Basics of General Chemistry
 The Covalent Molecule
 Bond polarity and MEP
 Shapes of molecules
 The VSEPR model
 Polar and non-polar molecules
I am polar,
you are
not!
Chapter 4
1
Polar Covalent Bonds
 Polar covalent bonds occur when a covalent bond is
formed between two atoms of differing
electronegativities
The more electronegative atom draws electron density closer to
itself
The more electronegative atom develops a partial negative charge
(d-) and the less electronegative atom develops a partial positive
charge (d+)
A bond which is polarized is a dipole and has a dipole moment
The direction of the dipole can be indicated by a dipole arrow
The arrow head is the negative end of a dipole, the crossed end is
the positive end
Chapter 4
2
Example: the Molecule HCl
 The more electronegative chlorine draws
electron density away from the hydrogen
 Chlorine develops a partial negative charge
Dipole moment   0
Chapter 4
3
Bond Polarity and MEP
 A map of electrostatic potential (MEP) is a way to
visualize distribution of charge in a molecule
Parts of the molecule which are red have relatively more electron
density or are negative

These region would tend to attract positively charged species
Parts of the molecule which are blue have relatively less electron
density or are positive

These region would tend to attract negatively charged species
The MEP is plotted at the van Der Waals surface of a molecule

This is the farthest extent of a molecule’s electron cloud and therefore indicates
the shape of the molecule
The MEP of hydrogen chlorine clearly indicates that the negative
charge is concentrated near chlorine

The overall shape of the molecule is also represented
Chapter 4
4
COVALENT
versus
POLAR COVALENT
IONIC POLAR COVALENT
COVALENT
> 1.7 1.7
0.4 < 0.4---0
Difference between E. N. values
Metal
and non-metal
Two non-metals
Chapter 4
5
Matching Game
Dipole moment   0
Od- — Cd+
Fluorine molecule
HI
Dipole moment  = 0
Chapter 4
6
Predicting Bond Polarity
Write the symbol δ+ and δ- over the atoms in the
following bonds and draw the corresponding dipoles:
δ-
δ+
δ-
δ+
O
N
O
H
δ+
H
δ-
C
δ-
C
δ+
H
Chapter 4
What’s the
net dipole
moment for
the
molecule?
7
Shapes of Molecules
Why shapes are important?
Shape determines the polarity of the molecule.
How do we know or predict the shapes?
CO2
CO32Od- — Cd+
Chapter 4
8
The VSEPR Theory
V alence Shell
E lectron
P air
theory (Sidgwick and Powell, 1940)
R________
epulsion
Best shape has the arrangement that minimizes electron repulsion.
How to predict the shape of a molecule/ion?
1.
2.
3.
4.
Write the Lewis structure.
Count the total coordination number around the central atom.
Predict the ideal shape using the total coordination number.
Get the real shape by “disappearing” the lone pairs.
Total coordination number =#Bonding atoms
Chapter 4
+
#lone pairs
9
The VSEPR Model
Total
Coordination
Number
Shape
(Ideal)
2
Linear
3
Trigonal
Planar
4
Tetrahedral
5
Trigonal
bipyramidal
6
Octahedral
Drawing
Chapter 4
Bond
Angle
10
In-Class Practice
Use VSEPR to predict shapes of the following molecules and ions.
Total
Coordination
Number
Shape
(Ideal)
CH4
4
tetrahedral
H2O
4
tetrahedral
SCl2 4
tetrahedral
NO2- 3
trigonal planar
Chapter 4
Drawing
Bond
Angle
11
Molecular Dipole
The dipole moment of a molecule can be measured
experimentally
The actual unit of measurement is a Debye (D) which is
equivalent to 1 x 10-18 esu cm
Molecular Dipole
 In molecules with more than two atoms the molecular
dipole is the sum of the bond dipoles
 Some molecules with very polar bonds will have no net
molecular dipole because the bond dipoles cancel out
 The center of positive charge and negative charge
coincide in these molecules
In carbon tetrachloride the
bond dipoles cancel and the
overall molecular dipole is 0
Debye
Molecular Dipole
 In chloromethane the C-H bonds have only small
dipoles but the C-Cl bond has a large dipole and the
molecule is quite polar
 An unshared pair of electrons on atoms such as
oxygen and nitrogen contribute a great deal to a
dipole
 Water and ammonia have very large net dipoles
Predicting Molecular Polarity
Prove the following molecules are either polar or nonpolar.
CS2
SO3
Linear, non-polar.
Trigonal planar,
non-polar.
O
Steps:
1.
2.
3.
4.
Write the Lewis structure.
Draw all bond dipoles.
Find molecular dipole (net bond dipole).
If molecular dipole = 0, non-polar. If not, polar.
O
Chapter 4
S
δ+
O
15
Predicting Molecular Polarity
Prove the following molecules are either polar or nonpolar.
PBr3
SO2
Chapter 4
16
Additional Homework:
4.97, 99, 103, 105, 106, 109, 111.
Chapter 4
17