Hydrides

Hydrogen forms binary hydrides -- compounds of hydrogen with one other element -- with a great many electropositive elements, both metals and non-metals. It also forms ternary hydrides involving two different metals at once, such as $LiBH_4$ and $LiAlH_4$. Depending on the nature of the bonding involved, hydrides are classified into three types: ionic, covalent and metallic. As a rule of thumb, hydrides formed with elements less electronegative than hydrogen tend to be ionic, while those formed with elements more electronegative than hydrogen tend to be covalent.

Ionic (saline) hydrides. These form with a strongly electropositive metal -- generally an alkali metal or an alkaline-earth metal, though not beryllium or magnesium -- by outright transfer of an electron from the metal to a hydrogen atom, giving discrete hydride ions ($H^-$) alongside metal cations ($M^{n+}$). They can be made by directly reacting the elements at around 400°C, and are salt-like, high-melting, white crystalline solids:

Covalent (molecular) hydrides. These form when hydrogen is attached to another element by a shared, rather than transferred, pair of electrons. Familiar examples with non-metals include methane, ammonia, water and hydrogen chloride. Covalent hydrides are further split into three categories:

• Electron-precise hydrides ($CH_4$, $C_2H_6$, $SiH_4$, $GeH_4$) -- the central atom's valence electrons exactly match the number needed for all its bonds, with no lone pairs or electron deficiency left over.
• Electron-deficient hydrides ($B_2H_6$) -- the central atom does not have enough electrons to form conventional two-centre, two-electron bonds to every hydrogen, so unusual bridge-bonding is needed instead.
• Electron-rich hydrides ($NH_3$, $H_2O$) -- the central atom has one or more lone pairs left over after bonding to all its hydrogens.

Because most covalent hydrides are made of discrete, small molecules held together only by comparatively weak intermolecular forces, they are typically gases or volatile liquids at room temperature. …