From a distance, they shot at each other. Close up, they cut each other with knives.

VOA: special.2009.09.03

And what is discussed is that for a 1 s hydrogen atom, that falls at an a nought distance away from the nucleus.

我们讨论了对于氢原子１ｓ轨道，它的最可能半径在距离原子核ａ０处。

麻省理工公开课 - 化学原理课程节选