Bonding and Molecular
Structure
1
Valence Bond Theory
In covalent bonding, orbitals overlap
Most primitive overlap between 2 sorbitals
– sigma (σ) bond
Electron density greatest along bond
axis
2
VB Theory Tenants
Orbitals overlap to form a bond
between 2 atoms
2 electrons, of opposite spin, can be
accommodated in the overlapping
orbitals
Higher probability of electrons being
found in space influenced by both
nuclei
3
What about more complex VB’ing?
HF still forms sigma bond, but
“skewed”
– Why?
– Which valence electrons of F are
involved in sigma bond? (Hint: what’s
F’s electron configuraiton?)
s-orbital bonds with p-orbital of F to
form sigma bond
4
What about VB’ing in orbitals
above 1s?
Orbital hybridization
– Mixing of s-, p-, and d-orbitals within atom to
form molecule
Two rules:
1) # hybrid orbitals = # atomic orbitals
mixed
2) hybrid orbitals more directed from central
atom toward terminal atoms than
unhybridized atomic orbitals  better
orbital overlap & stronger bond between
central & terminal atoms
Mixed hybridized orbitals yield electronpair geometry (VSEPR)
5
Hybridized Orbitals
2 electron pairs
– s-orbital and 1 p-orbital mix  sp (linear)
3 electron pairs
– s-orbital and 2 p-orbitals mix  sp2 (trigonal
planar)
4 electron pairs
– s-orbital and 3 p-orbitals mix  sp3 (tetrahedral)
5 electron pairs
– s-orbital, 3 p-orbitals, and 1 d-orbital mix  sp3d
(trigonal bipyramidal)
6 electron pairs
– s-orbital, 3 p-orbitals, and 2 d-orbitals mix 
sp3d2 (octahedral)
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Hybridized Orbitals: sp3
7
What about electron geometries?
For instance, NH3, H2O?
Lone pairs fill hybridized orbitals
– So, NH3 = sp3
– And, H2O = sp3
8
Practice
Give the hybridization of and energy
diagram for
– BeCl2
– BBr3
9
Double bonds
1 unhybridized p-orbital (on each
atom) allows for overlap
Electron densities above and below
sigma bonds (but not on axis like
sigma bonds)
– pi-bond
Can only exist if all atoms lie on
same plane
– Ethylene
10
Ethene
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Triple bonds
2 unhybridized p-orbitals (on each
atom) allow for overlap
2 pi-bonds formed
– Perpendicular to one another
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Ethyne
13
Recapitulate
Double bond consists of sigma- and
pi-bonds
Triple bond consists of sigma- and 2
pi-bonds
Pi-bond if unhybridized p-orbitals
remain on bonding atoms
Multiple bonds entail either sp or sp2
hybridization
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