In chemistry, a diatomic molecule is a compound made up of exactly two atoms. At standard temperature and pressure (STP), most diatomic species are gases, and a handful of solids transform into diatomic gases when heated.
A diatomic molecule contains two atoms. The common diatomic elements are H₂, N₂, O₂, F₂, Cl₂, Br₂, and I₂.
Hydrogen, nitrogen, oxygen, and the halogens fluorine, chlorine, bromine, and iodine form homonuclear diatomic molecules at room temperature. Nitrogen is especially noteworthy because its triple bond renders it exceptionally stable.
In contrast, noble gases such as helium and neon are monatomic, rarely forming molecules under ordinary conditions.
Metals remain crystalline solids at STP and do not form homonuclear diatomic molecules. While they can combine with nonmetals to produce ionic compounds (e.g., CuCl₂, Fe₂O₃), these structures typically contain more than two atoms.
Heteronuclear diatomic gases include carbon monoxide (CO), hydrogen chloride (HCl), and nitric oxide (NO). Despite differing nuclear species, they share the same two‑atom structure.
When heated, certain elements that are solid at room temperature become gaseous diatomic molecules. For example:
Even ionic solids such as sodium chloride (NaCl) can dissociate into diatomic gas molecules under extreme heat.
Oxygen, nitrogen, and other diatomic gases remain bonded as two‑atom molecules when cooled to liquid form, thanks to intermolecular forces that keep them together below their boiling points.
