Hund's rule
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Hund's rule is a principle of physical chemistry which states that 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).
In 2004, it was reported in PhysicsWeb (as cited in the "References" section) that researchers had synthesized the first organic molecule known to violate Hund's rule: 5-dehydro-m-xylylene (DMX).
References
- A glossary entry hosted on the web site of the Chemistry Department of Purdue University (http://www.chem.purdue.edu/gchelp/gloss/hundsrule.html)
- A PhysicsWeb article (http://physicsweb.org/article/news/8/2/5)Template:Chem-stub