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Topic: Valence bond theory and hybridization problems  (Read 3230 times)

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Offline Technicalhuman

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Valence bond theory and hybridization problems
« on: December 28, 2013, 04:54:09 AM »
Valence bond theory explains that chemical bonds are a result of overlaps of orbitals. So, when concerning the limitations of that theory we often use CH4 as an example. So using just VB theory, should we say that
A) carbon can only form 2 bonds as it only has 2 p orbitals that are singly filled. However, CH2 is not stable but CH4 is.
Or B) do we explain that after carbon gets excited, it has a valence electronic configuration of 2s1 2p3 so if the hydrogen bonds this way only 3 of the C-H bonds would have equal bond lengths and energies while the last one would have a stronger bond.

So essentially, when explaining the limitations of VB theory is the atom allowed to be excited (B explanation) or must bonding occur in its ground state (A explanation)? Or perhaps VB theory is not concerned with these matters and instead it just explains overlap of orbitals so both answers are correct?

As for hybridization, is there a general purpose for hybridization?

In H2O there is hybridization of oxygen but in H2S, there isn't. The explanation for this is that H2S has 3p orbitals that are much larger than the 2p orbitals in O. So when they bond without hybridizing the S atom, the repulsion is already at a minimum so there is no need for hybridization. While in H2O, the 2p orbitals being too small has to hybridize in order for the electron repulsion to be kept at a minimum.

So would the need for hybridization in each molecule different? Because in CH4 there is a need for hybridization or else the structure would not be stable as sp3 hybridized orbitals have stronger bonds than the bonding in 3 p orbitals and 1 s orbital. While in the H2O case hybridization is needed as it reduces electron electron repulsion. But in H2S even without hybridization there is already minimal repulsion so there is no need for hybridization? So i feel that each molecule has a different reason for needing hybridized orbitals but they all stem around making a species more stable and to allow for bonding to occur?

Thanks in advance for the help  :)

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