3.0 Bonding
Lister p 44-69
AQA AS Specification
Lessons
1-2
3
3-5
Topics
Bonding
•understand ionic bonding, covalent bonding, co-ordinate bonding and metallic
bonding in terms of electrons and forces.
Electronegativity
•understand electronegativity and that the electron distribution in a covalent
bond may not be symmetrical
• know that covalent bonds between different elements will be polar to different
extents
• understand qualitatively how molecules may interact by dipole forces and
hydrogen bonding
• understand the importance of hydrogen bonding in determining the boiling
points of compounds and the structures of some solids (e.g. ice) and to
understand changes of state
5-7
Bonding and structure
•recognise the four types of crystal and know the structures of the following
crystals: sodium chloride, magnesium, diamond, graphite, iodine and ice
•be able, in terms of electron pair repulsion, to predict the shapes of, and bond
angles in, simple molecules and ions.
3.4 (Lister) – Metallic bonding
Describe how bonding occurs in
metals
Properties of metals
The delocalisation of electrons
can be used to explain some
properties of metals
Strong forces between lattice ions and
electrons lead to high m.p. and malleability
3.1 Ionic bonding
Name the
three types of
strong bonds
Why
do elements in the same
between
group have similar patterns of
atoms
bonding?
Covalent - (Love)
Ionic - (Altruism)
Metallic -(Indifference)
Because of the octet rule. Atoms try
to gain noble gas configurations
Sodium has lost
negative
one aouter
charge
so become
a by
electron.and
It gains
stability
positive
giving anion.
electron to
Chlorine
a negative
chlorine. gains
Chlorine
also
charge
becomes
a
becomeand
stable
by gaining
negative
ion
one electron.
Ionic bonding in MgCl2
In each case, the ions are held
together by attractive electrostatic
forces.
They form giant structures
NaCl
Task
Draw dot cross diagrams to show how
CaCl2 is formed
The Octet Rule
http://liakatas.org/chemblog/?page_id=17#
Videos
Ionic bonding and orbitals
Properties of ionically bonded compounds
List the properties of ionic
compounds
Solid at room temperature
Giant structure
High melting points
Crystalline
Brittle - shatter easily
Conductors in liquid or solvated state
Polar, Hydrophilic
Why are they brittle?
Ions of the same charge
are now adjacent
3.2 Covalent bonding
If ionic bonds form between metals
and non-metals, between what class
of element do covalent bonds form?
Non-metals and non-metals
Nirvana
Why?
Electron loss
Electron gain
11
Why do covalent bonds form?
Covalent bonds often form
between atoms with too many
electrons in their valence shells
to give away, but not enough to
easily fill.
Thus they share electrons with their
neighbours, in such a way that including
the shared electrons the shells are full
Delocalizing electrons over two atoms
instead of one lowers the energy of the
system
Properties of covalent bonds
Colvalent bonding forms discrete
molecules. State 4 facts about
covalent bonds
Atoms share pairs of electrons
Each atom has a stable, noble gas
configuration
They have molecular formulae
The molecules are neutral
Lewis structures, are
similar to dot-cross
diagrams
Use the information above to
draw a dot-cross diagram for
methane
What holds covalent bonds together?
What force exists between two
atoms?
The Electrostatic force
Draw a diagram to show these
forces
The atomic separation of particles
in a nucleus is determined by the
balance of these forces
Extension
More about covalent bonds
Unlike Group 1 and group 7 elements,
most elements need to gain, lose or
share more than one electron
Draw a dot cross diagram for oxygen
Question
Draw dot cross
diagrams for the
following:
Hydrogen
Oxygen
Carbon dioxide
Carbon tetrafluoride
Properties
How strong is the inter molecular
attraction between covalently
bonded molecules?
Not very!
Low melting and boiling points
Often amorphous
List 3 properties of covalently
bonded molecules
Often poor conductors of electricity
Remain molecular if they dissolve in
water
Can be involved in other types of
bonding…….
Lone pairs
What is a lone pair?
Lone pairs occur in elements from
group 5, 6 and 7
Lone pair
How many lone
pairs does Oxygen
have?
Lone pairs
Lone pairs affect
the shape of the
molecule
Dative bonding
Lone pairs can take part in something
called “dative covalent bonding” (coordinate bonding)
Dative bonds are
indistinguishable from other
covalent bonds, however, an
ion or dimer may be formed.
How?
+
H
i.e.
H
N
H
H
3.3 Electronegativity
How desperate are you for this lesson to end? (tick one)
Very
Slightly
What group do you belong to?
Group I & VII
Not at all
Group II & VI
N/A
Group III & V
Group 4
If you answered “very”, then you
are like elements in Groups I and
VII. They are very desperate to
lose or gain an electron!
This demonstrates the concept of electronegativity:
"The power of an atom in a molecule to attract electrons to itself."
The Pauling scale
Increases
Decreases
Why are the halogens
missing?
What does electronegativity
depend upon?
Describe the trends of
electronegativity across the
PT
1. Nuclear charge
2. Distance between the nucleus
and the outer electrons
3. The shielding by inner electrons
Atomic radii – what’s the link?
Rules
1. The smaller the atomic radius, the closer the nucleus is to the shared
electrons, the larger the electronegativity
2. The larger the nuclear charge (for a given shielding effect), the greater the
electronegativity
Inequality
What happens if the partner of oxygen doesn’t
want to give away it’s electron??
The molecule becomes
polar
Polar molecules
What molecule is this?
HCl
Is it polar, or non polar?
polar
How many lone pairs
are there?
three
Electron probability
(density) map
What other bonding
possibilities are there
Question:
Which of the following molecules will be polar?
All of them, except CCl4
HCl, H2O, CCl4, CH2 O
How do we know if a molecule is going to be polar?
Step 1: Draw a reasonable Lewis structure for the substance
Step 1: If the difference in electronegativity for the atoms in a bond is
greater than 0.4, we consider the bond polar.
Step 3: If there are no lone pairs on the central atom, and if all the
bonds to the central atom are the same, the molecule is nonpolar.
If the central atom has at least one polar bond and if the groups
bonded to the central atom are not all identical, the molecule is
probably polar.
Step 4: Describe the polar bonds with arrows pointing toward the
more electronegative element. Use the length of the arrow to show
the relative polarities of the different bonds. Check for symmetry.
Example:
Is CO2 polar or non polar ?
Step 1: The Lewis structure for CO2 is
Step 2: The electronegativities of carbon and oxygen are
2.55 and 3.44.
The 0.99 difference in electronegativity indicates that the
C-O bonds are polar, BUT all of the bonds to the central
atom are the same, which indicates that the molecule
nonpolar.
If we put arrows into the geometric sketch for CO2, we
see that they exactly balance each other, in both direction
and magnitude.
This shows the symmetry of the bonds.
Example 2
Why is CCl4 non-polar?
The lewis structure is:
The molecular geometry
of CCl4 is tetrahedral
Even though the C-Cl bonds are polar,
their symmetrical arrangement makes the
molecule nonpolar.
Summary of bonding
3.5 Other bonding possibilities 
There are three types of intermolecular force

van der Waals

dipole-dipole forces

hydrogen bonding
Dipoles:
Individual bonds can be polar, but molecules with
polar bonds can also have a dipole moment caused
by all of the polar bonds in the molecule.
What is a turning
moment?
Turning moments are found
where asymmetric forces
operate
Dipoles
What causes the
intermolecular force?
An unequal distribution
of electron density due to
the high e-negativity of
Chlorine. It is an
electrostatic force
Uncharged molecule can still have an
electric dipole moment.
Electric Dipoles arise from opposite but
equal charges separated by a distance.
Molecules that possess a dipole moment
are called Polar molecules
Dipole –dipole forces
Dipole-Dipole forces exist between
neutral polar molecules
Dipoles affect the boiling points of a substance –
the reason why water is a liquid at room
temperature
Dipoles and symmetry
(Ext) Induced dipoles are the reasons for
induced charge in electrostatics.
At the molecular level, the proximity of a
charged particle can distort the electron cloud
of another neutral atom and the two will stick
together. This is called an induced dipole
A polar molecule is one with a permanent dipole moment.
A polar molecule must have a slightly positive end opposite a
slightly negative one.
What about symmetrical
molecules?
If a molecule is 'spherical' enough,
then each end of the molecule will
have the same properties and in must
be non-polar.
Van der Waals forces
What happens in non-polar
species?
In the noble gases, there is no
“molecular stickiness” so how can
they be liquefied?
Other forces exist – called
dispersive forces. They are very
important in non-polar molecules
and atoms, but exist in all atoms
and molecules.
Consider helium. Where would
you be likely to find the electrons
at a moment in time?
Transient dipoles
The movement of the
electrons, even in the He
atom, cause an
instantaneous dipole to be
formed.
The time-averaged dipole
moment of the atom is still
zero.
This dipole, however short lived, can
induce a dipole in a neighbouring atom,
causing a force.
This force is always attractive but even
shorter ranged (and weaker) than
permanent dipole-induced dipole forces.
True
Van der Waals
False
They are in addition to other types of force
They act only at certain time and in certain places
The size of vdW forces decreases with increasing Z
The increase in b.p with Z in Group 0 is due to vdW
vdW forces do not act between non-polar molecules
The bigger the molecule, the larger the vdW
vdW forces rely upon spherical symmetry
3.6 Hydrogen bonding
If van der Waals forces act between all molecules and atoms
(ie they are ubiquitous), what range do hydrogen bonds have
and are they as strong?
•H-bonds are a special case of permanent dipole-dipole
interactions.
•They are stronger than van der Waals forces and around 10%
as strong as covalent bonds
•Molecules with hydrogen bonds have higher boiling points
than molecules that don’t.
What are the two prerequisites for H-bonding?
A hydrogen atom covalently bonded to an
electronegative atom … N, O or F.
A lone pair of electrons on the electronegative
atom.
If only one of these
conditions is met,
you don’t get
hydrogen bonding.
Task
Decide what these molecules are and whether they
will take part in H-bonding
Ammonia
Has hydrogen bonds.Nitrogen is very
electronegative, and it has one lone
pair of electrons in ammonia
Methane
No hydrogen bonds. Carbon is not
very electronegative, and it has no
lone pairs of electrons in methane.
Water
Has hydrogen bonds.Oxygen is very
electronegative, and it has two lone
pairs of electrons in water
Hydrogen bonding in water
Boiling points of period 2 and 3 hydrides
Complete work
sheet 3.6
Why do the
hydrides of N, O
and F buck the
trend?
http://liakatas.org/chemblog/?page_id=17#
Videos
HSW – Hydrogen bonds in action
Why does ironing get rid of creases in
fabrics and paper?
The heat supplied, breaks the
hydrogen bonds between the
polymers in the fabrics. They re-form
when the heat is removed
Protein chains
Summary:
Summary of intermolecular bonding
Energy/kJ
mol-1
Van der Waals
3.7 States of matter
http://www.harcourtschool.com/activity/states_of_matter/
Describe three states of matter in terms of shape, spacing and movement,
including the evidence for the theory and any models that are used
Solid
Liquid
Gas
Arrangement
Regular
Random
Random
Evidence
Sharp edges on
crystals
Will change shape
and fill the bottom
of a container
A gas will fill a
container
Spacing
Evidence
Movement
Evidence
Close
Close
Far apart
Not easily
compressed
Not easily
compressed
Easily
compressed
Vibrating around
a point
Rapid jostling
Rapid
V. slow diffusion
Solids expand on
heating
Slow diffusion
Liquids evaporate
Rapid diffusion
Gases exert
pressure
Latent heat
Explain the following diagram in
terms of energy and molecules
Latent Heat of Fusion
solid => liquid
If you have a glass of a cool drink, well supplied with ice, you can expect
its temperature to drop until it is close to 0ºC. You also can expect (and
can easily check with a thermometer) that it will remain cold, regardless of
the outside temperature, as long as there remains some unmelted ice in
the drink. Only after all the ice has melted will the temperature of the drink
begin to rise.
Why is this?
 Energy must be
supplied to overcome
molecular attractive
forces.
Ice to water
 This energy is supplied
externally, normally as heat, and
does not increase the
temperature.
 Latent heat of
fusion is the amount
of heat required to
convert a unit mass
of solid into liquid
without a change in
temperature.
 As the effects are not
measurable as a temperature
change, we call this heat latent
heat, meaning “invisible” heat.
Melting
How is the melting temperature of a solid related to the
intermolecular bonding
Crystals and metals have
strong electrostatic
intermolecular bonds
Tungsten
Ice
Latent Heat of Vaporization
liquid => solid
water to water vapour
 Energy must also
be supplied to
overcome molecular
attractive forces in a
liquid.
 As for a solid, this energy is
supplied externally, normally as
heat, and does not increase the
temperature.
 Latent heat of
vapourisation is the
amount of heat
required to convert a
unit mass of liquid
into vapor without a
change in
temperature.
 As a fluid changes from liquid to
vapor at its boiling point, its
temperature will not rise above its
boiling point.
Evaporation
Particles in liquids and gases
move with a range of speeds
defined by the maxwell-boltzman
distribution
If the speed is high enough, then
the kinetic energy of the particle
will be enough to overcome the
attractive intermolecular forces.
Carbon allotropes
Name 4 allotropes of carbon
What is the difference between
graphite and diamond?
Diamond, graphite,
buckminster fullerine, soot
nanotubes (carbon fibre)
http://liakatas.org/chemblog/?page_id=17#
Videos (graphite)
Why is graphite
a conductor?
m.p.3820K
m.p.3948K
Fullerines and nanotubes
m.p >280 oC
3.8 Summary
1. Produce a table summarising the
properties of Covalent, ionic, metallic and
inter- molecular bonds.
2. How are these properties reflected in
physical characteristics, such as m.p.
and b.p, conductivity etc
3.9 Molecular and ionic shapes
VSEPR theory: Valence Shell Electron Pair Repulsion theory.
States that pairs of electrons arrange themselves around the central atom
so that they are as far apart from each other as possible.
http://liakatas.org/chemblog/?page_id=17#
Videos
Which of these
shapes contains a
central atom with a
lone pair?
VSEPR summary
Place these in order of increasing repulsion:
Lone pair - bonding pair
Bonding pair- bonding pair
Lone pair - Lonepair
Complete worksheet 3.9
**