That happens because of space issues that you were asked to do that, because you can always assume that all of the core orbitals are already going to be filled.

这是出于空间的考虑,要求你这么做是因为，你总是可以假定所有的,芯电子轨道都已经填满了。

麻省理工公开课 - 化学原理课程节选