Answer 1

(a) Interstitial compounds are those which are formed when small atoms like H, C, N, B etc. are trapped inside the crystal lattice of metals.
(b) They have melting point higher than metals due to stronger metal-non metal bonds or compared to metal-metal bonds in pure metals. 

Answer 2

Interstitial compounds :
Interstitial compounds are formed when small atoms such as H, C or N are trapped inside the crystal lattices of metals. They are usually nonstoichiometric and are neither typically ionic nor covalent.