|
In atomic physics, Hund's rules refers to a set of rules that German physicist Friedrich Hund formulated around 1927, which are used to determine the term symbol that corresponds to the ground state of a multi-electron atom. The first rule is especially important in chemistry, where it is often referred to as, simply, Hund's Rule. The three rules are:〔G.L. Miessler and D.A. Tarr, Inorganic Chemistry (Prentice-Hall, 2nd edn 1999) (0138418918 ), pp. 358–360 〕〔T. Engel and P. Reid, Physical Chemistry (Pearson Benjamin-Cummings, 2006) (080533842X ), pp. 477–479〕〔G. Herzberg, Atomic Spectra and Atomic Structure (Dover Publications, 1944) (0486601153 ), p. 135 (Although, Herzberg states these as being two rules rather than three.)〕 # For a given electron configuration, the term with maximum multiplicity has the lowest energy. The multiplicity is equal to , where is the total spin angular momentum for all electrons. Therefore, the term with lowest energy is also the term with maximum . # For a given multiplicity, the term with the largest value of the total orbital angular momentum quantum number has the lowest energy. # For a given term, in an atom with outermost subshell half-filled or less, the level with the lowest value of the total angular momentum quantum number (for the operator ) lies lowest in energy. If the outermost shell is more than half-filled, the level with the highest value of is lowest in energy. These rules specify in a simple way how usual energy interactions dictate the ground state term. The rules assume that the repulsion between the outer electrons is much greater than the spin–orbit interaction, which is in turn stronger than any other remaining interactions. This is referred to as the LS coupling regime. Full shells and subshells do not contribute to the quantum numbers for total , the total spin angular momentum and for , the total orbital angular momentum. It can be shown that for full orbitals and suborbitals both the residual electrostatic term (repulsion between electrons) and the spin–orbit interaction can only shift all the energy levels together. Thus when determining the ordering of energy levels in general only the outer valence electrons must be considered. ==Rule 1== (詳細はPauli exclusion principle, two electrons cannot share the same set of quantum numbers within the same system; therefore, there is room for only two electrons in each spatial orbital. One of these electrons must have (for some chosen direction ''z'') ''S''''z'' = ½, and the other must have ''S''''z'' = −½. Hund's first rule states that the lowest energy atomic state is the one that maximizes the sum of the ''S'' values for all of the electrons in the open subshell. The orbitals of the subshell are each occupied singly with electrons of parallel spin before double occupation occurs. (This is occasionally called the "bus seat rule" since it is analogous to the behaviour of bus passengers who tend to occupy all double seats singly before double occupation occurs.) Two different physical explanations have been given〔I.N. Levine, Quantum Chemistry (Prentice-Hall, 4th edn 1991) (0205127703 ), pp. 303–304〕 for the increased stability of high multiplicity states. In the early days of quantum mechanics, it was proposed that electrons in different orbitals are further apart, so that electron–electron repulsion energy is reduced. However, accurate quantum-mechanical calculations (starting in the 1970s) have shown that the reason is that the electrons in singly occupied orbitals are less effectively screened or shielded from the nucleus, so that such orbitals contract and electron–nucleus attraction energy becomes greater in magnitude (or decreases algebraically). 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「Hund's rules」の詳細全文を読む スポンサード リンク
|