ionic (saline) hydride

When you push two electrons onto an atom that already has only one, something strange happens: hydrogen, normally the partner that loses or shares its electron, instead becomes the one that takes an electron. Combine a very electropositive metal — sodium, potassium, calcium — with hydrogen, and the metal hands over its outer electron so completely that hydrogen ends up as a genuine negative ion, the hydride ion H-. The result is a white, salt-like, crystalline solid that looks and behaves like an ionic salt — which is why these are called saline hydrides.

Inside sodium hydride, NaH, the lattice is built of Na+ and H- ions packed in the same rock-salt arrangement as ordinary table salt: each ion surrounded by six of the opposite charge. The H- ion is unusual because it is large and loosely held — two electrons crowding around a single proton repel each other strongly, so the ion is soft and easily gives an electron away. That makes H- a powerful base and a strong reducing agent. The clearest signature is its violent reaction with water: H- snatches a proton from H2O to form hydrogen gas, NaH + H2O gives NaOH + H2.

Ionic hydrides matter as compact stores of reactive hydride. Calcium hydride, CaH2, is sold as a drying agent because it tears water apart so eagerly; lithium aluminium hydride and sodium borohydride (covalent relatives that deliver H-) are everyday reducing reagents in synthesis. The honest caveat: only the most electropositive metals — Groups 1 and 2, except the small, polarizing beryllium and somewhat magnesium — form truly ionic hydrides. Less electropositive elements form covalent or metallic hydrides instead, so the H- ion is a privilege of the most reactive metals.