An empirical rule for determining the order in which atomic orbitals are filled. This rule, which was put forward by Erwin Madelung (1881–1972) in 1936, has 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 orbital quantum number (see atom). The second states that for two orbitals with equal values of n + l the order of filling is the order of increasing n. The first part arises because increasing the value of n + l increases the number of nodes of the wave function. The second part arises because the approximate Fock degeneracy of many-electron atoms means that the value of n has a stronger effect on the energy than the value of l. There are some exceptions to Madelung’s rule, such as the electron configurations of copper and chromium; most of these exceptions are due to the extra stability associated with d and f subshells that are either completely or half full.