Quantum mechanics has given us the means
determine the electron structure of isolated atoms.
For instance, the electron structure of an isolated
carbon atom can be written as:
Energy
2p
2s
1s
Based on this model, we would predict that carbon
atoms should form two covalent bonds (since covalent
bonds involve the overlap of half-filled orbitals).
However, in nature we find that carbon atoms do not
normally behave in this manner. Instead carbon atoms
form 4 bonds.
Hybridization adjusts the concept of electron
structures of various atoms, including carbon
atoms, to make them consistent with the way they
are observed to bond in nature.
The basic idea of hybridization is indicated by the
name. A hybrid in biology is an offspring of parents
with different characteristics. For instance, a mule is a
hybrid of a a horse and a donkey.
Hybridization in chemistry involves combining
atomic orbitals to form a new set of “hybrid”
orbitals. These new orbitals will have some of the
properties of the different atomic orbitals which go
into forming them.
Let’s take a carbon atom to see how the formation of the
hybrid orbitals creates a new set of orbitals with some
properties of the atomic orbitals.
Hybridized Carbon atom in a
compound
Isolated Carbon atom
2s
1s
hybridization
Energy
Energy
2p
2sp3 (hybrid orbital)
1s
Notice that the hybrid orbitals are “crosses” between the
low energy s orbital and the higher energy p orbitals.
Also notice that the hybrid orbitals all are equal in
energy.
After forming the hybrid orbitals, the electrons must be
distributed among the new orbitals. Since these hybrid
orbitals are equal in energy, the electrons must
distributed according to Hund’s Rule.
Energy
2sp3
1s
In the case of a carbon atom, the four valence
electrons are distributed among the four hybrid
orbitals. This produces 4 half-filled orbitals capable
of forming 4 bonds.
To summarize, we have seen that the atomic orbitals
found in isolated atoms undergo a change
(hybridization) when they are surrounded by other
atoms in a compound.
These changes in the orbitals allow scientists to explain
the bonding of various atoms in nature.
However, we need to remember that carbon atoms are
not the only atoms to undergo hybridization. Let’s
look at the electron configuration for an isolated
nitrogen atom and see if hybridization can be used to
explain how it bonds to form ammonia (NH3).
2s
1s
Energy
Energy
2p
2sp3
1s
In the hybrid orbital, there are three lone electrons to
bond, and a lone pair . . . . . . . just like the Lewis
structure.
Hybridization Theory is also capable of explaining
molecules such as PF5 which contain expanded octets.
If we determine the Lewis diagram for this molecule, we
find the following:
It is not possible to explain this structure
without hybridization theory!
However, by utilizing hybridization we can explain how
the phosphorus atom is able to form 5 bonds.
We can take 5 atomic orbitals from the P atom and
“cross” them to form 5 equal hybrid orbitals.
3d
Energy
4s
3p
3s
hybridization
Energy
3d
4s
3sp3d (hybrid orbital)
We can then reassign the five
valence electrons to the new hybrid
orbitals using Hund’ Rule. This
creates 5 half filled orbitals capable
of overlapping with the F atoms to
form PF5
In the hybrid orbital, there are
five lone electrons to bond. . . . .
. . just like the Lewis structure.
Hybridization theory is a way of explaining the shapes of
molecules which are found in nature (and predicted by the
VSEPR theory.
The different shapes can be explained by the different
types of hybridization.
domain = a bond set or a lone pair
The domain of the molecule determines the type
of hybridization which the central atom must
undergo.
The following chart shows the type of hybridization
which can be used to explain the various shapes found in
nature and predicted by the VSEPR theory
# domains
2
3
domain name
linear
trigonal planar
type of hybridization
(number)
2 orbitals (called sp hybrids)
3 orbitals (called sp2 hybrids)
4
5
6
tetrahedral
trigonal bipyramidal
octohedral
4 orbitals (called sp hybrids)
5 orbitals (called sp3d hybrids
6 orbitals (called sp3d2 hybrids)
3
atomic orbitals
(formed from)
one s & one p
one s & two p's
one s & three p's
one s & three p's & one d
one s & three p's & two d's
Notice that the name of the hybrid orbitals is determined
by the atomic orbitals which were combined to form
them.
For instance: sp3 hybrids were formed from one s orbital
and three p orbitals.
Now see if you can use what you have learned to
predict the hybridization of some other
compounds. Let’s start with carbon dioxide
Formula:
CO2
Lewis Diagram:
domain number:
two
domain geometry
linear
Hybridization
sp hybridization
Remember double or triple bonds count as 1 domain in VSEPR
theory.
Now try the sulfate ion
Formula:
2-
SO42-
Lewis Diagram:
domain number:
four
domain geometry :
tetrahedral
Hybridization
sp3 hybridization
2-
Now try the carbonate ion
Formula:
2-
CO32-
2-
Lewis Diagram:
domain number:
domain name:
Hybridization
three
trigonal planar
sp2 hybridization
RESONANCE STRUCTURES – SWEET!!!!
Now try the sulfur hexafluoride
Formula:
SF6
Lewis Diagram:
domain number:
domain shape:
Hybridization
six
octohedral
sp3d2 hybridization