$$ \begin{array}{r} \operatorname{Cr}: 1 \mathrm{~s}^2 \quad 2 \mathrm{~s}^2 \quad 2 \mathrm{p}^6 \quad 3 \mathrm{~s}^2 \quad 3 \mathrm{p}^6 \quad 3 \mathrm{~d}^5 \quad 4 \mathrm{~s}^1 \\ \quad \ell=1 \quad \ell=1 \quad \ell=2 \end{array} $$ electrons having $\ell=1 \Rightarrow 12$ electrons having $\ell=2 \Rightarrow 5$