An electron configuration explorer

The filling order, Hund's rule and the spin-only formula fit in a few lines of code. You will sort the subshells into filling order, count unpaired electrons, and find the magnetic moment of an ion.

The idea. Subshells fill in order of $n + l$, the smaller $n$ first for a tie. A subshell of $2l + 1$ orbitals with $k$ electrons has $k$ unpaired up to half full, then $2(2l + 1) - k$. $\mu = \sqrt{n(n + 2)}$ BM.

Your tasks.

1. Sort subshells into filling order.
2. In unpaired, return the unpaired electrons for k electrons in a subshell with quantum number l.
3. Find mu_co, the moment of $\text{Co}^{2+}$, which is $3d^7$.