It is an aggregate of four polar bonds, but they are symmetry arranged in space.

这是四个极性键，按空间对称聚集的结果。

麻省理工公开课 - 固态化学导论课程节选

But he also realized that when they did hit something what they hit what unbelievably massive but also that that mass was concentrated So eventually, this is what we have come to call the nucleus of an atom.

碰到的一定是十分重的东西,而且那些质量,要聚集在非常非常小的空间内,这个就是我们称作,原子核的东西。

麻省理工公开课 - 化学原理课程节选