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Subject: Re: why hybridisation occurs and how does it actually take place?

Date: Thu Jul 17 10:59:26 2014
Posted by Dan Berger
Position: Faculty Chemistry/Science, Bluffton University

carbon undergoes sp3 hybridisation, thereby resulting in 4 unpaired electrons which are paired further by 4 H atoms finally forming CH4 (methane). but my question is why does that occur,why carbon can't simply form CH2 to pair up its 2 unpaired electrons


This question has been partly addressed before on MadSci.

However, there are some things that should be addressed here:

Carbon does not "undergo" sp3 hybridization; carbon forms bonds that are as stable as possible in a given situation. We can model that bonding in terms of hybrid atomic orbitals, which overlap with atomic orbitals from neighboring atoms. We can also model it in other ways. But models are ways of making sense of reality; they are not reality themselves.

Under the right circumstances, carbon does form CH2. But e.g. CH2 + H2 is unstable with respect to forming CH4 -- CH2 is unstable in the presence of anything that can form bonds to carbon -- and so CH2 is never found in nature except when it hasn't any opportunity to react with anything else (e.g. in space).

CH2 vs CH4

Ultimately that's what it comes down to: those "paired" 2s electrons on carbon are not nearly as stable as you might think they are, left to themselves. They'd much rather form bonds.

Why is this? It's a brute fact: carbon with four bonds, four pairs of electrons, an "octet," is much more stable than carbon with two bonds, three pairs of electrons. We can model that stability in various ways, and explain it in terms of the mathematics of quantum mechanics, but ultimately nature presents us with facts and we try to fit them into a model that allows us to predict more facts.

Incidentally, hybridization by itself is just a method of generating atomic orbital descriptions that can be used to describe bonding; it's not an explanation of anything. (Hybridization is taken from valence-bond theory, which is one of two quantitative bonding theories in chemistry and can be used to "explain" bonding, by calculating that A is lower in energy than B.)


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