Which cation from following develops least magnetic moment?

Step-by-step solution

The magnetic moment depends on the number of unpaired electrons n via = n(n+2) . The cation with the fewest unpaired electrons will exhibit the smallest magnetic moment. For Cu ²⁺ , the electronic configuration is [ Ar ] 3d^9 with n = 1 unpaired electron. Cr ³⁺ is [ Ar ] 3d^3 with n = 3 , Co ²⁺ is [ Ar ] 3d^7 with n = 3 , and Fe ²⁺ is [ Ar ] 3d^6 with n = 4 . Since Cu ²⁺ has only one unpaired electron, it possesses the least magnetic moment.