这是二级电离能，这就是说，电子数第二多的电离能。

That is to say the ionization energy of the second most electron.

经常你们会被问到关于电离能。

So, oftentimes you'll just be asked about ionization energy.

对，好，我们再找最低的电离能。

Yeah. OK. We're looking for the lowest ionization energy.

因此，我们把电离能，最低的原子放在中间。

So, we keep the atoms with the lowest ionization energy in the center.