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As a part of the list of Hund's rules, Hund's Rule of Maximum Multiplicity is an observational rule that a greater total spin state usually makes the resulting atom more stable. The rule, discovered by Friedrich Hund in 1925, is of important use in atomic chemistry, spectroscopy, and quantum chemistry.
The increased stability of the atom, most commonly manifested in a lower energy state, arises because the high-spin state 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[1]. 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.[2]
See also
It is general rule that if a group of n or fewer 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 unfavorable process; energy must be expended in order to make it occur.
External links
References
- ^ I.N. Levine, Quantum Chemistry (Prentice-Hall, 4th edn 1991) [ISBN 0205127703], pp. 303-304
- ^ Slipchenko, L.; Munsch, T.; Wenthold, P.; Krylov, A. (2004). "5-Dehydro-1,3-quinodimethane: a hydrocarbon with an open-shell doublet ground state". Angewandte Chemie (International ed. in English) 43 (6): 742–745. doi:. PMID 14755709.
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