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The Aufbau principle states that, hypothetically, electrons orbiting one or more atoms fill the lowest available energy levels before filling higher levels (e.g., 1s before 2s). In this way, the electrons of an atom, molecule, or ion harmonize into the most stable electron configuration possible. ''Aufbau'' is a German noun that means "construction". The Aufbau principle is sometimes called the building-up principle or the Aufbau rule. The details of this "building-up" tendency are described mathematically by atomic orbital functions. Electron behavior is elaborated by other principles of atomic physics, such as Hund's rule and the Pauli exclusion principle. Hund's rule asserts that even if multiple orbitals of the same energy are available, electrons fill unoccupied orbitals first, before reusing orbitals occupied by other electrons. But, according to the Pauli exclusion principle, in order for electrons to occupy the same orbital, they must have different spins (-1/2 and 1/2). A version of the Aufbau principle known as the nuclear shell model is used to predict the configuration of protons and neutrons in an atomic nucleus.〔Cottingham W.N. and Greenwood D.A., ''An introduction to nuclear physics'' (Cambridge University Press 1986) ISBN 0 521 31960 9. chap.5 ''Ground state properties of nuclei: the shell model''〕 ==Madelung energy ordering rule== The order in which these orbitals are filled is given by the ''n + ℓ rule'', also known as the Madelung rule (after Erwin Madelung), or the Janet rule or the Klechkowski rule (after Charles Janet or Vsevolod Klechkovsky in some, mostly French and Russian-speaking, countries), or the diagonal rule.〔(Electron Configuration ) from WyzAnt〕 Orbitals with a lower ''n + ℓ'' value are filled before those with higher ''n + ℓ'' values. In this context, ''n'' represents the principal quantum number and ''ℓ'' the azimuthal quantum number; the values ''ℓ'' = 0, 1, 2, 3 correspond to the ''s'', ''p'', ''d'', and ''f'' labels, respectively. The rule is based on the total number of nodes in the atomic orbital, ''n + ℓ'', which is related to the energy. In the case of equal ''n + ℓ'' values, the orbital with a lower ''n'' value is filled first. The fact that most of the ground state configurations of neutral atoms fill orbitals following this ''n + ℓ, n'' pattern was obtained experimentally, by reference to the spectroscopic characteristics of the elements. The Madelung energy ordering rule applies only to neutral atoms in their ground state, and even in that case, there are several elements for which it predicts configurations that differ from those determined experimentally. Copper, chromium, and palladium are common examples of this property. According to the Madelung rule, the 4s orbital (''n + ℓ'' = 4 + 0 = 4) is occupied before the 3d orbital (''n + ℓ'' = 3 + 2 = 5). The rule then predicts the configuration of 29Cu to be 1s22s22p63s2 3p64s23d9, abbreviated ()4s23d9 where () denotes the configuration of Ar (the preceding noble gas). However the experimental electronic configuration of the copper atom is ()4s13d10. By filling the 3d orbital, copper can be in a lower energy state. Similarly, chromium takes the electronic configuration of ()4s13d5 instead of ()4s23d4. In this case, chromium has a half-full 3d shell. For palladium, the Madelung rule predicts ()5s24d8, but the experimental configuration ()4d10 differs in the placement of two electrons. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Aufbau principle」の詳細全文を読む スポンサード リンク
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