Based on atomicity, molecules can be classified as:                   
*Monoatomic (composed of one atom). Examples include He (helium), Ne (neon), Ar (argon), and Kr (krypton). All noble gases are monoatomic. 
* Diatomic (composed of two atoms). Examples include H (hydrogen), N (nitrogen), O (oxygen), F (fluorine), and Cl (chlorine). Halogens are usually diatomic. 
*Triatomic (composed of three atoms). Examples include O (ozone).
*Polyatomic (composed of three or more atoms). Examples include S.
Atomicity may vary in different allotropes of the same element.
The exact atomicity of metals, as well as some other elements such as carbon, cannot be determined because they consist of a large and indefinite number of atoms bonded together. They are typically designated as having an atomicity of 1.
The atomicity of homonuclear molecule can be derived by dividing the molecular weight by the atomic weight. For example, the molecular weight of oxygen is 31.999, while its atomic weight is 15.879; therefore, its atomicity is approximately 2 (31.999/15.879 ≈ 2).