The number of atoms constituting a molecule is known as its atomicity.
Depending upon the number of atoms, molecules can be:
Monatomic- A molecule which contains only one atom. Example: Helium, neon, etc.
Diatomic- A molecule which contains two atoms. Example: Hydrogen molecule, nitrogen molecule, etc.
Triatomic- A molecule which contains three atoms. Example: Carbon dioxide, water, etc.
Molecules containing more than 3 atoms are considered as polyatomic molecules.