Use Hund's rule to derive the electronic configuration of `Ce^(3+)` ion, and calculate its magnetic moment on the basis of 'spin - only' formula.

Explanation

Ce (Z = 58), with the electronic configuration `[Xe]4f^(1)5d^(1)6s^(2)`, loses electrons form 5d and 6s - orbitals to form `Ce^(3+)`.
`therefore` Electronic configuration of `Ce^(3+)=[Xe]4f^(1)`
So magnetic moment of `Ce^(3+)` with one unpaired electron is given by, `mu = sqrt(n(n+2))BM`, where n = 1
`= sqrt(1(1+2))=sqrt(3)=1.73 BM`