In chemistry, a hydride is formally the anion of hydrogen (H), a hydrogen ion with two electrons. In modern usage, this is typically only used for ionic bonds, but it is sometimes (and has been more frequently in the past) applied to all compounds containing covalently bound H atoms. In this broad and potentially archaic sense, water (H2O) is a hydride of oxygen, ammonia is a hydride of nitrogen, etc. In covalent compounds, it implies hydrogen is attached to a less electronegative element. In such cases, the H centre has nucleophilic character, which contrasts with the protic character of acids. The hydride anion is very rarely observed.
Almost all of the elements up to Cf form binary compounds with hydrogen, the exceptions being He, Ne, Ar, Kr, Pm, Rn, Fr, and Ra. Exotic molecules such as positronium hydride have also been made.