Other Sigma Bonds – Chemical Bonding

00:00 Let's have a look at another example of a sigma bond. Let's take example hydrofluoric acid, or HF. Note what we said before: The atom configuration, or the electron configuration, for hydrogen is 1s1. The electron configuration for fluorine is, as you can see, 1s2, which is a helium nucleus; 2s2; 2p5. Fluorine needs to gain one electron; hydrogen can give one electron. Or, as we've said, they can share them covalently. And in this case, a hydrogen–fluorine sigma molecular orbital can be formed. It's formed from the interaction of a s atomic orbital with, in this case, a p atomic orbital on the fluorine. Note the difference in shape. H has a single s orbital, shown in green.

01:02 The p orbital lobe, which must be in the direction of the s orbital for the hydrogen, must also be green, i.e. it must be in phase. When those two orbitals overlap, it is possible to form a sigma molecular bond. And this is shown here. Note we have the s orbital of the hydrogen, which was originally a sphere, now overlapping with a p orbital of the fluorine. Also pay attention to that lobe of the fluorine p orbital which was out of phase, which is an orange color. That lobe has now got a lot smaller. This is still an example of a sigma orbital.

01:50 Bear in mind the orientation. Before, we were talking about a p orbital which was in line, which could facilitate overlap of the lobe which was in phase with the s orbital. If, on the other hand, we look at a p orbital, which is at 90 degrees to that orbital we were originally looking at, then you can see that overlap of the in-phase lobe is no longer possible. This cannot occur and indeed gives a non-bonding interaction. And the best way to explain this, the best way to find an analogy, is to consider the idea of interference of a radio signal. If you have two radio signals which are coherent with each other, they result in constructive interference, and this is, in this case, desirable for interaction to occur. If, on the other hand, your two signals in the electromagnetic spectrum are out of phase, you end up with destructive interference, and this is undesirable.