A diatomic molecule is a stable group of two atoms connected by chemical bonds. Diatomic molecules that are comprised of the same element are called diatomic elements. There are five diatomic elements that exist as gases at standard temperature and pressure: hydrogen, nitrogen, oxygen, fluorine and chlorine.
Additionally, at elevated temperatures, bromine and iodine also exist as diatomic elements. Astatine, the heaviest of the halogens, is theorized to form a diatomic molecule, but this has not been confirmed.
Diatomic compounds are two atom molecules with different elements. Some examples include carbon monoxide, nitric oxide, silicon monoxide and hydrogen chloride. Diatomic molecules are the most plentiful type of compound in the universe.