The oxidation number of `+6` thermodynamically preferred for

Options

A. Cr
B. Mo
C. W
D. both (2) and (3)

Answer

D

Explanation

Group 6 consists of the elements Cr, Mo, W and Sg which are silvery, lustrous, hard but fairly soft (when pure) and have very high melting points and low volatility. The melting point of W is the next highest to C. The group oxidation state of `+6` is exhibited by all the three elements (Cr, Mo and W) in their oxoanions. For Mo and W, the O.N. of `+6` is thermodynamically preferred . However, for Cr, the `+6` state is highly oxidizing, the O.N. of `+3` is most stable for Cr in aqueous solution and in complexes. `Cr(+2)` is strongly reducing. Despite their thermodynamic instability, kinetic factors enable serveral `Cr(+6)` compounds to exist. The most important of these are the chromates and dichromates. Chromium is used in leather industry and in the manufacture of stainless steel and chrome plating (Cr provides a shiny protective coating for Fe and steel surface. Cr metal is not inert in itself, instead, it has a very thin, tough, transparent oxide coating that confers the protection). Mo is used in X- ray tubes. Compounds of Mo and W are employed as cataysts.