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, O3
. 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.