Sigma, Pi, Hybridization and Shape

Sigma bonds, Pi bonds.

Shapes of molecules.

Hybrid orbitals.

How do they all fit together?

I was really confused with this information when I was a student. It wasn’t covered when I did my biochemistry degree and I only felt like I really understood things until I started teaching about the concepts … so I really hope this blog post helps your understanding of things.

Where do I start? Well, the three above concepts (Sigma & Pi, shape and hybrid orbitals) are, in my opinion connected … Except they are not directly connected in the syllabus.

Hybrid orbitals is an AHL concept, covered in section 14.2, shapes of molecules are found in 4.3 and 14.1 and hybridization in 14.2 as well. So I guess you could argue that they are at least close together.

Let me start with sigma and pi bonds. I always tell my students that all this is doing is describing the shape of the bond.

Sigma bonds are formed when:

two s orbitals combine

or

an s and p orbital combine where the p is sideways on

or

where an sp1, 2 or 3 hybrid combines with another sp1, 2 or 3 or an s orbital or a p orbital

The shape looks:

Source: ZooFari; Raster: Anselm H. C. Horn (CC-BY-SA 3.0 ), via Wikimedia Commons

Sp3 hybrid orbitals are formed when the s and p orbitals in carbon combine to become equal. Carbon, in the ground state has the arrangement 1s2 2s2 2p2. Remember, there are three p orbitals so in the ground state, there is only two electrons in two separate p orbitals. When carbon reacts, the s orbital gains a bit of energy, the p orbitals lose a bit of energy and they all become equal. As they are equal, the electrons rearrange themselves to move into each empty orbital and it is called an sp3 hybrid.

If only three orbitals combine, there is an sp2 hybrid (leaving one electron in a p orbital) and if only two orbitals combine, an sp hybrid (leaving two electrons in two p orbitals).

These unhybridized p electrons can combine with another p electron to form a Pi bond. The Pi bond is again, the name of the shape and is found as an electron cloud above and below the sigma orbital. This forms the double bond.

Source: JoJanderivative work: Vladsinger (CC-BY-SA 3.0), via Wikimedia Commons

If there are two p orbitals, as in the case of the sp hybrid, and triple bond forms and the Pi bond is found above and below the sigma bond and on either side of it.

Shape is related to hybridization. A tetrahedral shape will be due to sp3 hybridization, sp2 hybridization forms a trigonal planar shape and a sp hybrid a linear shape.

Is this how you were taught hybridization? Do you have any questions on it? If so, please post them below as I would love to hear them.

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