There is a huge difference between the energies in the outermost shell and the inner shells, which tells you that it's unlikely that any electrons except those in the outermost shell are going to be active.

最外层和最里层所具有的能量,有很大差异，而这告诉我们有一点是不太可能的,那就是除了最外层的电子,别的电子都是应该是活泼的。

麻省理工公开课 - 固态化学导论课程节选

And, one other thing that I failed to point out, if you take a look at the energies associated with the outermost electrons, in this case, lithium, you see it's 0.5 megajoules per mole, and then what's the 6.26?

还有另外一点我忘记提到的是，如果你观察离最外层,的电子的能量，在这种情况，对于锂，你可以得到，它是每摩尔0。5兆焦，那么6。26是什么呢？

麻省理工公开课 - 固态化学导论课程节选