Hund's rules [SEP] named after
Hund's rules
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 simply as Hund's Rule.
The three rules are:
1. For a given electron configuration, the term with maximum multiplicity has the lowest energy. The multiplicity is equal to formula_1,
. Similarly for organic molecules, the same rule predicts that the first triplet state (denoted by T in photochemistry) is lower than the first excited singlet state (S), which is generally correct.
However Hund's rules should not be used to order states other than the lowest for a given configuration. For example, the titanium atom ground state configuration is ...3d for which a naïve application of Hund's rules would suggest the ordering F  P  G  D  S. In reality, however, D