Define atomicity. What will be the atomicity of ozone and ammonia?

The total number of atoms that constitute a molecule is known as atomicity.

The chemical compound ozone($O_3$) is composed of 3 oxygen atoms. Therefore, its atomicity is three.

The chemical formula of ammonia is ${NH}_3$, and there are 3 hydrogen atoms and 1 nitrogen atom. Therefore, the atomicity of ammonia is four.