So, you already have a hint that methyl groups are never in the middle, they always have to be on the outside.

那么，你已经得到了一个提示,那就是甲基永远都不可能在中间，它们总是在外侧。

麻省理工公开课 - 化学原理课程节选

If I look at something that goes as n squared, if this is the edge of the nucleus here and if this is r1, 4 it says when n goes to two the radius goes to four.

如果在原子核外侧,我们发现某一半径和n的平方成正比，也就是说当n为2时半径等于。

麻省理工公开课 - 固态化学导论课程节选