Diatomic
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Diatomic molecules are molecules formed exactly two atoms, of the same or different elements. Diatomic elements are those that almost exclusively exist as diatomic molecules, known as homonuclear diatomic molecules, in their natural elemental state, not chemically bonded with other elements. Examples include H2 and O2. 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. (They can be remembered as H.O.N. and the Halogens.) 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. The seven primary diatomic elements (excluding Hydrogen) form a '7' shape on the periodic table, beginning with Nitrogen.
The bond in a homonuclear diatomic molecule is non polar and 100% covalent.
Examples of heteronuclear diatomic molecules include carbon monoxide (CO) and nitric oxide (NO).