Are we all set? Back up.

都就位了,往后退

耶鲁公开课 - 古希腊历史简介课程节选

Because we have paired set in a 2 s orbital, so all we're left essentially is two electrons that are available for bonding.

因为我们有一对，在2s轨道里已经配对了,所以只剩下两个电子可以用来成键。

麻省理工公开课 - 化学原理课程节选