Properties of Polar and Non-polar Molecules
Knowing whether or not a molecule is polar or non-polar can give us insight
into its various physical properties. (i.e. melting point and boiling point)
Some Terminology
Intramolecular Forces: forces within a molecule that hold the
atoms together in a chemical bond
Intermolecular Forces: attractive forces between two distinct
molecules
Effect on Physical Properties
Since melting and boiling involves the separation of molecules from
one another, it makes sense that any substance with intermolecular forces
would have an increased melting and boiling point. (the values for water
are high compared to the values for carbon dioxide)
Hydrogen Bonding (weak forces of attraction)
Hydrogen "bonding" actually refers to the intermolecular forces that
exist between polar molecules containing hydrogen atoms (slightly positive)
and atoms such as O, N, and F (slightly negative). The hydrogen bond is
weaker than the intramolecular bonds of the molecule.
Example: Water
The solid line represents covalent bonds
The dashed lines represent hydrogen bonds
Properties of water:
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Exists as liquid, solid and gas naturally
Most common substance on earth
It is the universal solvent
High specific heat due to hydrogen bonding ( the ability to absorb a lot of
heat with very little change in temperature since hydrogen bonds must
be broken)
 Ice is less dense the liquid water due to hydrogen bonding
 High boiling point and melting point due to hydrogen bonding
Structures of Solids/Crystals
The arrangement of ions/atoms/molecules can result in 2 possible shapes:
1. Crystalline solid:
 ordered in well-defined arrangements of particles. (flat surfaces or
regular shapes)
 examples: diamond, quartz, salt
2. Amorphous solid:
 Irregular or random arrangement of particles (no orderly structure).
 Examples: glass, rubber
Solids (crystals or amorphous) are the result of ionic bonds, covalent
bonds, intermolecular attractions between molecules or metallic bonding.
Ionic Crystals
 anions and cations held together by electrostatic interactions
 array of positive and negative ions arranged such that every positive ion
has only negative neighbours and vice versa.
 strength of crystal depends on the charge and size of the ions involved.
 Examples: table salt NaCl and Epsom salts Mg(SO4)2
Network Solids AKA Network Crystals AKA Covalent Crystals
 Involves atoms that are covalently bonded (intramolecular forces) in an
extensive three-dimensional network.
 There are 5 network crystals (random order):
1. Diamond C4
2. Graphite C3
3. Silicon dioxide SiO2
4. Boron nitride BN
5. Silicon carbide SiC
Molecular solid
 Consists of molecules held together by intermolecular forces (hydrogen
bonding in ice)
 Example: wax and ice
Metallic solids
 Consist of atoms of metals held together via metallic bonding
 depends on the electronic structure of the metals.
 array of metal cations in a 'sea' of valence electrons
 example of metallic solids - zinc, iron
 example of mixtures of metals (Alloys): stainless steel, brass.
 two types of alloys: substitutional alloy and interstitial alloys.
Diamond versus Graphite
Diamond and graphite are both made up of carbon atoms that are bonded
in different ways. This difference in arrangement is responsible for their
unique properties. Thus diamond and graphite are allotropes.
Allotropes - different forms of the same element.
Properties of diamond
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All four valence electrons in carbon are involved in bonding.
Each carbon atom is bonded to 4 other carbons.
Hardness due to the strong covalent bonds.
Does not conduct electricity (no electrons available)
Properties of Graphite
 Only 3 of the 4 valence electrons are involved in bonding.
 Carbons are arranged in six-membered rings to form layers. (two
dimensional layers)
 The layers are held together by intermolecular forces.
 Covalent bonds account for hardness.
 It is slippery to the touch due to the fact that the layers slide past each
other easily because of the weak intermolecular forces.
 Does conduct electricity (free electrons in structure)