Molecular Structure
Molecular geometry is the
general shape of a molecule or the
arrangement of atoms in three
dimensional space.
Physical and chemical properties
depend on the geometry of a
molecule.
Molecular Structures
3-D Model
3-D Drawing
VSEPR Model
The Valence Shell Electron Pair Repulsion model
predicts the shapes of molecules and ions by
assuming that the valence shell electron pairs are
arranged as far from one another as possible to
minimize the repulsion between them.
3
VSEPR Model
H
N
:
H
H
Electron Pair Geometry –
is determined by the number and
arrangement of all electron pairs
(bonding and lone) around the
central atom.
Molecular geometry –
is determined by the arrangement of
atoms (or bonding electron pairs
only) around the central atom.
In molecules with no lone pairs,
Electron Pair Geometry = Molecular Geometry
AXE shorthand notation:
• A - central atom
• X - terminal atoms
• E - lone pair electrons
Predicting Molecular
Geometry: VSEPR


# e- pairs
Only five basic shapes.
When a lone pair replaces an atom, the molecular
geometry changes as well as the angles.
2
3
4
5
6
6
Predicting Molecular
Geometry: VSEPR
1. Draw the Lewis structure.
2. Determine how many electron pairs (bonded and non-bonded) are
around the central atom. **Treat a multiple bond like a single
bond when determining a shape.
3. (Write the AXE shorthand notation.)
4. Determine the electron pair geometry (**one of the five basic
shapes).
5. If the molecule has lone pairs around the central atom, then
determine the molecular geometry. (This is a subset of the
electron geometry.)
Predicting Molecular Geometry
Example 1: BeCl2
1. Draw the Lewis structure
Cl
Be
Cl
2. Two electron pairs around the
central atom.
Two bonded and Zero lone pairs.
(electron pair geometry = molecular geometry)
Geometry is LINEAR. Bond angle is 180°.
Predicting Molecular Geometry
Example 2: BF3
..
:F:
..
:F
..
B
..
F:
..
Three electron pairs around the central
atom.
Three bonded and Zero lone pairs.
(electron geometry = molecular geometry)
Geometry is TRIGONAL PLANAR.
Bond angle is 120°.
Predicting Molecular Geometry
Example 3: SO2
AX3E0
O
S
AX2E1
O
Three electron pairs around the
central atom.
Two bonded and One lone pairs.
S
(electron geometry = trigonal planar)
Molecular geometry = BENT
Bond angle <120⁰.
O
O
Tetrahedral (Electron Geometry)
Four e- pairs about central atom
bond
pairs
lone
pairs
4
0
3
1
2
2
tetrahedral
..
triangular
pyramidal
..
angular
(bent)
Model
Predicting Molecular Geometry
Example 4: CH4
AX4E0
H
H
C
H
H
Four electron pairs around the
central atom. Zero lone
(electron pair geometry = molecular geometry)
Molecular geometry = TETRAHEDRAL
pairs.
Bond angle is 109.5⁰.
Predicting Molecular Geometry
Example 5: NH3
H
N
H
AX4
AX3E1
H
Four electron pairs around the
central atom.
Three bonded and One lone pair.
(electron geometry = tetrahedral)
N
H
H
H
Molecular geometry = TRIGONAL PYRAMIDAL
Predicting Molecular Geometry
Example 6: H2O
AX4
H
O
AX2E2
H
Four electron pairs around the
central atom.
Two bonded and Two lone pairs.
O
(electron geometry = tetrahedral)
Molecular geometry = BENT
H
H
Predicting Molecular Geometry
Tetrahedral - bond angles
Order of increasing repulsion:
bonding pair-bonding pair
<
bonding pair-lone pair
<
lone pair-lone pair
Predicting Molecular Geometry
Example 7: PF5
:
: :
:F
P
F:
F:
:
:F:
: : : :
:F:
Five electron pairs around the
central atom.
electron and molecular geometry=
Zero lone pairs.
TRIGONAL BIPYRAMIDAL
Predicting Molecular Geometry
Example 8: SF6
:
S
: :
: :
:
:
:F
:F:
:
F:
:
:F
F:
:
:F:
Six electron pairs around the central atom.
Six bonded and Zero lone pairs.
(electron geometry = octahedral)
Molecular geometry = OCTAHEDRAL
Molecular Geometry
Dipole Moment and Polarity
d-
d+
d-
O
C
O
nonpolar, bp=-79C
dipole moment, μ = 0 D
d-
O
polar, bp=100C
d+
H
H d+
+
Net
dipole
dipole moment, μ = 1.85 D
Molecular Geometry
Dipole Moment and Polarity
In general, a molecule is polar if:


it is not a basic VSEPR shape (has lone pairs of
electrons)
Ex: H2O, bent (polar)
or if the terminal atoms/groups in a
basic VSEPR shape differ.
Ex: CH2Cl2, tetrahedral (polar)
Dipole Moment and Molecular
Geometry
Molecules that exhibit any asymmetry in the
distribution of electrons would have a nonzero net
dipole moment. These molecules are considered
polar.
Non polar
Polar
VSEPR shape
identical atoms
VSEPR shape
atoms differ
Dipole Moment and Molecular
Geometry
Molecular Geometry
Dipole Moment and Polarity
Non polar
+
PF3Cl2
PF5
Non polar
VSEPR shape
identical atoms
Atoms differ. BUT can
be divided into
nonpolar VSEPR
shapes:
linear + triangular
planar
PF4Cl
Polar
Polar
Atoms differ. Doesn’t
divide into nonpolar
VSEPR shapes
VSEPR
shape
atoms differ
PF3Cl2
Dipole Moment and Molecular
Geometry
F
Cl
ClF3
F
T-shaped
No symmetry → polar
F
F
XeF4
F
..
Xe
..
F
F
S:
F
F
F
F
F
Xe:
Square Planar
Symmetric → non polar
F
SF4
SeeSaw
No symmetry → polar
XeF2
Linear
Symmetric → non polar
Molecular Geometry
Dipole Moment and Polarity
CO, PCl3, BCl3, GeH4, CF4

Which compound is the most polar?

Which compounds on the list are non-polar?