Hybridisation

Have you wondered why Carbon has an electronic configuration of 1s²2s²2p² and yet it requires 4 covalent bonds to be formed with it so as to obtain a stable octet configuration?

When we consider the electronic configuration of Carbon, it has 2 single electrons in two of the three p orbitals. Therefore, shouldn't Carbon obtain a stable octet configuration by forming three covalent bonds using just the orbitals in the 2p subshell? Thus, shouldn't Carbon forms 3 covalent bonds; 2 equal sharing ones and 1 dative?

Since, Carbon makes use of all its valence electrons for bonding, it implies that there must be mixing of the orbitals in the 2s and 2p subshells, which result in an re-ordering such that all the valence electrons are singly placed in orbitals; without any pairing.

This mixing of orbitals is called hybridisation; where two (at least) different types of orbitals are mixed to produce a hybrid. (e.g. an illustration below to show mixing of an s and p orbital producing a hybrid orbital.)
Hybridisation does not come free. In-order to mix the s and p orbital, there must be energy investment which results in the excitation of the electrons and orbitals.

Why would it be favourable for the orbitals be willing to be excited, where the lower the potential energy, the more stable it is? Well, a possible explanation is that the energy used to mix the orbitals is compensated by the energy released from the formation of strong covalent bonds. - [This idea is actually used in the Born Haber cycle.]

In this entry, I will also be taking the opportunity to elaborate more about spacial arrangement of sp² and sp³ orbitals.

In the mixing to obtain sp³ orbitals, we will have to make use of the s orbital and three p orbitals. Thus, when this is done, we will obtain four sp³ orbitals.

From the above picture, the angle between two sp³ orbitals is 109.5^o . In addition, this orbital can only have head on overlapping, thus allowing the formation of sigma bonds. Hence, this explains why the bond angle in methane is 109.5^o. Notice that it is the bigger loop of the hybrid orbital which is used for overlapping (Why?).

While to form sp² orbital, there is a mixing of the s orbital and two p orbital. Therefore, we will obtain three sp² orbitals. Leaving one p orbital unmixed with the rest.

The arrangement of the three sp² orbitals and the p orbital is that the former lies on the same plane at a angle of 120^o separating the orbitals. While, the latter is found perpendicular to the plane where the sp² orbitals lies. The spacial orientation of the these orbital allows for three sigma bonds and one pi-bond to be formed.

Lastly, the sp orbtials is formed by mixing one s orbital and one p orbital. Leaving two p orbitals not mixed. The sp orbitals are found on the same plane while the p orbitals are found perpendicular to this plane.

Therefore, more often than not, multiple bonds are formed due to hybridisation has occurred. Although, one should be mindful about considering whether the bond energy released is sufficient to compensate the energy requirement to mix orbitals.

In this article, we have basically touched about mixing of s and p orbitals only. There are other examples where there is mixing of s, p and d orbitals, but this is beyond our current scope.

In conclusion, hybridisation theory can also be used to supplement what is known from VSEPR. It aids in the explanation of the bond angles which we observed. Thus, with the knowledge of hybridisation theory, it helps to reconcile the knowledge we have from "Atomic Structure" and "Chemical Bonding".

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^{Article written by Kwok YL 2009.}

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