Generally, group 6, 7, 8 and 9 has low affinity towards hydrogen when they are in their normal oxidation states. This prevents the formation of metal hydrides. But chromium $(\mathrm{Cr})$ is an exception. The $\mathrm{Cr}$ absorb hydrogen in almost $1: 1$ stoichiometric quantity. During the formation of such compounds hydrogen atoms occupy the interstitial sites present in crystals of metals.