Determining the orbitals that overlap to form a sigma bond

Question

In the chapter on molecular orbitals, my chemistry book mentions three types of sigma bonds: those formed by s-s ($\ce{H2}$), s-p ($\ce{HCl}$), p-p ($\ce{Cl2}$) hybrid-s ($\ce{CH4}$) and hybrid-hybrid ($\ce{C2H6}$) overlap. How is it possible to predict the type of atomic orbital overlap by which a specific σ bond is formed?

Answer 1

The basic answer is go by what is available.

• Hydrogen does not have any other occupied orbitals than s-orbitals. Hence it must use an s-orbital for bonding. This goes both directions.

• Carbon requires four bonds. In order to achieve this, it must hybridise in some way, otherwise, an electron pair would be in an s-orbital and an empty p-orbital would be unavailable for bonding. Hence carbon uses hybrid orbitals in both directions.

• Chlorine could hybridise, but it doesn’t have to fulfill its bonding needs. In general, an electron pair in an s-orbital is more stable than one in a p-orbital because it is ‘closer to the nucleus’. (That layman’s argument is bad but gets us a long way.) So chlorine will happily use its p-orbitals for bonding.

  This goes both ways, by the way, so a carbon-chlorine bond ($\ce{C-Cl}$) is a hybrid-p sigma bond.

• Finally, let’s consider oxygen and nitrogen. They would love to just bond with their p-orbitals, but even if hydrogen is the bonding partner that leads to too much steric strain. Therefore, they hybridise just enough to alleviate the steric stress while still maintaining a maximum s-character for a/the lone pair. So those would love to form bonds with their p-orbitals but typically can’t (so they must use hybrid orbitals). Phosphorus and sulfur are large enough to generally allow bonding only with p-orbitals.