Molecular Geometry


The valence shell electron pair repulsion model (VSEPR model) assumes
that electron pairs repel one another. This produces a set of geometries
which depend only on the number of valence shell electron pairs and not
on the atoms present.
To determine the molecular geometry
 Draw the Lewis structure
 Count the number of electron pairs (bond pairs and lone pairs but
count multiple bonds as one pair)
 Arrange electron pairs to minimise repulsion
 Name the geometry from the atom positions
1
No e Pairs
(Lewis
Structure)
Arrangement of
Electron Pairs
No of 
Bond
Pairs
No of
Lone
Pairs
X
2
0
Linear
BeCl2, CO2, N3
3
0
Trigonal planer
BCl3, SO3, CO3
2
1
Angular
SO2, O3, NO2
4
0
Tetrahedral
CH4, NH4 , PO4
3
1
Trigonal pyramid
H3O , NH3,
XeO3
2
2
g
Angular
2
S
h
a
p
e
Molecular geometry
Examples
X
3
4
X
-
2-
-
+
3-
+
-
H2O, NH2 , ClO2
X
5
5
0
4
1
3
2
Trigonal
bipyramidal
PCl5, SF5
SF4, PBr4
6
X
-
"See-saw"
ClF3, XeF3
T-shaped
-
-
+
-
ICl2 , XeF2
2
3
6
0
Linear
Octahedral
SF6, SiF6 , AsF6
5
1
Square pyramidal
IF5, SF5 , SbF5
4
2
Square planer
ICl4 , XeF4
2-
-
-
-
2-
2
Examples and Questions
O C O
Linear
Cl
O
F
P Cl
Cl
S
Tetrahedral
F
F
F
'See-saw'
F
Cl
F
F
F
'T'-shaped
F
F
I
F
F
Square pyramid
Give the molecular geometry of BeCl2, PCl5, HCO2-, NH4+
3
1
Multiple Bonds
Single bonds are ‘sigma’ bonds. Double bonds consist
of one sigma and one ‘pi’ bond


A Sigma () bond is a bond that has a cylindrical shape about
the bond axis.
A Pi () bond is a bond that has an electron distribution above
and below the bond axis.
4
Larger Covalent Molecules
Similarities in structure may not be immediately obvious
CH 3
CH 3
N
CH3
N
O
N
O
HO
OH
H3CO
O
OH
morphine
H3COCO
OCOC
H3
codeine
heroin
CH2N(CH3)2
H2C
Methadone:
C
CH3
H3C
N
O
C
CH2CH3
O
5
Identify procane (an aesthetic)
O
N
CH3
H3CO
C
Cocaine
O
COOH
H3COOC
CH3
H3C
N
O
H2N
Benzocaine
C
OCH3
O
O
H2N
C
CH2CH3
N
O
O
HO
CH2CH3
NH
COOH
6
2
Enzymes
Enzymes facilitate chemical reactions in cells.
They are very shape specific.
7
Polar Covalent Bonds
For a heteronuclear bond such as in H2O, the oxygen attracts electrons
much more strongly than H so that the electron cloud is pulled more
tightly about the oxygen atom, hence that end of the molecule
experiences a slight build-up of negative charge relative to the
hydrogen atoms.
8
Polar Covalent Bonds
In Polar Bonds…



In polar bonds, the electron transfer is not complete; the
electron pair is still shared; but gives rise to partial charges
(+/-). This is called a polar covalent bond.
The degree to which a covalent bond is polar depends on the
electronegativities of the bonded atoms.
Electronegativity is an empirical scale that represents the ability
of an atom, when in a compound, to attract the electrons of a
chemical bond towards itself.
9
3
Electronegativity

Electronegativity increases across a period and decreases
down a group.

Why are there no
values of
electronegativity
assigned to the
Noble gases?
10
Types of Intermolecular Forces
1. London forces (dispersion forces) are the weak
attractive forces between molecules resulting from the small,
instantaneous dipoles that occur because of the varying
positions of the electrons during their motion about nuclei.
11
Types of Intermolecular Forces
2. A dipole–dipole force is an
attractive intermolecular force resulting
from the tendency of polar molecules to
align themselves such that the positive
end of one molecule is near the negative
end of another.
3. Dipole – induced dipole. The
interaction between a dipole and an
induced dipole.
(Van der Waal's forces is a general term for those intermolecular forces
that include dipole–dipole and London forces.)
12
4
Types of Intermolecular Forces
4. Hydrogen bonding is a weak to moderate attractive
force that exists between a hydrogen atom covalently
bonded to a very electronegative atom and a lone pair of
electrons on another, electronegative atom: F, O or N.
13
Types of Intermolecular Forces
5. Ion-dipole forces result when an ion and a polar
molecule interact. This is the dominant intermolecular
force when an ionic compound dissolves in water.
6. Ion-induced dipole forces arise from the interaction
of the charge on an ion with the electron cloud on a
molecule. The charge distorts the electron cloud inducing
a dipole.
14
Magnitude of forces
kJ mol-1
15
5
Which has the higher b.p.?
(a) CO2 or SO2
• Dispersion forces increase with MW  Greater in SO2 than in CO2
• SO2 is polar and has dipole-dipole forces whereas CO2 is non-polar
•These combine so that SO2 has the higher b.p.
bp
(b) CH3OCH3 or CH3CH2OH
• Dimethyl ether and ethanol have the same molecular formula but
different structural formula. Same MW  similar London Forces.
• There is a H atom in ethanol bonded to an electronegative ion (O)
and therefore H-bonding is important.
• Ethanol will have the higher BP.
16
Anaesthetics
F
Sevoflurane
F
 Quite insoluble in blood, so very quick to
get into brain and act but need high
concentrations and expensive.
 Used as an induction agent
H
C
F
F
H
O
C
F
H
F
F
Halothane
 More soluble in blood, so not as quick
acting but lasts longer.
 Used as a maintenance agent to keep
animal ‘under’
F
F
H
C
C
F
Cl
Br
17
Vanquin and worms
CH3
CH3
N
R
N
CH3
H
H 3C
N
CH3
How vanquin works
 Blue portion is ionic and ensures solubility as drug ingested.
 In intestine the pH changes causing the red portion to
separate, this is lipophilic and is absorbed by the worms and
kills them.
 The worms let go and are flushed out of body.
18
6
Colloids and Surface Chemistry
Solution
homogeneous
mixture
Suspension
heterogeneous
mixture
particles are
molecules
particles
visible to eye
Colloid
size 1-1000 nm
particles invisible to eye,
remain suspended
19
Characteristics of Colloids




Very large surface area
May have a charged surface
Scatter light (Tyndall effect)
Classified in terms of dispersed substance (s, l, g) in
dispersing medium (s, l, g)
20
Classification of Colloids
Dispersed
Phase
Liquid
Dispersing
Medium
Gas
Name of
Colloidal System
Liquid Aerosol
Solid
Gas
Aerosol
Dust, smoke
Gas
Liquid
Foam
Suds, whipped cream
Liquid
Liquid
Emulsion
Cream, milk, mayo
Solid
Liquid
Sol
Paints, jellies, sewage
Common Examples
Mist, clouds, fog
Gas
Solid
Solid Foam
Marshmallow
Liquid
Solid
Solid Emulsion
Butter, cheese
Solid
Solid
Solid Sol
Opals, some alloys
21
7
Surface Tension

Many properties of liquids give us direct information
about the forces that exist among the particles

The molecules in the interior of the droplet are surrounded by other
molecules however those at the liquid
q
surface are subject
j
to attractions
only from one side and from below

The effect of this uneven pull on the surface molecules tends to draw them
into the body of the droplet assuming a spherical shape – minimum
surface area

The resistance of a liquid to an increase in its surface
area is called surface tension of the liquid.
Polar solvents tend to have fairly high surface tension

22
Lipids



Lipids are water-insoluble substances that can be extracted
from cells by non-polar organic solvents such as benzene
and ether
 fats
 phospholipids
 waxes
 steroids
Fats that are esters of glycerol and are called triglycerides
Saponification is the hydrolysis of triglycerides to glycerol and fatty
acids. In the lab this is done with an aqueous solution of sodium
hydroxide to form carboxylate salts - soaps
23
Micelles

Fatty acids ions have a long
nonpolar tail and polar head

Th
These
iions fform micelles
i ll iin water
t

A “soap” solution is not a true solution; it does not contain
individual fatty acids anions dispersed in water but rather
groups of ions (micelles)

Thus a soap-water mixture is a suspension of micelles in
water. Because the relatively large micelles scatter light,
soapy water looks cloudy
24
8
Surfactants

Soap can be viewed as an emulsifying
agent, since it acts to suspend the normally
incompatible grease in the water.

Because of this ability to assist water in
‘wetting’ and suspending nonpolar materials
soap is called a wetting agent or surfactant.

Bile is the natural surfactant and aids
absorption of fats through the gut.
25
Phospholipids



Phospholipids are similar in structure to fats in that they
are esters of glycerol. However unlike fats they contain
only two fatty acids. The third ester linkage involves a
phosphate group, which gives phospholipids two distinct
parts:
 long non-polar tail
 polar substituted phosphate “head”
Phospholipids tend to form bilayers in aqueous solution
with the tails in the interior and the polar heads
interfacing with the polar water molecules
The bilayers of larger phospholipids can close to form
vesicles
26
Cells

Phospholipids form a significant
portion of cell membranes

The cell membrane:

Protects the workings of the cell from the extracellular fluid that
surround it
 Allow nutrients and other necessary chemicals to enter the cell
and waste products to leave

The most widely accepted model of this transfer of nutrients and waste
is called the fluid mosaic model: small uncharged molecules such as
water, oxygen and carbon dioxide diffuse freely through the bilayer,
while other substances pass through “gates and passages” provided
27
by specific proteins embedded in the membrane
9