Molecular shapes
Balls and sticks
Learning objectives
 Apply VSEPR to predict electronic geometry
and shapes of simple molecules
 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
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)
Choices are limited
 Groups of charge range from 2 – 6
 Only one electronic geometry in each case
 More than one molecular shape follows from
electronic geometry depending on number of lone
pairs
 One surprise: the lone pairs occupy more space
than the bonded atoms (with very few exceptions)
 Manifested in bond angles (examples follow)
 Molecular shape selection (particularly in trigonal
bipyramid)
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)
 BCl3 provides 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:
• 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
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.
Consider some examples
 In CO2 (linear molecule) the two polar bonds
oppose each other exactly
 In chemical tug-o-war there is stalemate
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
Rules of thumb for evaluation of
polarity
 Presence of one lone pair of electrons
 Only one polar bond
 Always polar molecules
 Two or more polar bonds
 Do polar bonds perfectly oppose?
 If no, polar molecule
Two bonds
 Equal bonds oppose
(linear)
 Nonpolar (CO2)
 Unequal bonds oppose
(linear)
 Polar (HCN)
 Equal bonds do not
oppose (bent)
 Polar (H2O)
Three bonds
 Equal bonds oppose in
trigonal planar
arrangement
 Nonpolar
 Unequal bonds in
trigonal planar
arrangement
 Polar