c
\({Np}=1 {~s}^{2} 2 {~s}^{2} 2 {p}^{6} 3 {~s}^{2} 3 {p}^{6} 4 {~s}^{2} 3 {~d}^{10} 4 {p}^{6} 5 {~s}^{2} 4 {~d}^{10} 5 {p}^{6} 6 {~s}^{2}\)
\(4 {f}^{14} 5 {~d}^{10} 6 {p}^{6} 7 {~s}^{2}-5 {f}^{4} 6 {~d}^{1}\)
Total no. of \('f'\) electron \(=14 {e}^{-}+4 {e}^{-}=18\)