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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
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So we can also name this orbital, and this orbital we're going to call sigma 1 s star.
我们也可以，给这个轨道命名。
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Also, it is cylindrically symmetric around the bonding axis, so this is how we know that it's a sigma orbital.
此外，它关于键轴是圆柱对称的，这就是为什么我们知道它是sigma轨道。
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sigma 2pz And this is now sigma of 2pz.
现在这是。
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And a sigma bond forms any time you have two orbitals coming together and interacting on that internuclear axis.
当你把两个轨道合在一起,并在核间轴上有相互作用时,就形成了sigma键。
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So it's going to be a sigma bond, 1s and we have oxygen 2 s p 3 and hydrogen 1 s.
它是sigma键，我们有氧2sp3和氢。
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So let's fill it out in this way, 2p keeping in mind that we're going to fill sigma out the pi 2 p's before the sigma.
让我们这样填上去，记住我们先填π,轨道再填。
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If we think about that, that's a sigma bond, right, they're coming together along the nuclear axis.
如果我们考虑它的话，它是sigma键，对吧，它们沿着核轴方向靠近。
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If we have the molecule ethane, then what we're going to have first is our sigma bond that we described between the two carbons.
如果我们有乙烷分子，那我们首先有,碳碳之间的sigma键。
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sigma STUDENT: Sigma.
学生：
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sigma Here is the sigma.
这就是。
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So z equals 7 -- this is the cut-off where, in fact, the sigma orbital is going to be higher in energy than the pi 2 p orbitals.
所以z等于7-这是分界点，实际上，sigma轨道能量,要比π2p轨道高。
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And again, I want you to have practiced drawing these out in the form you always need to start with the sigma and then write the number of the orbital.
同样的，我希望你们,练习画这种图,你总是要从sigma开始,然后写出轨道数目。
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First of all, this is the two s orbitals in hydrogen, 1s plus 1s smearing to give us this sigma molecular orbital.
首先，这是氢气中的两个s轨道，1s与1s轨道重叠,产生sigma分子轨道。
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The reason that it's sigma is if you look at the bonding axis here, is that there is no nodal plane along the bonding axis.
它是sigma键的原因是,因为如果你看键轴，上面是没有节面的。
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If we have a double bond, we know we need to have only one sigma bond, and we're also going to have one pi bond.
如果我们有双键，我们知道我们需要一个sigma键，还需要一个π键。
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So some p orbitals form pi molecular orbitals, and some form sigma p orbitals.
有些p轨道会形成π分子轨道，有些会形成p轨道。
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A triple bond, again is going to have one sigma bond on the internuclear axis.
一个三键，同样的也有沿着核间轴sigma键。
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So in terms of the carbon hydrogen bond, it's a sigma bond, - because we define it -- any time we are bonding to an atom, we have to keep redefining our bond axis to whatever two atoms we're talking about.
对于碳氢键，它是sigma键，因为我们定义它-,任何时候我们有原子成键，我们要重新定义键轴方向,为我们所讨论的两个原子方向。
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But keep in mind sigma orbitals have no nodal planes along the bond axis.
但记住sigma轨道沿着,键轴方向是没有节点的。
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So it's stigma carbon s p 2, carbon s p 2.
所以它是sigma碳sp2碳sp2。
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In contrast, if we talk about a double bond, what we're now talking about is having both a sigma bond and also one pi bond. And I apologize, I intended to set this up right before class, but that didn't happen today.
相反，如果我们讨论一个双键，我们现在讨论的是,一个sigma键和π键，抱歉，我应该在课前就把这个装好的，今天没装好。
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This will be called lowercase sigma.
用小写的sigma表示。
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PROFESSOR: Yup, it's a sigma bond.
教授：它是sigma键。
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There is a sigma orbital.
有一个sigma轨道。
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Any time two orbitals come straight on together in that internuclear axis, you're going to have a sigma bond.
任何时候两个轨道,在核间轴上直接到一起，你就能得到sigma键。
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And then this means we'll have a total of sigma1s two electrons in our hydrogen molecule, so we can fill both of those into the sigma 1 s orbital, the bonding orbital. We don't have to put anything into the anti-bonding orbital, so that's great.
我们可以把这两个,都填入，轨道里去，成键轨道,我们不需要把什么放到反键轨道里去，这很好。
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Again, we're going to take the linear combination of those p atomic orbitals and make what are called pi or some more sigma molecular orbitals.
同样的，我们需要得到,原子p轨道的线性组合,然后组成我们所说的,π分子轨道或者sigma分子轨道。
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sigma2sp It's going to be a sigma bond. So, we have sigma 2 s p, carbon 2 s p. So they're two s p bonds combining.
是sigma键，我们有，碳2sp。，所以它们是两个sp键结合。
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So, if we think about what bonds are in this molecule, sp2 we actually have six of these sigma carbon s p 2, carbon s p 2 bonds.
如果我们考虑这个分子里都有什么键，我们有六个sigma碳，碳sp2键。
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