The number of atoms present in one molecule of the substance is called its atomicity. Depending on the number of atoms present in a molecule, the following types of molecules are known:
(i) Monoatomic molecules : The noble gases exist as single atoms and, therefore, their molecules are monoatomic.
(ii) Diatomic molecules : The molecules consisting of two atoms, e.g., O2, N2, Cl2, H2, Br2,I2, etc.
(iii) Triatomic molecules : The molecules consisting of three atoms, e.g., 03 (Ozone molecule).
(iv) Tetratomic molecules : The molecules consisting of four atoms, e.g., P4 (Phosphorus molecule).
(v) Polyatomic molecules : The molecules consisting of more than four atoms, e-g; S8 (Sulphur molecule).
Note: All of the above types are the molecules of an element, which means one, two or more atoms of the same element existing as one species in the free state.