Classification of Hydrides: Ionic, Covalent and Interstitial

When hydrogen combines with another element, the compound formed -- a hydride -- can fall into any of three structurally very different classes, depending chiefly on the electronegativity, size and electronic structure of the element hydrogen is bonded to.

Ionic (saline) hydrides. Formed by the most strongly electropositive metals -- the alkali metals of Group 1, and the heavier alkaline earth metals calcium, strontium and barium in Group 2 -- by direct combination with dihydrogen on heating: $2\text{Na} + \text{H}_2 \rightarrow 2\text{NaH}$; $\text{Ca} + \text{H}_2 \rightarrow \text{CaH}_2$. Here the very electropositive metal transfers its valence electron(s) essentially completely to hydrogen, giving a crystalline ionic lattice built from the metal cation and the discrete hydride ion, $\text{H}^-$ (hydrogen with a filled $1s^2$ shell, isoelectronic with helium). Being ionic solids, they have comparatively high melting points, conduct electricity when molten (liberating $\text{H}_2$ gas at the anode, confirming that hydrogen is present as the negative ion, $\text{H}^-$), and react vigorously with water to liberate dihydrogen: $\text{NaH} + \text{H}_2\text{O} \rightarrow \text{NaOH} + \text{H}_2\!\uparrow$. Beryllium and magnesium, being far more polarizing on account of their small size, do not form simple ionic hydrides; their hydrides ($\text{BeH}_2$, $\text{MgH}_2$) are covalent/polymeric instead.

Covalent (molecular) hydrides. Formed when hydrogen combines with a p-block nonmetal or metalloid by sharing electrons rather than transferring them completely, giving discrete covalent molecules -- for example $\text{CH}_4$, $\text{NH}_3$, $\text{H}_2\text{O}$, $\text{HF}$ and $\text{B}_2\text{H}_6$. Being simple molecular substances held together in the bulk only by comparatively weak intermolecular forces, they are typically gases, liquids or low-melting solids at room temperature, in sharp contrast to the ionic hydrides. …