化学键的形成来源于一个满壳层之外的电子，也有可能是形成于一个不完全的壳层或满壳层上的一个电子空穴。

One gets a contribution from an electron outside closed shells and also a possible contribution coming from an incomplete shell or a hole in a closed shell.

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这个想法是由化学中的化学键理论想到的。在这个理论里，人们通常认为原子中形成满壳层的电子对化学键完全没有贡献。

The idea was suggested by the chemical theory of valency in which one is used to the idea of electrons in an atom forming closed shells which do not contribute at all to the valency.

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So to figure out bonding electrons, -- what we take is that number 18, which is our total number of electrons we need to fill valence shells, and we subtract it from our number of valence electrons, which is 10.

那么为了找出成键电子，我们将十八，也就是填满所有价壳层,所需要电子的总个数，减去我们所有的价电子的个数，也就是十。

麻省理工公开课 - 化学原理课程节选