called energy levels or shells.

2. Each shell can hold electrons up to some limits and each shell is associated with a fixed amount of energy.

b. Bohr’s model:

C. The three subatomic particles are electrons, protons, and neutrons. Electrons are negatively charged, protons are positively charged, neutrons are neutral.

OR

(i) The subscripts (lower figures) represent the atomic number and the superscripts (upper figures) represent the mass number.

(ii) The factor is responsible for the change in the superscripts 16, 17 and 18, though the element is the same number of neutrons.

(iii) The usual name for such atoms of an element is isotopes.

(iv) The nuclear composition of is that it has 8 protons and 10 neutrons.