This is because most fraternities and sororities are named after a series of Greek letters, such as Alpha Delta Pi or Sigma Phi.

VOA: special.2009.09.25

And so both of these lobes together constitute a pi bond.

上下两片叶一起组成了一根π键。

麻省理工公开课 - 固态化学导论课程节选

So that's exactly what our definition of a pi bond is, so we have one sigma bond, and one pi bond.

这是我们对π键的定义，我们有一个sigma键，和一个π键。

麻省理工公开课 - 化学原理课程节选

n So the velocity is given by this product of the quantum number n Planck constant 2 pi mass of the electron time the radius of the orbit, which itself is a function of n.

速度是量子数,普朗克常数2π乘以轨道半径的值,它自身也是n的函数。

麻省理工公开课 - 固态化学导论课程节选

So in the first case of this first bond here that I've put in a square, what type of a bond is this, is the sigma or pi?

在这第一个键里,我用方框把它标出来了，这个键是什么类型的，sigma还是π？

麻省理工公开课 - 化学原理课程节选

All right, so what we see here is we have our sigma bond that's along the internuclear axis here, but we also have a pi bond, because each of these atoms now has electrons in it's in a p orbital, so we're going to overlap of electron density above and below the bond.

这里我们看到sigma键,是沿着核间轴的，但我们还有一个π键，因为每个原子的p轨道上,都有电子，所以电子密度在键的上面,和下面都有电子密度交叠。

麻省理工公开课 - 化学原理课程节选