$(a)$ Define atomicity.
$(b)$ Give an example for an element which is monoatomic and a polyatomic molecule.

(N/A) Atomicity is defined as the total number of atoms present in one molecule of an element or a compound.
$(b)$ Monoatomic molecule: $He$ (Helium) or $Ne$ (Neon) are examples of monoatomic molecules because they consist of a single atom.
Polyatomic molecule: $S_8$ (Sulphur) is an example of a polyatomic molecule because it consists of eight atoms.