Molecular shapes
Balls and sticks
Learning objectives
 Apply VSEPR to predict electronic geometry
and shapes of simple molecules
 Determine molecule shape from electronic
geometry
 Distinguish between polar and nonpolar
bonds in molecules
 Predict polarity of simple molecules from
bond polarity and molecular shape
Roadmap to polarity
 Establish skeleton of
molecule
 Determine Lewis dot
structure using S = N – A
 Determine electronic
geometry using VSEPR
 Identify molecular
geometry from molecular
 Count number of polar
bonds
 Perform polarity analysis
Stage 2: Valence shell electron pair repulsion
 Lewis dot structure provides 2D sketch of
the distribution of the valence electrons
among bonds between atoms and lone
pairs; it provides no information about
molecular shape
 First approach to this problem is to consider
repulsion between groups of electrons
(charge clouds)
Electron groups (clouds) minimize
potential energy
 Valence shell electron pair
repulsion (VSEPR)
 Identify all groups of charge: nonbonding pairs or bonds (multiples
count as one)
 Bonded atoms – single, double
or triple count as 1
 Lone pairs count as 1
 Distribute them about central
atom to minimize potential energy
(maximum separation)
Electronic geometry:
Counting groups of charge
 Identify central
atom. Many
molecules have
more than one.
 Central atom has
more than one atom
bonded to it
Choices are limited




Groups of charge range from 2 – 4
If octet expanded can get 5 or 6
Only one electronic geometry in each case
More than one molecular shape follows from
electronic geometry depending on number of lone
pairs
Total number of groups dictates
electronic geometry
 Octet rule:
 Two – linear
 Three – trigonal planar
 Four – tetrahedral
 Additional possibilities (expand octet):
 Five – trigonal bipyramidal
 Six - octahedral
Stage 3: Molecular shape:
 What you get from electronic
geometry considering atoms
only
 Same tetrahedral electronic
geometry – different
molecular shape
1. Establish electronic
structure using Lewis dot
model
2. Determine electronic
geometry using VSEPR
model
3. Determine molecular
shape from electronic
geometry
4. Determine molecule
polarity using symmetry
model
Two groups: linear
 Except for BeH2, all cases with two groups involve
multiple bonds
Three groups: trigonal planar
 Two possibilities for
central atoms with
complete octets:
 Trigonal planar (H2CO)
 Bent (SO2)
 BH3 is example of
trigonal planar with
three single bonds
 B is satisfied with 6
electrons
Four groups: tetrahedral
 Three possibilities:
 No lone pairs (CH4) tetrahedral
 One lone pair (NH3) –
trigonal pyramid
 Two lone pairs (H2O) –
bent
 Note: lone pairs need
more room:
• H-N-H angle 107°
• H-O-H angle 104.5°
• Tetrahedral angle 109.5°
Representations of the tetrahedron
Groups of
charge
Lone electron Electronic
pairs
geometry
Molecular
shape
2
0
Linear
Linear
3
0
Trigonal planar Trigonal planar
3
1
Trigonal planar
4
0
Tetrahedral
Tetrahedral
4
1
Tetrahedral
Trigonal
pyramid
4
2
Tetrahedral
Bent
Bent
Molecules with multiple centers
 A central atom is any atom with more than one atom
bonded to it
 Perform exercise individually for each atom
 Electronic geometry and molecular shape will refer only to
the atoms/lone pairs immediately attached to that atom
Important properties related to
polarity
 Solubility: polar molecules dissolve in polar
solvents; nonpolar molecules dissolve in nonpolar
solvents
 Oil (nonpolar) and water (polar) don’t mix
 Ammonia (polar) dissolves in water
 Melting and boiling points
 Polar substances have high intermolecular forces:
 Melting and boiling points are much higher than with
nonpolar substances (H2O is a liquid, CO2 is a gas)
Roadmap to polarity
 Establish skeleton of
molecule
 Determine Lewis dot
structure using S = N – A
 Determine electronic
geometry using VSEPR
 Identify molecular
geometry from molecular
 Count number of polar
bonds
 Perform polarity analysis
Polar bonds and polar molecules
 Not all molecules
containing polar bonds
will themselves be polar.
 Need to examine the
molecular shape
 Ask the question:
 Do the individual bond
polarities cancel out?
 If so, non polar. If not,
polar.
How to determine if polarity
cancels out
 In CO2 (linear molecule) the two polar bonds
oppose each other exactly
 In chemical tug-o-war there is stalemate
H2O: The most important polar molecule
 In BF3 the three bonds cancel out – tug of war stalemate
 In H2O (bent) the polar bonds do not directly oppose – no
stalemate
 Lone pair also adds some component
 Overall net polarity
 Consequence of polarity: H2O is a liquid, CO2 is a gas
Symmetry and polarity
 If the molecule “looks”
symmetrical it will be
nonpolar
 If the molecule “looks”
non-symmetrical it will
be polar