The lowest-energy arrangement, or ground-state electron configuration, of an atom is a listing of the orbitals occupied by its electrons. We can predict this arrangement by following three rules. Rule 1 The lowest-energy orbitals fill up first, according to the order 1s → 2s → 2p → 3s → 3p → 4s → 3d, a statement called the aufbau principle. Note that the 4sorbital lies between the 3pand 3dorbitals in energy. Rule 2 Electrons act in some ways as if they were spinning around an axis, somewhat as the earth spins. This spin can have two orientations, denoted as up (h) and down (g). Only two electrons can occupy an orbital, and they must be of opposite spin, a statement called the Pauli exclusion principle. Rule 3 If two or more empty orbitals of equal energy are available, one electron occupies each with spins parallel until all orbitals are half-full, a statement called Hund’s rule.
John McMurry, Organic Chemistry 8th ed. (2012), Ch. 1 : Structure and Bonding