A covalent bond is any time we have a pair of electrons that is shared between two different atoms.

所谓共价键就是我们有两个原子,共用一对电子。

麻省理工公开课 - 化学原理课程节选

So in oxygen we have a similar situation where, in fact, we are not going to promote any of the electrons because we have two lone pair electrons no matter what we do.

在氧中，情况很类似，我们不能激发电子,因为无论如何我们都有两个孤对电子。

麻省理工公开课 - 化学原理课程节选

So, using our simple valence bond theory, what we would expect is that we want to pair up any unpaired electrons in methane with unpaired electrons from hydrogen and form bonds.

利用简单的价电子成键理论，我们预计,要把所有甲烷中没有配对的电子,和氢原子中没有配对的电子配对来形成键。

麻省理工公开课 - 化学原理课程节选