Elements in the first column all have one electron in their outermost shell. Those in the second column have two outer electrons, and so on.

第一列元素最外电子层内均有一个电子，第二列元素则含有两个电子，依此类推。

youdao

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