(a) Atoms of some elements are not able to exist independently. For such elements atoms form molecules and ions. In case of metals and inert gases atoms can exist independently.

Atoms of metals and inert gases:

Eg., \(\frac{Na, Mg, Al}{Metal}\) \(\frac{He, Ne, Ar}{Inert\; gases}\)

Non-metal: E.g., H₂, Cl₂, P₄, S₈

Exceptional non-metal C

(b) The number of atoms constituting a molecule is known as its atomicity.

E.g.,O₃ —> atomicity is 3

O₂ —> atomicity is 2

(c) Polyatomic ions: When more than two atoms combine together and act like an atom with a charge on it is called polyatomic ion.

E.g., OH^–, NO₃^–, NH₄⁺