The number of atoms present in a single molecule is termed as its atomicity. It can be classified into three types.\tThe molecules of many elements are composed of only a single atom of that element. They are termed as monoatomic. For instance, Ar (Argon), He(Helium). So their atomicity is one.\tOn the other hand the molecules of many elements are composed of two atoms of that element. For instance, oxygen can never exist independently and exist as O2 molecule known as diatomic So its atomicity is 2.\tThe molecules of many elements are composed of three atoms of that element. They are termed as triatomic For instance, O3, CO2. So their atomicity is 3.\tThe molecules of many elements are composed of four atoms of that element. They are termed as tetra atomic For instance, P4, SO3. So their atomicity is 4.