Use Hund's rule to derive the electronic configuration of Ce3+ion and calculate its magnetic moment on the basis of a spin only formula.

The atomic number of Cerium (Ce) is Z = 58.

The electronic configuration of 58Ce = [Xe]54 4f1 5d1 6s2

And, the electronic configuration of Ce3+= [Xe]54 4f1 , i.e., there is only one unpaired electron, i.e., n = 1. 

Now, the magnetic moment on the basis of spin only formula is given as: 

μ = √n(n+2) BM  

⇒ μ = √1(1+2)

⇒ μ = √3 BM  

⇒ μ = 1.73 BM

Note: Bohr magneton or BM is a unit for expressing the magnetic moment of an electron caused by its orbital or spin angular momentum.