1. IB Chemistry Power Points
Topic 4
Bonding
www.pedagogics.ca
LECTURE
Intermolecular Forces and
Physical Properties

2. Much taken from
AN INTRODUCTION TO
BONDING
and
SHAPES OF MOLECULES
Great thanks to
JONATHAN HOPTON & KNOCKHARDY PUBLISHING
www.knockhardy.org.uk/sci.htm

3. Intermolecular Forces
Intermolecular forces collectively describe the
attractions BETWEEN the unit particles that make
up an element or compound.
The nature of the intermolecular forces depends
on the structure of the substance in question.
WARNING: be very specific in your language usage
when answering “explain” type questions.
The stronger the intermolecular forces, the greater
the forces of attraction. This affects properties of
substances such as melting and boiling point.

4. Complex Structures and Intermolecular Forces
In general - intermolecular forces in complex structures are
strong, chemical bonds that involve valence electrons.
• metallic bonds (in metallic structures)
• ionic bonds (in ionic compounds)
• covalent bonds (in giant covalent network structures)
Simple Structures and Intermolecular Forces
In general - intermolecular forces in simple molecules are
weak, electrostatic attractions between particles.
• Van Der Waals forces
• Dipole – Dipole interactions
• hydrogen “bonds”

5. Intermolecular Forces – Ionic Bonds
Ionic compounds are generally visualized as solids consisting
of anions and cations held together by electrostatic
attractions in a crystal lattice structure.
In molten NaCl, the ions have
sufficient energy to overcome
(“break”) the ionic bonds such
that the ions are no longer held
in fixed positions (note: they
are still attracted to each other)

6. Intermolecular Forces – Metallic Bonds
Metallic structures are generally visualized as
solids consisting of fixed cations held in place by
mutual attractions for a “sea” of valence electrons.

7. Intermolecular Forces – Metallic Bonds
Metallic bonds are ELECTROSTATIC attractions between
positive metal ions and negative valence electrons.
Heating a metal leads to an increase in the space between
the metal ions (thermal expansion). Increased energy of
ions, increases vibration, overcomes intermolecular forces,
and allows them to move apart. When melting occurs, the
ions are no longer “fixed” in position.

8. Intermolecular Forces – Giant Covalent Structures
The intermolecular forces in covalent networks (giant
molecules, macromolecules) are covalent bonds.
In diamond, each carbon atom
is covalently bonded to 4 other
carbon atoms. Collectively,
these 4 bonds create extremely
strong intermolecular forces.
It is difficult to imagine a
molten diamond – where the
bonds have been broken.

9. more on macromolecules - allotropes
How the atoms are bonded together in macromolecules can
affect the properties of the substance. Different bonded
forms are called allotropes. For example, three allotropes of
pure carbon are shown below.
Diamond Graphite C60
Buckminsterfullerene

10. more on macromolecules
Pure silicon and silicon dioxide (quartz) have similar structures
to diamond.
Silicon Silicon Dioxide

11. Basic Structures and Physical Properties

12. Simple Molecular Structures and
Intermolecular Forces
The intermolecular forces between simple molecules are
much weaker than the covalent bonds that bind the atoms
together to make the molecule itself.
Be very careful with language use here.
The strong intermolecular forces in ionic, metallic, and
giant covalent structures are chemical bonds. The weak
intermolecular forces between simple molecules are
NOT chemical bonds but are sometimes referred to as
“physical bonds”.

13. Simple Molecular Structures and
Intermolecular Forces
Intermolecular forces between simple covalent
molecules are collectively called Van der Waals
forces.
Some texts, and the IBO often refer to only the
weakest type of these forces as VDW forces (be aware)
VDW forces - an electrostatic attraction
between opposite dipoles in two different
molecules.

14. Non-Polar Molecules – weak VDW forces
attractions from temporary separations of charge
force of attraction increases with molecular
weight Mr

15. Polar Molecules – stronger attractions
Dipole-Dipole attraction
between oppositely
charged regions of
neighboring POLAR
molecules. For example
HCl

16. Hydrogen “bonding” – strongest attractions
Hydrogen bonding occurs between
positive hydrogen dipoles and the
lone pairs of oxygen and nitrogen
atoms.
Look for O-H and N-H bonds in
molecules!
Remember - Not a chemical “bond”
Hydrogen bonding in Kevlar

17. Hydrogen Bonding in Water

18. Hydrogen Bonding between Ammonia and Water

19. Physical Properties
Melting, boiling points, volatility,
electrical conductivity, solubility

20. Melting point, boiling point, volatility
The stronger the intermolecular forces, the greater
the forces of attraction.
Results in increased melting and boiling points,
and decreased volatility (ease of evaporation).
In general, metallic, ionic, and giant covalent
structures have very high m.p., b.p. and low
volatility.
Trends in these properties in simple covalent
molecules are important to understand.

22. Covalent Molecules and Boiling Points
The greater the attraction between dipoles the more energy must be put
in to separate molecules resulting in higher boiling points.
Mr °C Mr °C
CH4 16 -161 H2O 18 +100
Boiling points SiH4 32 -117 H2S 34 -61
of hydrides GeH4 77 -90 H2Se 81 -40
SnH4 123 -50 H2Te 130 -2
Those in red
illustrate NH3 17 -33 HF 20 +20
hydrogen PH3 34 -90 HCl 36.5 -85
bonding AsH3 78 -55 HBr 81 -69
SbH3 125 -17 HI 128 -35

23. BOILING POINTS OF HYDRIDES
100 H2O
The higher than expected boiling
points of NH3, H2O and HF are due to
intermolecular HYDROGEN BONDING
BOILING POINT / C°
HF
0 Mr
50 100 140
NH3
-160

24. Electrical Conductivity
Conductivity means “movable charge”.
Metals conduct: valence electrons are free to move
Molten ionic compounds, and aqueous solutions
conduct: ions are free to move
Simple covalent structures do not conduct
Giant covalent structures do not conduct (exception
silicon and graphite)

25. Summary
Conductivity means “movable charge”.
Metals conduct: valence electrons are free to move
Molten ionic compounds conduct: ions are free to move
Simple covalent structures do not conduct
Giant covalent structures do not conduct (exception
silicon and graphite)