90.

What are hydrides? Give their types.

The binary compounds of hydrogen with metals and non-metals are called hydrides. The hydrides are of three types:

(i) Ionic or salt like hydrides: These hydrides are formed by the highly electropositive elements of Group 1 and Group 2 except Be and Mg. They are formed by the transfer of electrons from the metals to the hydrogen atom. As such they are ionic compounds (Li⁺H^-) and thus behave like salts. For example, LiH, NaH, CaH₂.

(ii) Molecular or covalent hydrides: These are formed by elements having higher electronegativity than hydrogen i.e. by the p-block elements. These are formed by sharing of electrons between the element and hydrogen atom. As such they are covalent or molecular compounds. For example, B₂H₆, NH₃, H₂O, H₂S etc.

(iii) Metallic or interstitial hydrides: The d-block and f-block elements combine with hydrogen to form non-stoichiometric interstitial hydrides. Hydrogen is in the atomic state which occupies interstitial holes in close-packed metal structures. The composition of an interstitial hydride changes with temperature and pressure. These hydrides give out hydrogen easily and hence act as strong reducing agents.