Unpaired electron - Unpaired electron

This refers to electrons that are not paired within an atom or molecule. In rare gases with closed shell structures and many stable molecules, electrons with opposite spins occupy orbitals in pairs according to the Pauli principle. However, atoms other than rare gases tend to have electron arrangements in which the electron spins are parallel due to Hund's rule, resulting in unpaired electrons. In addition, molecules with an odd number of electrons (e.g. NO, free radicals) and even molecules with an even number of electrons can have unpaired electrons (e.g. O ₂ , biradicals). In general, unpaired electrons pair with other unpaired electrons to form stable covalent bonds, so free radicals have high chemical reactivity. Molecules with unpaired electrons are paramagnetic and give electron spin resonance absorption.

Source: Morikita Publishing "Chemical Dictionary (2nd Edition)" Information about the Chemical Dictionary 2nd Edition

一つの原子あるいは分子のなかで対をなしていない電子をいう．閉殻構造をもった希ガスや多くの安定な分子では，パウリの原理に従って逆向きのスピンをもった電子が対をなして軌道を占有している．しかし，希ガス以外の原子では，フントの規則により電子のスピンが平行になる電子配列をとりやすく，不対電子をもつ．また，奇数個の電子をもつ分子(例：NO，遊離基)はもちろん，偶数個の電子をもつ分子でも不対電子をもつことがある(例：O₂，ビラジカル)．一般に不対電子は，ほかの不対電子と対をつくって安定な共有結合をつくるので，遊離基は化学反応性が大きい．不対電子をもった分子は常磁性を示し，電子スピン共鳴吸収を与える．

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