Bonding & Molecular Structure
Physical Science
K Warne
Atoms and molecules (simple and giant)
Carry out all of the following tasks with respect to the following elements &
compounds: oxygen, water, petrol, sugar, carbon (diamond & graphite), table
salt and copper metal.
 Find out which of these (or any other) substances found in atomic form.
 Define a molecule.
 Write formula for each of these substances using:
o Molecular formula for simple molecules
o Empirical formulae for giant molecules
 Draw these molecules showing both space filling and ball and stick representations.
 Group these substances into:
o
Simple molecules (relatively few atoms per molecule):
o Giant molecules (millions of atoms per molecule):
Describe the difference between inter and intra molecular forces. State which of these
forces are involved in phase changes. Explain the boiling point of water in relation to the
other hydrides of group six.
.  Link the macroscopic properties of these substances to their microscopic (particle)
Intermolecular forces
• Two different types
of bonds occur in
water.
• Intramolecular
– Between hydrogen
atoms and oxygen
atoms inside the
molecules Covalent
bonds
• Intermolecular
– Between two
different water
molecules Hydrogen
bonds
H
+
H
O -
Bond Polarity in Water
The oxygen atom
has greater
electronegativity so
it is surrounded by
greater electron
density than the
hydrogen atoms.
+
-
+
H
O
H
O -
+
H
H
+
The water molecule is a
DIPOLE - it has two oppositely charged “poles”.
-
Hydrogen Bonding
•
The oppositely charged poles on
the water molecules attract each
other.
•
This electrostatic attraction
constitutes Hydrogen bonding.
•
It is the strongest form of
intermolecular attraction.
•
Hydrogen bonding exists only
between molecules in which
hydrogen is bonded to a very
electronegative atom (H-O-X, HNX2 or H-F).
•
Result in abnormally high boiling
points.
Example:
Water
H
+
H
O -
Molecular solids
Strong covalent bonds
Iodine - I2
PROPERTIES
• Low melting points
• Brittle
• Soluble in non-polar solvents
• Non - conducting
Weaker intermolecular bonds
Covalently bonded molecules held together by weaker intermolecular bonding.
IMF vs Mp & Bp
Molecular
Mass
(Mr g.mol-1)
Flourine
19 x 2 =
F2 pale yellow gas
38
Chlorine
Cl2 pale green gas
35.5 x 2 =
Bromine
80 x 2 =
Br2 red volatile liquid
160
Iodine
127 x 2 =
I2 purple solid - sublimes
254
71
Mp & Bp increases with SIZE
and molecule MASS
..
Halogen : X .
X2 (diatomic) . .
Mp / Bp
(oC)
-220 / -188
-101/ -35
-7 / 59
114 / 184
Density
1cm x 1cm x 1cm
= 1cm3

High density – solids
Low density – gases
Many particles per cm3
Few particles per cm3
Density = mass/volume (g.cm-3)
Effect of Temp
Maxwell-Boltzman Curve
Average Ek
Increases
• Temperature is a measure of average kinetic energy.
• As temperature rises more particles have high energy.
Effect of Temp
Maxwell-Boltzman Curve
Average Ek
• Temperature is a measure of average kinetic energy.
• As temperature rises – particles move faster – and further apart –
substances expand – used in a thermometer.
Viscosity
• Viscosity is a measure of
how thick (viscous) and
sticky a liquid is.
• Viscosity reduces the
ability of a liquid to flow.
• Liquids that flow readily
(water) have a low
viscosity.
• Viscosity is a function of
(depends on) the attractive
forces of the molecules of
the liquid.
• Strong forces – high
viscosity
• Temperature also greatly
affects viscosity: as
Kinetic energy enables particles
temperature increases,
to overcome forces.
viscosity decreases.
Bonding & Molecular Structure
• Atoms and molecules (simple and giant)

 State that the only substances found in atomic form are the noble gases
 Describe a molecule as a group of two or more atoms that are attracted to each other
by relatively strong forces or bonds
 Give examples of molecules based on the above description e.g.
o Small molecules (relatively few atoms per molecule): oxygen, water, petrol, sugar,
o Giant molecules (millions of atoms per molecule): diamond, a sodium chloride
crystal, a metal crystal like a piece of copper, …
 Recognize molecules from models (space filling, ball and stick, …)
 Draw diagrams to represent molecules using circles to represent atoms
 Represent molecules using
o Molecular formula for small molecules, e.g. O2, H2O, C8H18, C12H22O11, …
o Empirical formulae for giant molecules, e.g. C, NaCℓ, Cu,
 Give the formula of a molecule from a diagram of the molecule and vice versa
.
Describe the difference between inter and intra molecular forces
 • Linking macroscopic properties of materials to micro (particle) structure