If we have a higher z effective, it's pulled in tighter, we have to put in more energy in order to eject an electron, so it turns out that that's why case 2 is actually the lowest energy that we need to put in.

而如果有效核电量更高，原子核的束缚也就更紧，我们不得不输入更多的能量来打出一个电子，这就是第二种情况,所需要输入的,能量更少的原因。

麻省理工公开课 - 化学原理课程节选