And the reason that they're the least sheilded is because they can get closest to the nucleus, so we can think of them as not getting blocked by a bunch of other electron, because there's some probability that they can actually work their way all the way in to the nucleus.

它们最不容易被屏蔽的原因,是因为他们可以更加接近原子，所以我们可以认为它们,最不容易被其它原子阻挡住，因为它们有一定的概率,离原子核非常近。

麻省理工公开课 - 化学原理课程节选