Valence Bond
Theory and
Hybridization
Linus Pauling
• Author: “Nature of the
chemical bond”
• Received Nobel prize
in 1954 for his work
• Introduced concept of
orbital hybridization
Valence bond theory –
the basic idea
Two half-filled orbitals overlap to form
a covalent bond. The electrons in this
new probability density are then shared
by both atoms (equally attracted to both
nuclei). Arrange themselves to have
maximum overlap of their half filled
orbitals, producing a bonding orbital of
lowest energy!
Two s orbitals
overlapping
Other overlaps
– all form covalent bonds
- all overlapping orbitals must be halffilled
- the new probability distribution formed
can only have a max of two electrons
Types of covalent bonds
• Orbitals can overlap in two main
ways creating two different types
of covalent bonds
• Sigma bonds & Pi bonds
The σ bond
Electron density
is between the
nuclei of the
overlapping
atoms
Single bonds
σ bonds
The π bond
Electron
densities are
above and below
the nuclei of the
bonding atoms
Only p orbitals
can form pi
bonds!
Double Bond
one π bond & one σ bond
Triple bond
two π bond & one σ bond
The benzene ring
Promotion &
Hybridization
• Certain atoms can change their
electron configuration in order to
bond and form a wide variety of
compounds
• This “change” in electron
configuration takes place in two
steps: Promotion of an electron to
a higher energy orbital &
hybridization or blending of
orbitals creating a new type of
orbital for bonding
Promotion
• Most of the time
atoms exist in
their “ground
state” but, in
certain cases
the instant
before bonding
promotion
takes place
allowing more
bonding
spaces:
Hybridization
The merging of orbitals
•Merging orbitals must all be half filled
•No orbitals are “lost” due to merging – if
you blend one s orbital and one p orbital you
will end up with TWO hybrid orbitals!
Hybridization
Continued
• Mix at least 2 nonequivalent atomic
orbitals (eg. s and p). Hybrid orbitals have
different shapes from original atomic
orbitals
• Covalent bonds are formed by:
– Overlap of hybrid orbitals with atomic orbitals
– Overlap of hybrid orbitals with other hybrid
orbitals
Example: Hybridization in
carbon to form methane (CH4)
Types & Names of hybrid
orbitals
• The type of hybrid orbital depends upon
the orbitals which have been blended
sp2 Hybridization in BF3
_ _ _
↓
2s2
2p1
Unhybridized Boron
For BF3, 3 hybrid orbitals are needed, so 3
atomic orbitals are required as follows: (s + p
+ p) = sp2
_
sp2
_
sp2
_
sp2 Hybridized Boron
3 sp2 orbitals needed
to form 3 sigma bonds
sp Hybridization in BeCl2
__ _
↓
2p Unhybridized Be
2s2
• For BeCl2, 2 hybrid orbitals are needed, so 2
atomic orbitals are required as follows: (s + p )
= sp
_
_
sp sp Hybridized Be
Once hybridization has occurred –
hybridized orbitals are ready to bond –
just like regular orbitals
Bonding in hybridized orbitals
Because of their shape,
hybrid orbitals can only
undergo sigma bonding.
Shapes & Hybrids – a little
trick :0)
Hybridization Involving d Orbitals
promote
3s
3p
3d
unhybridized P atom
P = [Ne]3s23p3
3s
3p
3d
vacant d orbitals
hybridize
Ba
F
Be
F
P
five sp3d orbitals
F
3d
Be
F
Be
F
Ba
Trigonal bipyramidal
degenerate
orbitals
(all EQUAL)
Sigma and Pi Bonds
• Single Bond = 1 sigma bond
• Double Bond = 1 sigma bond and 1 pi
bond
• Triple Bond = 1 sigma bond and 2 pi
bonds