In the ground state of an ion, the electrons fill the orbitals by Hund's rule, and all the unpaired electrons have the same spin. Each electron has a spin quantum number $m_s = +\tfrac12$ or $-\tfrac12$. For which of the following ions is the sum of $m_s$ over all its electrons equal to $2$ or $-2$? The atomic numbers are V 23, Fe 26, Co 27 and Cu 29.