Hund's rule states that the most stable arrangement of electrons : (for a ground state electron configuration)

According to Hund's rule, all orbitals in a sublevel must be solely occupied before any orbital can be double occupied. The same spin is shared by every electron in singly occupied orbitals (to maximize total spin).
According to Hund's rule, the electron configuration with the greatest number of unpaired electrons that all have the same spin in a degenerate orbital is the most stable.