Charles Janet) Klechkowsky rule (after Vsevolod Klechkovsky) Wiswesser's rule (after William Wiswesser) aufbau approximation or diagonal rule Here n represents the principal quantum number and the azimuthal quantum number; the values = 0, 1, 2, 3 correspond to the s, p, d, and f subshells, respectively. The subshell ordering by this rule is 1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p, 5s, 4d, 5p, 6s, 4f, 5d, 6p, 7s, 5f, 6d, 7p, 8s, 5g, ... For example titanium (Z = 22) has the ground-state configuration . Other authors write the subshell always in order of increasing n, such as Ti (Z = 22) . This can be called "leaving order", since if this atom is ionized, electrons leave approximately in the order 4s, 3d, 3p, 3s, etc. For a given neutral atom, the two notations are equivalent since only the subshell occupancies have physical significance. Subshells with a lower n + value are filled before those with higher n + values. In the case of equal n + values, the subshell with a lower n value is filled first. In general, subshells with the same n + value have similar energies, but the s-orbitals (with = 0) are exceptional: their energy levels are appreciably far from those of their n + group and are closer to those of the next n + group. This is why the periodic table is usually drawn to begin with the s-block elements. The Madelung energy ordering rule applies only to neutral atoms in their ground state. There are twenty elements (eleven in the d-block and nine in the f-block) for which the Madelung rule predicts an electron configuration that differs from that determined experimentally, although the Madelung-predicted electron configurations are at least close to the ground state even in those cases. One inorganic chemistry textbook describes the Madelung rule as essentially an approximate empirical rule although with some theoretical justification, based on the Thomas–Fermi model of the atom as a many-electron quantum-mechanical system. Exceptions in the d-block The valence d-subshell "borrows" one electron (in the case of palladium two electrons) from the valence s-subshell. A special exception is lawrencium 103Lr, where the 6d electron predicted by the Madelung rule is replaced by a 7p electron: the rule predicts , but the measured configuration is . For example, in copper 29Cu, according to the Madelung rule, the 4s subshell (n + = 4 + 0