Valence bond theory



In molecular orbital theory.

History

In 1916, valence crouched in pre-wave-mechanical terms. Resonance theory was criticized as imperfect by Soviet chemists during the 1950's.[3]

Theory

A valence bond structure is similar to a electrons should be in the bond region. Valence bond theory views bonds as weakly coupled orbitals (small overlap). Valence bond theory is typically easier to employ in ground state molecules.

The overlapping atomic orbitals can differ. The two types of overlapping orbitals are sigma and pi. hybridization.

VB theory today

Valence bond theory now complements Molecular Orbital Theory (MO theory), which does not adhere to the VB idea that electron pairs are localized between two specific atoms in a molecule but that they are distributed in sets of molecular orbitals which can extend over the entire molecule. MO theory can predict aromatic properties of molecules as due to resonance between Kekule, Dewar and possibly ionic structures, while molecular orbital theory views it as delocalisation of the π-electrons. The underlying mathematics are also more complicated limiting VB treatment to relatively small molecules. On the other hand, VB theory provides a much more accurate picture of the reorganization of electronic charge that takes place when bonds are broken and formed during the course of a chemical reaction. In particular, valence bond theory correctly predicts the dissociation of homonuclear diatomic molecules into separate atoms, while simple molecular orbital theory predicts dissociation into a mixture of atoms and ions.

More recently, several groups have developed what is often called Hartree-Fock reference wavefunction.

Applications of VB theory

An important aspect of the VB theory is the condition of maximum overlap which leads to the formation of the strongest possible bonds. This theory is used to explain the covalent bond formation in many molecules.

For Example in the case of F2 molecule the F - F bond is formed by the overlap of p orbitals of the two F atoms each containing an unpaired electron. Since the nature of the overlapping orbitals are different in H2 and F2 molecules, the bond strength and bond lengths differ between H 2 and F2 molecules.

In HF molecule the covalent bond is formed by the overlap pf 1s of H and 2p orbital of F each containing an unpaired electron. Mutual sharing of electrons between H and F results in a covalent bond between HF.

References

  1. ^ Murrel, JN, Kettle, SF Tedder, JM "The Chemical Bond", John Wiley & Sons (1985) ISBN 0-471-90759-6
  2. ^ Walter Heitler - Key participants in the development of Linus Pauling's The Nature of the Chemical Bond.
  3. ^ I. Hargittai, When Resonance Made Waves, The Chemical Intelligencer 1, 34 (1995))
 
This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Valence_bond_theory". A list of authors is available in Wikipedia.