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Hund's rule of maximum multiplicity, Friedrich Hund discovered this rule and others referred to collectively as Hund's rules for atomic energy levels. They are important for spectroscopy and quantum chemistry.

Hund's rule of Maximum Multiplicity is a principle (1925) of atomic chemistry which states that a greater total spin state usually makes the resulting atom more stable, most commonly manifested in a lower energy state, because it forces the unpaired electrons to reside in different spatial orbitals. A commonly given reason for the increased stability of high multiplicity states is that the different occupied spatial orbitals create a larger average distance between electrons, reducing electron-electron repulsion energy. In reality, it has been shown that the actual reason behind the increased stability is a decrease in the screening of electron-nuclear attractions. Total spin state is calculated as the total number of unpaired electrons + 1, or twice the total spin + 1 written as 2s+1.

As a result of Hund's rule, constraints are placed on the way atomic orbitals are filled using the Aufbau principle. Before any two electrons occupy an orbital in a subshell, other orbitals in the same subshell must first each contain one electron. Also, the electrons filling a subshell will have parallel spin before the shell starts filling up with the opposite spin electrons (after the first orbital gains a second electron). As a result, when filling up atomic orbitals, the maximum number of unpaired electrons (and hence maximum total spin state) is assured.

In 2004, researchers reported the synthesis of 5-dehydro-m-xylylene (DMX), the first organic molecule known to violate Hund's rule.^[1]

List of Hund's rules

It is general rule that if a group of n or less electrons occupy a set of n degenerate orbitals, they will spread themselves out among the orbitals and give n unpaired spins. This is Hund's Rule or The Rule Of Maximum Multiplicity. It means that pairing of electrons is an unfavourable process; energy must be expended in order to make it occur.

• A glossary entry hosted on the web site of the Chemistry Department of Purdue University

1. ^ L Slipchenko et al. Angew. Chem. Int. Ed. 2004 43 742