Ch 13: Covalent Bonding
Section 13: Valence-Shell Electron-Pair Repulsion
1. Recall the rules for drawing Lewis dot structures
2. Remember the special situations:
- Resonance structures
- Formal charges
- Sub-octets and expanded octets
3. Use the best LDS to determine the electron pair
geometry and molecular shape of the molecule/ion
Remember
LDS Mechanics
Three steps for “basic” Lewis structures:
1. Sum the valence electrons for all atoms to
determine total number of electrons (add e- for
any net negative charge or remove e- for any net
positive charge)
2. Use pairs of electrons to form a bond between
each pair of atoms (bonding pairs).
3. Arrange remaining electrons around atoms (lone
pairs or multiple bonds) to satisfy the “octet rule”
for each atom (“duet” rule for hydrogen).
Remember
LDS Rules of Thumb
1. In a polyatomic molecule, the atom that can make the
most number of bonds typically goes in the center.
• This atom is also typically the least electronegative atom in the
molecule.
2. H can only form one bond, so it goes on the outside of
the molecule...H is a terminal atom.
3. If O and H both appear in a chemical formula, there’s a
good chance that they are bonded to each other.
4. When several C atoms appear in the same formula, they
are probably bonded to each other in a chain.
– In other situations the C atoms can form a closed loop, or
branching structures…we’ll come back to that in Ch 21! =)
Remember
Beyond the Octet Rule
• There are numerous exceptions to the octet rule.
• We’ll deal with two classes of violation here:
– Sub-octet systems (less than 8 electrons)
– Valence shell expansion (more than 8 electrons)
Remember
Sub-Octet Systems
• Some atoms (for example: Be, B, and Al) can
form stable molecules that do not fulfill the
octet rule.
F
F
B
F
F
F
B
F
• Experiments demonstrate that the B-F bond
strength is consistent with single bonds only.
Remember
Valence Shell Expansion
• For third-row elements (“Period
3”), the energetic proximity of
the d orbitals allows for the
participation of these orbitals in
bonding.
• When this occurs, more than 8
electrons can surround a thirdrow element.
• Example: ClF3 (a 28 e- system)
F obeys octet rule
F
F
Cl
F
Cl is breakin’ the law! It has
10 electrons.
Remember
Valence Shell Expansion (cont.)
• Typical atoms that demonstrate valence-shell
expansion are P, S, and larger halogens (Cl,
Br, and I).
• Example: PCl5
40 e-
Cl
Cl
Cl
P
Cl
Cl
Remember
Valence Shell Expansion (cont.)
Lewis-dot structure & valence shell expansion:
• As before, assign electrons to bonds and lone pairs to give
each atom an octet.
• Assign any remaining electrons to elements with accessible d
orbitals.
• When it is necessary to exceed the octet for one of several
third-row or higher) atoms, put extra electrons on the central
atom.
• Formal charge is used to discriminate between multiple LDSs.
• DO NOT expand an octet unless you HAVE to!!
Example: I3-
I
I
I
22 e-
I
FC = 0
I
-1
I
0
I
I
-2
+1
I
0
What Do Molecules Look Like?
The Lewis Dot Structure approach provides
some insight into molecular structure in terms
of bonding, but what about 3D geometry?
Recall that we have two types of electron
pairs: bonding and lone.
Valence-Shell Electron-Pair Repulsion
(VSEPR). 3D structure is determined by
minimizing repulsion of electron pairs.
Electron pairs
(both bonding
and lone) are
distributed
around a central
atom such that
electron-electron
repulsions are
minimized.
Electron pairs (both bonding and lone) are
distributed around a central atom such that
electron-electron repulsions are minimized.
2 electron pairs
3 electron pairs
4 electron pairs
Period 1, 2
5 electron pairs
6 electron pairs
Period 3 & beyond
Arranging Electron Pairs
• Must consider both bonding and lone pairs when
minimizing repulsion.
• Example: CH4 (bonding pairs only)
H
H C H
H
Lewis Structure
VSEPR Structure
Arranging Electron Pairs (cont.)
Example: NH3 (both bonding and lone pairs).
H
H N
H
Lewis Structure
VSEPR Structure
Note:
“electron pair geometry” vs.
“molecular shape”
VSEPR Structure Guidelines
The previous examples illustrate the strategy for applying
VSEPR to predict molecular structure:
1. Construct the Lewis Dot Structure
2. Arrange bonding/lone electron pairs in space such that
repulsions are minimized (electron pair geometry).
3. Name the molecular shape from the position of the
atoms.
VSEPR Shorthand:
1. Refer to central atom as “A”
2. Attached atoms are referred to as “X”
3. Lone pair are referred to as “E”
Examples:
CH4: AX4
NH3: AX3E
H2O: AX2E2
BF3: AX3
VSEPR: 2 electron pairs
Linear (AX2): angle
between bonds is 180°
Experiments show that molecules with
multiple bonds can also be linear.
Example: BeF2
F Be F
Multiple bonds are treated as a
single effective electron group.
F
Be
F
F Be F
180°
More than one central atom?
Determine shape around each.
VSEPR: 3 electron pairs
Trigonal Planar (AX3):
angle between bonds
is 120°
Multiple bond is
treated as a single
effective electron
group.
Example: BF3
F
F
F
B
120°
F
B
F
F
VSEPR: 4 electron pairs (cont.)
Tetrahedral (AX4): angle
between bonds is ~109.5°
Example: CH4
H
109.5°
H C H
H
tetrahedral e- pair geometry AND tetrahedral molecular shape
Bonding vs. Lone pairs
Bond angle in a tetrahedral arrangement of electron pairs
may vary from 109.5° due to size differences between
bonding and lone pair electron densities.
bonding pair is constrained by
two nuclear potentials; more
localized in space.
lone pair is constrained by
only one nuclear potential;
less localized (needs more
room).
VSEPR: 4 electron pairs
Trigonal pyramidal (AX3E): Bond angles are <109.5°,
and structure is nonplanar due to repulsion of lone pair.
Example: NH3
H
H N
H
107°
tetrahedral e- pair geometry; trigonal pyramidal molecular shape
VSEPR: 4 electron pairs (cont.)
Classic example of tetrahedral angle
shift from 109.5° is water (AX2E2):
104.5o
“bent”
tetrahedral e- pair geometry; bent molecular shape
VSEPR: 4 electron pairs (cont.)
Comparison of CH4 (AX4), NH3 (AX3E), and H2O
(AX2E2):
AX2E
AX3E
AX2E2
1. Refer to central atom as “A”
2. Attached atoms are referred to as “X”
3. Lone pair are referred to as “E”
Molecular vs. Electron-Pair Geometry
H
O
N
C
F
H
H
H
Central Atom
Compound
Carbon, C
CH4
Nitrogen, N
NH3
Oxygen, O
H2O
Fluorine, F
HF
Electron-Pair Geometry Molecular Shape
What is the electron-pair geometry and the
molecular shape for HCFS?
a) trigonal planar, bent
b) trigonal planar, trigonal planar
c) tetrahedral, trigonal planar
d) tetrahedral, tetrahedral
VSEPR: Beyond the Octet
Systems with expanded valence shells will have
five or six electron pairs around a central atom.
F
Cl
Cl
Cl
F
P
Cl
F
S
F
F
Cl
F
90°
F
120°
90°
F
F
S
F
F
F
90°
VSEPR: 5 electron pairs
• Consider the structure of SF4 (34 e-, AX4E)
• What is the optimum arrangement of electron pairs around S?
F
F
F
F
??
S
F
F
S
F
F
F
S
F
F
F
Compare e– pair angles
lone-pair / bond-pair:
two at 90o, two at 120o
bond-pair / bond-pair: four at 90o, one at 120o
three at 90o
three at 90o, three at 120o
Repulsive forces (strongest to weakest):
lone-pair/lone-pair > lone-pair/bond-pair > bond-pair/bond-pair
VSEPR: 5 electron pairs
• Driving force for last structure is to maximize the
angular separation of the lone pairs.
I3- (AX2E3):
5-electron-pair geometries
AX4E
AX3E2
AX2E3
our
previous
example
VSEPR: 6 electron pairs
Which of these is the more likely structure?
See-saw
Square Planar
6-electron-pair geometries
AX5E
AX4E2
our
previous
example
Molecular Dipole Moments
We can use VSEPR to
determine the polarity of a
whole molecule.
1. Draw Lewis structures to
determine 3D arrangement
of atoms.
2. If one “side” of the molecule has
more EN atoms than the other,
the molecule has a net dipole.
Shortcut: completely
symmetric molecules will not
have a dipole regardless of
the polarity of the bonds.
Molecular Dipoles
The C=O bonds have dipoles
of equal magnitude but
opposite direction, so there is
no net dipole moment.
The O-H bonds have dipoles of
equal magnitude that do not
cancel each other, so water
has a net dipole moment.
Molecular Dipoles (cont.)
symmetric
asymmetric
symmetric
Molecular Dipole Example
• Write the Lewis dot and VESPR structures for
CF2Cl2. Does it have a dipole moment?
Advanced VSEPR Application
Molecules with more than one central atom…
methanol (CH3OH)
H
H C O
H
tetrahedral e- pairs
tetrahedral shape
tetrahedral e- pairs
bent shape
H
The VSEPR Table
# e- pairs
e- Geom.
Molec. Geom.
2
AX2
BeF2
linear
3
AX3
BF3
trigonal planar trigonal planar
AX2E
O3
trigonal planar bent
AX4
CH4
tetrahedral
tetrahedral
AX3E
NH3
tetrahedral
pyramidal
AX2E2
H2O
tetrahedral
bent
4
linear
The VSEPR Table
# e- pairs
5
6
e- Geom.
AX5
PF5
trigonal
bipyramidal
AX4E
SF4
AX3E2
ClF3
AX2E3
I3-
AX6
SF6
trigonal
bipyramidal
trigonal
bipyramidal
trigonal
bipyramidal
octahedral
AX4E2
XeF4
octahedral
Molec. Geom.
trigonal
bipyramidal
see saw
T-shaped
linear
octahedral
square planar
What is the expected shape of ICl2+?
A. linear
C. tetrahedral
B. bent
D. square planar