In Cu, all the d-electronsarepaired `(3d^(10)4s^(1))`. In Cr , all the d-electrons are unpaired `( 3d^(5) 4s^(1))`. Hence, d-delectron repulsion in Cu are much greater than those in Cr. Therefore ,Cu atom is larger size than Cr. In `Cu^(2+) (3d^(9))`, d-d electron repulsions decrease due to presenceofone unpaired d-electron. Moreover, the electrons are attracted by 29 protons of the nucleus whereas in `Cr^(2+)`, three unpaired electrons are still presenct but they are attractedby only 24 protons of the nucleus . Thus , `Cu^(2+)` is smaller in size than`Cr^(2+)`.
