Because aluminum ions have charge of plus three and oxygen ions have charge of minus two.

因为铝离子为正三价,氧为负二价。

麻省理工公开课 - 固态化学导论课程节选

So, we can say that -- if we have neon here and we want to think about what's isoelectronic, -- f minus would be isoelectronic. We also have oxygen -- what would the charge on oxygen be? Um-hmm, right. 2 minus.

那么，我们可以说--如果我们这里有氖原子,而我们想知道与它等电子的是什么，负一价的氟离子就是与它等电子的，我们还有氧-,氧应该是几价的？没错，负二价。

麻省理工公开课 - 化学原理课程节选