Classification of Molecules

The molecules of an element are constituted by the same type of atoms. Molecules of many elements, such as argon (Ar), helium (He) etc. are made up of only one atom of that element. But this is not the case with most of the nonmetals. For example, a molecule of oxygen consists of two atoms of oxygen and hence it is known as a diatomic molecule, O2. If 3 atoms of oxygen unite into a molecule, instead of the usual 2, we get ozone. The number of atoms constituting a  molecule is known as its atomicity.

Metals and some other elements, such as carbon, do not have a simple structure but consist of a very large and indefinite number of atoms bonded together. Let us look at the atomicity of some non-metals.

Atomicity of some elements : 

Atoms of different elements join together in definite proportions to form molecules of compounds.

For example, a molecule of water consists of two atoms of hydrogen and one atom of oxygen to form a triatomic molecule H₂O.

Molecules of some compounds : 

Molecule of Element:

The molecules of an element are constituted by the same type of atoms.

Atomicity

The number of atoms constituting a molecule is known as its atomicity. We know that hydrogen exists as H₂ in a free state. So, hydrogen’s atomicity would be 2.

Example:

Molecules of Compounds:

Atoms of different elements join together in definite proportions to form molecules of compounds.

For example, the ratio by a number of atoms for water is H: O = 2: 1 hence giving rise to formula H₂O.