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

That is to say it is confined to the atom.

也就是说电子会被束缚在原子内。

麻省理工公开课 - 固态化学导论课程节选

Core electrons are all those electrons held in really tight with the nucleus in the inner shells, whereas the valence electrons are only those electrons that are in the outer-most shell, or at your highest value of n of the principal quantum number.

芯电子是那些,在内壳层被原子核束缚得非常紧的电子，而价电子只包括,最外层的电子，或者说主量子数，n，的值最大的那些电子。

麻省理工公开课 - 化学原理课程节选

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