Valance Bond Theory
According to this theory a covalent bond is shaped between
the two atoms by the overlap of half filled valence atomic orbitals of every
atom containing one unpaired electron. A valence bond structure is similar to a
Lewis structure, but where a single Lewis structure cannot be written, several
valence bond structures are used. All of these VB structures represents a
specific Lewis structure. This combination of valence bond structures is the
main point of resonance theory. Valence
bond theory considers that the overlapping atomic orbitals of the participating
atoms form a chemical bond. Because of the overlapping, it is most probable
that electrons should be in the bond region. Valence
bond theory views bonds as weakly coupled orbitals (small overlap). Valence
bond theory is normally easier to employ in ground state molecules.
The overlapping atomic orbitals can be different. The two
types of overlapping orbitals are sigma and pi. Sigma bonds occur when the
orbitals of two shared electrons overlap head-to-head. Pi bonds occur when two
orbitals overlap when they are parallel. For example, a bond between two
s-orbital electrons is a sigma bond, because two spheres are always coaxial. In
terms of bond order, single bonds have one sigma bond, double bonds consist of
one sigma bond and one pi bond, and triple bonds contain one sigma bond and two
pi bonds. On the other hand, the atomic orbitals for bonding may be hybrids.
Often, the bonding atomic orbitals have a character of several possible types
of orbitals. The methods to get an atomic orbital with the proper character for
the bonding is called hybridization.
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