Although `Cr^(3+)` and `Co^(2+)` ions have same number of unpaired electrons but the magnetic moment of `Cr^(3+)` is 3.87 B.M. and that of `Co^(2+)` is 4.87 B.M. Why ?

Explanation

`Cr^(3+)` ion has the configuration `3d^3`. Because of symmetrical electronic configuration `(t_(2g)^3)` there is no orbital contribution to magnetic moment. However, there is appreciable orbital contribution for `Co^(2+)` ion `(t_(2g)^4 (e_g^2) )`