8.12 What are interstitial compounds? Why are such compounds well known for transition metals?

Transition metals are large in size and contain lots of interstitial sites. Transition elements

can trap atoms of other elements (that have small atomic size), such as H, C, N, in the

interstitial sites of their crystal lattices. The resulting compounds are called interstitial

compounds. 

The transition metals form interstitial compounds because there are vacant spaces in the lattice of transition metals which can be filled by small atoms like H, C, N, etc.