An empirical rule for determining the order in which atomic orbitals are filled. This rule, which was proposed in 1936 by Erwin Madelung, comes in two parts. The first part states that the order of filling is the order of increasing n + l, where n is the principal quantum number and l is the azimuthal quantum number. The second part states that for two orbitals with an equal value of n + l the order of filling is the order of increasing n. The first part arises because increasing n + l increases the number of nodes of the wavefunction, and hence increases the energy. The second part arises because the approximate Fock degeneracy of many-electron atoms means that the value ofn has a stronger effect on the energy than the value of l.