Hello Student,

In Valence Bond Theory, the formula to calculate spin only magnetic moment is-

u=√(n*(n+2)); Where u= the value of magnetic moment in units of Bohn Magneton (BM), n= number of unpaired electrons

When n=3; u=√(3*(3+2))=√3*5=√15=3.873BM

So for n=3 the value of magnetic moment is 3.873 BM.

Hope this helps:)

All the best!!!