How are multiple bonds formed? We have learned that the number of electron pairs between two atoms indicate the number of electrons between the two atoms. It is easy to appreciate how a pair of electrons can be shared between two atoms. However, how is it possible for multiple pairs of electrons to be shared between two atoms?
Furthermore, when multiple bonds are formed, the strength of the each bond in the multiple bond is not identical. For example the C=O has two bonds between the C and O atoms; these two bonds are not equal in strength; one is weaker and the other is stronger.
Multiple bonds are formed when there is a sigma bond with the remaining bonds to be pi bonds. Hence, for example, a C=O contains one sigma bond and one pi bond. Do note that all single bond are sigma bond (the reason can be gathered from the later part of the post).
Sigma bonds are obtained when the orbital overlap in a head-on fashion. This can be illustrated by the diagram below.
Furthermore, when multiple bonds are formed, the strength of the each bond in the multiple bond is not identical. For example the C=O has two bonds between the C and O atoms; these two bonds are not equal in strength; one is weaker and the other is stronger.
Multiple bonds are formed when there is a sigma bond with the remaining bonds to be pi bonds. Hence, for example, a C=O contains one sigma bond and one pi bond. Do note that all single bond are sigma bond (the reason can be gathered from the later part of the post).
Sigma bonds are obtained when the orbital overlap in a head-on fashion. This can be illustrated by the diagram below.
While pi bonds are obtained when the orbital overlap in a side-on fashion. This type of bonding can also seen from the diagram below.
As the extend of orbital overlap in the sigma bond's case is greater than in the pi bond's case, hence it is not surprising that the sigma bond is stronger than the pi bond. This, therefore, accounts for the differing bond strength in the two bonds of the C=O.
An interesting fact, although this require some visualisation and hopefully this diagram helps: It is not possible to form a double/triple bond between two atoms if there is no sigma bond existing in the first place. Hence, essentially, all multiple bonds must have one sigma bond and the rest the bonds are pi.
In conclusion, in the "A" level curriculum, it is good to know that there are three bond types that we need to know: Single bond (contains only sigma bond), double bond (one sigma and one pi) and triple bond (one sigma and two pi).
An interesting fact, although this require some visualisation and hopefully this diagram helps: It is not possible to form a double/triple bond between two atoms if there is no sigma bond existing in the first place. Hence, essentially, all multiple bonds must have one sigma bond and the rest the bonds are pi.
In conclusion, in the "A" level curriculum, it is good to know that there are three bond types that we need to know: Single bond (contains only sigma bond), double bond (one sigma and one pi) and triple bond (one sigma and two pi).
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Article written by Kwok YL 2009.
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