Which amongst the following pairs is not an example of

SOLUTION

Molecules are formed by the combination of two or more atoms. Given below are few molecules along with their constituent atoms.
$$\begin{array}{cc}\text{Molecule}& \text{Constituent atom)}\\ O_2& \text{2 O}\\ H_{2}O& \text{2 H, 1 O}\\ N{H}_3& \text{1 N, 3 H}\\ HCl& \text{1 H, 1 Cl}\end{array}$$ Sodium and potassium are atoms and not molecules.

However, oxygen gas $\left(O_2\right)$, ammonia $\left(N{H}_3\right)$, water $\left(H_{2}O\right)$ and hydrochloric acid $\left(HCl\right)$ are molecules as they are formed by the combination of more than one atoms.