How does the Valance Bond Theory Differ from the lewis Concept??

Question

How does the Valance Bond Theory Differ from the lewis Concept??

Answer 1

Valence Bond Theory was denied.

Lewis Concept is widely accepted.

Neither have been proven to be correct, but Lewis has not been proven wrong.

Answer 2

"The Lewis concept is much better."

Better for what?  It depends what you want to do.  You can't use Lewis theory to offer a quantitative rationale for the bond angle in PH3, you can't use it to make sense of any transition-metal complex, you can't use it to rationalize spectroscopy, and anything with delocalized electrons is a giant and inelegant kludge with Lewis.

"Valence Bond Theory was denied."

What?  By whom?  When?  If so, why is it still in every first-year chemistry textbook on the market?

"Neither have been proven to be correct, but Lewis has not been proven wrong."

Um, you don't prove theories to be correct.  That's not how science works.  You develop a theory to explain observations, you design new experiments to test them, you throw them out or modify them if those experiments yields results inconsistent with the predictions.  They never get "proved".  

OK, on to the question.

The Lewis model of bonding defines a bond as a pair of electrons shared between two atoms.  Its explanation for why that's a good thing is based in classical electrostatics.  It arrives at an "octet rule" based on empirical observation: there's no reason for atoms to like eight electrons, but a lot of them do.  (Except when they don't.)  It doesn't offer, in an of itself, any explanation of molecular geometry, it only describes the physical localized position of valence electrons in a compound to rationalize connectivity and qualitative trends in bond lengths.

It predates quantum mechanics.  If you want a quick and qualitative explanation for a broad and qualitative observation, Lewis is the way to go.  But it's extremely limited.

Valence bond theory takes that basic idea and combines it with an understanding of atomic orbital wavefunctions derived from a quantum mechanical treatment of atoms.  It describes a bond as the result of the overlap of two orbitals on two atoms.  Although rarely presented this way any longer, it can be rigorously mathematical, using detailed calculations of overlap integrals to quantitatively rationalize bond lengths, strengths, and angles.  It permits a methematical combination of canonical atomic orbitals into hybrid wavefunctions, rationalizing different bond angles and strengths far more precisely and with a far more sophisticated model than Lewis provides.

If you want to know why some C-H bonds are stronger than others, why NH3 and PH3 have the different exact bond angles they do, why the first and second bonding interactions of a double bond are not equal, or a host of other really important things to understand about molecules, chemists, to this day some 70 years after its development, still use valence bond theory to make sense of chemical and physical properties of molecules.

It's still nowhere near as successful or powerful as various implementations of molecular orbital theory, but it has the advantage of not making your head explode, which MOT often does to first year students.  

Answer 3

The Lewis concept is much better...