Question

From group 6 only one metal forms hydride. This metal is:

A. Mo

B. W

C. Cr

D. Co

Answer 1

Metal hydrides are compounds of one or more metal cations $\left( {{M}^{+}} \right)$ and hydride anions $\left( {{H}^{-}} \right)$ . The hydride is formed when hydrogen accepts electrons from the metal but in case of Group 6, 7, 8 and 9 the hydrogen donates its one valence electron to metal thus they do not form hydrides. A metal which can make a bond with hydrogen without accepting its one valence electron can form metal hydride.

Complete answer:

The low affinity of the elements of the group 6,7,8 and 9 towards hydrogen when in their normal oxidation states prevent the formation of their metal hydrides. This is known as hydride gap. The region of the periodic table from group 7 to 9 is referred to as the hydride gap as they do not form hydrides. Examples of such elements are as follows: Mo, W and Mn, Fe, Co, Ru etc.

In the group 6, Chromium $\left( Cr \right)$ is an exception as it is the only group 6 element to form a metal hydride. The metals like chromium absorb hydrogen in almost $1:1$ stoichiometric quantity. During the formation of these type compounds hydrogen atoms occupy the interstitial sites present in the crystals of metals which are not available in case of other elements of the group.

Hence, the correct answer is option C which is Cr metal.

Note: Transition metal hydrides are chemical compounds which contain a transition metal bonded to hydrogen. Most transition metals form hydride complexes and some are significant in various catalytic and synthetic reactions.