So that you take every sodium and you count all of its nearest neighbors, next nearest neighbors, next next nearest neighbors, not only on a line but in the plane and above and below in all three dimensions and go through that calculation, it is even more negative.

你考虑到每一个钠原子,你要考虑到它最近的原子，第二近的原子，第三近的原子，不是在一条线而是在一个面上,以及面上与面下的三维空间,再完成计算，这将会更负。

麻省理工公开课 - 固态化学导论课程节选

So pi bonds have electron density both above and below the bond axis, but they actually have a nodal plane at this z, this bond axis here.

键在键轴之上,和之下都有电子密度，但它们在z方向有节面，这是键轴的地方。

麻省理工公开课 - 化学原理课程节选