Hydrides are binary compounds of hydrogen. These can be classified in four groups viz :

(i) Ionic hydrides e.g., $\mathrm{N}\mathrm{a}\mathrm{H},\mathrm{ }\mathrm{C}\mathrm{a}{\mathrm{H}}_2,\mathrm{ }\mathrm{L}\mathrm{i}\mathrm{H}\mathrm{ }\mathrm{e}\mathrm{t}\mathrm{c}.$

(ii) Covalent hydrides e.g., ${\mathrm{B}}_2{\mathrm{H}}_6,\mathrm{ }\mathrm{N}{\mathrm{H}}_3,\mathrm{ }\mathrm{S}\mathrm{b}{\mathrm{H}}_3\mathrm{ }\mathrm{e}\mathrm{t}\mathrm{c}.$

(iii) Polynuclear hydrides e.g., $\mathrm{L}\mathrm{i}\mathrm{A}\mathrm{l}{\mathrm{H}}_4,\mathrm{ }\mathrm{N}\mathrm{a}\mathrm{B}{\mathrm{H}}_4\mathrm{ }\mathrm{e}\mathrm{t}\mathrm{c}.$

(iv) Interstitial hydrides, in which hydrogen is trapped in the interstial spaces of transition metals.