The element samarium has atomic number 62. Find the number of unpaired electrons in the $\mathrm{Sm}{}^{3+}$ ion (give the answer as an integer).

Sm is $[\mathrm{Xe}]4f^{6}6s^2$. Losing the two $6s$ electrons and one $4f$ electron gives $\mathrm{Sm}{}^{3+}$ with $4f^5$. The five $4f$ electrons occupy five of the seven orbitals singly (Hund's rule), so there are 5 unpaired electrons.