University Physics with Modern Physics (14th Edition)

No two electrons are allowed to occupy the same quantum mechanical state in the helium atom. In the ground state, both electrons have n = 1. Then there is only one possible value for the orbital quantum number, l = 0, and only one possibility for the orbital magnetic quantum number, $m_l=0$. The two electrons cannot share every quantum number, so the fourth possibility, spin, must differ. One must be "spin up" and the other must be "spin down".