Diatomic molecules are molecules formed of two atoms of the same element. Diatomic elements are those that almost exclusively exist as diatomic molecules in their natural elemental state, not chemically bonded with other elements. Examples include H2 and O2. Diatomic elements rarely exist in their atomic form. Earth's atmosphere is comprised almost completely (99%) of diatomic oxygen (O2) and nitrogen (N2).
Oxygen also exists as the triatomic molecule ozone (O3).
The diatomic elements are hydrogen, nitrogen, oxygen, and the halogens: fluorine, chlorine, bromine, iodine, and astatine. Astatine is so rare in nature, its most stable isotope has a half-life of only 8.1 hours, that it is usually not included.