Atomic radius of Cu is greater than that of Cr but ionic radius of `Cr^(2+)`. Give suitable explanation.

Explanation

In Cu, all the d-electrons are paired `(3d^(10)4s^1)` in Cr, all the d-electrons are unpaired `(3d^54s^1)` hence, d-d electron repulsions in Cu are much greater than those in Cr. Hence, Cu atom is larger in size than Cr. In `Cu^(2+)(3d^9)`, d-d electron repulsions decrease due to presence of one unpaired d-electron. moreover, the electrons are attracted. By 29 protons of the nucleus whereas in `Cr^(2+)` three unpaired electrons are still present but they are attracted by only 24 protons of the nucleus. Thus, `Cu^(2+)` is smaller in size than `Cr^(2+)`