The inner electrons are so tightly bound that they are for all intents and purposes, immobilized when it comes to reactivity.

内部电子被紧紧地束缚着,因此无论怎样，当进行反应时，它都是固定的。

麻省理工公开课 - 固态化学导论课程节选

So, that means the electrons are tightly bound.

所以那意味着这些电子是被紧紧地束缚着的。

麻省理工公开课 - 固态化学导论课程节选