The compounds of hydrogen with metals or non-metals are called as Hydrides. They are classified into different types depending upon their behaviour and nature of bonding.

(i) `text(Ionic or Saline hydrides)` : Ionic hydrides or salt-like hydrides. The compounds of Hydrogen with more electropositive metals like alkali metals and alkaline earth metals are ionic hydrides. They are formed by transfer of electron from metal to hydrogen. For example: `LiH`, `NaH`, `CaH_2`, `SrH_2` etc.

(a) They are crystalline solids.

(b) They have high melting and boiling points.

(c) They conduct electricity in molten state and release hydrogen gas at anode.

(d) They react with water and release hydrogen gas and thus act as powerful reducing agents

`NaH + H_2O -> NaOH + H_2`

The thermal stability decreases with increasing size of cations. Thus

`LiH` > `NaH` > `KH` > `RbH` > `CsH` > `CaH_2` > > `SrH_2` > `BaH_2`

(ii) `text(Covalent or Molecular Hydrides)` : The compounds of hydrogen with more electronegative elements or `p`-Block elements are covalent or Molecular hydrides.

For example: `HCl`, `H_2O`, `CH_4`, `PH_3` etc.

(a) They are generally soft, with less melting and boiling points.

(b) They are poor conductors of electricity.

(c) They release hydrogen on decomposition and thus act as reducing agents.

(d) Some of them react with water and release hydrogen gas.

`B_2H_6 + 6H_2O -> 2B(OH)_3 + 6H_2 uparrow`

(iii) `text(Interstitial hydrides or Metallic hydrides)` : The hydrogen atoms occupy the interstitial spaces of transition elements and hence they are interstitial hydrides. They are nonstoichiometric in nature.