Download Valence Bond Theory

Valence Bond Theory
When quantum mechanics is applied to a single atom
the result is a 3-d probability distribution which
indicates the likely location of an electron.
… such as an s orbital, or a p orbital.
When quantum mechanics is applied to a molecule
(with several atoms) it becomes more complicated.
Valence bond theory Valence bond theory says that
electrons in a covalent bond occupy a region that is the
overlap of individual atomic orbitals. For example, the
covalent bond in molecular hydrogen can be thought of as
result of the overlap of two hydrogen 1s orbitals.
The two hydrogen atoms will approach until they are
at the perfect distance from each other. At this point
the system will have the least amount of energy and the
highest amount of stability.
- the atoms
are more
stable
together than
apart
- too close …
energy
increases due
to repulsion
H
H
1s1
1s1
- the orbitals are overlapped
When an s and a p orbital overlap, the s and p lobe
combine to form a larger lobe, while the opposite side of
the p orbital is reduced in size.
- the orbitals are still
arranged in the x, y and z
directions.
ex
NH3
N
H
H
H
1s2 2s2 2px1
1s1
ex
PCl3
P
1s2 2s2 2px2 2py2 2pz2 3s2
3px1
3py1
3pz1
Cl
1s2 2s2 2px2 2py2 2pz2 3s2
3px2
3py2
3pz1
Cl
1s2 2s2 2px2 2py2 2pz2 3s2
3px2
3py2
3pz1
Cl
1s2 2s2 2px2 2py2 2pz2 3s2
3px2
3py2
3pz1
2py1
2pz1
1s1
1s1
The problem with molecular orbital diagrams is they
do not accurately represent the geometry of the
molecule. According to the orientation of the atomic
orbitals …
each of the Cl’s in the
PCl3 molecule should
be arranged 90 o from
each other.
- one Cl along each of
the x, y and z axis
- this is not the case!