Hi aspirant,

To understand this we must understand the difference between an atom and a molecule. Atom is the smallest unit of ordinary matter that forms a chemical element which maybe solid-liquid or gas. The atoms are divided into three parts internally but that is not taken into general consideration while division. Now talking about molecules, they are electrically neutral group of two or more atoms held together by chemical bonds. These molecules may be same or different from each other.

So if we say 100 atoms of a, it refers to 100 singular particles of same nature. Eg-- N

100 molecules of B means that 100  groups of different atoms come together to form a unit. Eg--N2

Hope it helps!!