$(a)$ Define atomicity.
$(b)$ State the atomicity of the following molecules:
$(i)$ Oxygen,$(ii)$ Phosphorus,$(iii)$ Sulphur,$(iv)$ Argon.

(N/A) Atomicity is defined as the total number of atoms present in one molecule of an element or a compound.
$(b)$ The atomicity of the given molecules is as follows:
$(i)$ Oxygen $(O_2)$: Diatomic (contains $2$ atoms).
$(ii)$ Phosphorus $(P_4)$: Tetra-atomic (contains $4$ atoms).
$(iii)$ Sulphur $(S_8)$: Polyatomic (contains $8$ atoms).
$(iv)$ Argon $(Ar)$: Monoatomic (contains $1$ atom).