Hund's Rule states that electrons will occupy empty orbitals in the same sublevel before they pair up in the same orbital.
The distribution of electrons of an atom or molecule (or other physical structure) in atomic or molecular orbitals.
Orbital: A region around the nucleus of an atom where there is a high probability of finding an electron.
Sublevel: An energy level defined by quantum theory. In chemistry, sublevels refer to energies associated with electrons.