The isolated atomic hydrogen anion, H⁻. It consists of a singly charged positive nucleus and two electrons. The electron-electron repulsion almost overwhelms the nuclear-electron attraction. Thus, the “extra” electron is held weakly and is readily donated. Ionic salts containing this large and easily polarized ion are highly reactive, strongly basic, and powerfully reducing. This makes them important reagents despite the fact that they are readily destroyed by the presence of the relatively acidic compound water (H₂O) or by exposure to the relatively oxidizing dioxygen (O₂) as found in air.