Valence Bond Theory and Molecular
Geometry
Valence Bond Theory
Valence bond theory describes covalent bond
in terms of the overlap of atomic orbitals.
Consider the H2O molecule:
Central atom O:
H2:
1s
1s
1s
Orbital overlap
suggests that
the bond angle
is 90.
HF:
2p
2p
1s
F2 :
1s
2s
2p
2p
2p
Hybrid Orbitals
sp Hybrid Orbitals in BeF2
• Hybrid orbitals are mixtures of atomic orbitals
with intermediate energy.
• The number of s, p, and d orbitals in the
mixture equals the number of hybrid orbitals.
s
+
p
Linear geometry is achieved using two sp
hybrid orbitals.
Be atom:
s
p
p
p
s
p
p
p
p
p
Promotion:
sp
Hybridization:
Animation
sp sp
sp
sp2 Hybrid Orbitals in BF3
Promotion:
p
p
p
s
p
p
p
sp2
sp2
sp2
Hybridization:
sp2
sp2
sp2
p
sp
Tetrahedral geometry is achieved using four
sp3 hybrid orbitals.
C atom:
s
sp
sp3 Hybrid Orbitals in CH4
Trigonal planar geometry is achieved using
three sp2 hybrid orbitals.
B atom:
2p
2s
Promotion:
s
p
p
p
s
p
p
p
sp3
sp3
Hybridization:
sp3
sp3
sp3
sp3
sp3
sp3
1
sp3d and sp3d2 Hybridization
Covalent Bond Formation
A s (sigma) bond results from end-to-end
overlap of orbitals. The maximum electron
density lies along the bond.
Trigonal bipyramidal
geometry is achieved
using five sp3d hybrid
orbitals.
Overlap of
pz orbitals
in O2:
Octahedral geometry is
achieved using six sp3d2
hybrid orbitals.
pz
pz
Covalent Bond Formation
A p (pi) bond results from side-to-side overlap
of p orbitals. The electron density is zero along
the bond.
s and p Bonding
H H
Diagram the bonding in C2H4: H C = C H
Trigonal planar - sp2
p
Overlap of
py orbitals
in O2:
H
sp2
H
sp2
s bond
py
s and p Bonding
H
sp2
p bond
s and p Bonding

Diagram the bonding in HCN: H C  N
O

Linear - sp
p
Diagram the bonding in CH2O: H C H
Trigonal planar - sp2
p
H
C
sp
N
s bond
H
sp2
sp
p
p bond
A triple bond consists of a s and two p bonds.

py
H
C
C
H
O
C
sp2
sp2
s bond
p bond
2