Particles Substances are Made of
1. Atoms and Molecules (simple and giant)
We know that all matter is made up of particles and that these particles can consist of a variety of different parts. Once these particles are broken down, we find that the building block of all matter is the atom. The largest atom is caesium and its atomic radius is 0.235nm
1
(0.235 x 10 -9 m). That means that over 4200
If we take the thickness of one hair to be 60 microns (60x10 -6 m), then
255 000 atoms of caesium will fit side by side in that hair! million atoms of caesium (or just under 10 million millionths of a gram of caesium!) will fit across one metre if they are put side by side.
There are very few elements that can be extracted from the earth or atmosphere in atomic form. Some are simply too reactive, for example when potassium is extracted from reacting potassium chloride with sodium, 2 the potassium reacts easily with water and oxygen, so needs to be stored under paraffin to prevent it from reacting.
We have explored a number of things relating to the properties of matter
– purity, electrical and heat conduction and magnetism. All of these and other properties are dependent on two things:

the way in which the atoms or ions make up the particle
– often known as intramolecular forces or chemical bond.

the way in which these particles are held together – often known as intermolecular forces.
Note that the terms intra and intermolecular forces are not really accurate in their own right but are used in their historical sense. To understand this further, we will need to explore the three broad categories into which the bonding of elements is split:

Ionic bonding – involves a transfer of electrons and occurs between a metal and a non-metal. The ionic bond is between the positive and negative ions.

Covalent bonding
– involves a sharing of electrons and occurs between a non-metal and non-metal.

Metallic bonding
– occurs between a metal and metal.
In all three bonding types, giant molecules can be formed. In the case of covalent substances, a network of atoms forming a regular pattern is formed. In the case of ionic and metallic substances, lattices are formed.
1

Note that:

there are many covalent substances that form simple structures and the forces holding these molecules together are referred to as intermolecular forces.

the formula of an ionic substance represents the ratio in which the ions are found in the giant solid. Some texts draw a distinction between this and a molecule by calling it a ‘formula unit’.
We will refer to it as a ‘molecule’. The ball and stick diagram 3 alongside shows each ion having around it six oppositely charged ions.
The ratio of sodium ions to chloride ions is 1:1 so the formula of a ‘molecule’ of sodium chloride is NaCl. This is also known as the empirical formula (simplest formula) of sodium chloride.
Cubic structure of sodium chloride
Activity 8 Carbon and Graphite
South Africa is one of the world’s leading producers of diamonds – a special form of carbon.
Read the paragraph below and answer the questions that follow.
There are 2 allotropes (substances with the same chemical formula, but a different structural formula) of carbon, namely diamond and graphite. In the case of diamond, each carbon shares electrons with four other carbons forming a three-dimensional structure. In the case of graphite, a layered molecule is formed where three of the four electrons are shared between other carbon atoms. The fourth electron forms part of a set of delocalised electrons that are free to move within each layer. The ball and stick diagrams 4 below show this effect.
Layers of Graphite Structure of Diamond
2

1. Why do you think graphite conducts electricity, while diamond does not?
2. Why is diamond used on tools to facilitate cutting but graphite is used in pencils?
3. Why do you think the melting points of both diamond and graphite are close to 4000°C?
Summary of the Differences Between the Different Types of Structures
All substances can be broken up into three broad categories of structures as detailed below:
Examples
GIANT IONIC
‘MOLECULES’
All ionically bonded substances, e.g. NaCl,
CaCO
3




GIANT MOLECULAR
MOLECULES
Diamond, C
Graphite, C
Silica, SiO
2
All solid metals
SIMPLE
MOLECULAR
MOLECULES

Carbon dioxide,
CO
2

Oxygen, O
2

Water, H
2
O

Sugar, C
12

Sulphur, S
H
22
O
11
8
Particles making the substance up
Positive ions or cations bonded electrostatically or ionically to negative ions or anions
Atoms of carbon, formula units of SiO or atoms of metals
2
Molecules of CO
O
2
, H
2
O, C
12
H
22
2
O
,
11 or atoms of S forming a ring structure
Melting and boiling points
Phase
Hard to break therefore have very high melting and boiling points
All exist as solids
High melting and boiling points because much energy needed to break bonds
All exist as solids
Relatively low, indicating weaker forces of attraction holding the particles together
All phases, but S
8 only exists as S
8
in the solid phase
Conductor of electricity when solid
Non-conductors – there are no free ions
Metals and graphite are conductive as there are free electrons. Diamond is non-conductive
Non-conductors
Conductor of electricity when molten
Conductors – ions are free to move
All metals conduct, the rest do not
Non-conductors
Solubility in water
Most are soluble and therefore also conduct electricity – electrolysis results
Insoluble Varies
3

The diagrams below show the ring structure of the sulphur molecule (S
8
) in ball and stick as well as space filling mode.
Side View of the S
8
Molecule (ball and stick)
Top View of the S
8
Molecule (space filling)
Activity 9 Giant and Simple Structures
1. The table below gives some information about four substances. Giving your reasons, state which one: a) is ionic.
b) is metallic.
c) has a giant molecular structure.
d) has a simple molecular structure.
5
SUBSTANCE
MELTING
POINT (

C)
BOILING
POINT (

C)
ELECTRICAL CONDUCTION as solid as liquid
A 1083 2567 good good
B
–182 –164 poor poor
C
D
1723
993
2230
1695 poor poor good poor
4

2. Write down the numbers (1) to (10) and fill in the missing words from the list below.
Words can only be used once. hardness giant strongly covalent melting point weaker continuously difficult easier boiling point ionic
The structure of a substance means the way its atoms, molecules or ions are joined together. The structure of a substance decides properties such as
…….(1)……., ……..(2)……., and ………(3)……… . A simple molecular structure contains ……..(4)…….. molecules. The bonds between molecules of these substances are ….. (5)….. than the bonds between the atoms. In a/an
…….(6)…….. structure the ions are bonded together …….(7)…….. The structure goes on ……..(8)………. and makes a ……..(9)…….. structure. The ions are
……..(10)……. to break apart, and the structure is strong.
2. Intermolecular Forces and Kinetic Molecular Theory
Kinetic molecular theory provides us with a model on how to view three of the phases of matter, namely solids, liquids and gases. A diagram 6 representing the positioning of the particles relative to each other is shown.
SOLID LIQUID GAS
Note the following about kinetic molecular theory:

Density refers to the number of units (either molecules or atoms) that exist in a certain volume.

An increase in temperature causes particles of a substance to vibrate more. The
 greater the vibration, the greater the chance that intermolecular forces can be broken and a resulting change of phase.
Temperature is therefore a measure of the average kinetic energy of particles.
The reason for needing to use an
Many types of tomato sauces that in essence are liquids, retain a certain average is that particles within a substance all have different kinetic energies because they all vibrate at shape after being squeezed
(toothpaste does). Their viscosities are high. Such substances are different speeds.
Viscosity is a measure of how a substance (usually a liquid or gas) thixotropic . They can become more liquid after shaking.
5

resists flow. Liquids such as water and petrol have a low viscosity but golden syrup has a high viscosity. Increasing the temperature of a fluid will reduce its viscosity.
The table below summarises differences between solids, liquids and gases using the given criteria.
SOLID LIQUID GAS
Touching each other little space between particles
– very Touching but some have moved away and cause open spaces
Very few particles touch
– distances between the particles around 10 times the diameter of each of the particles
Orderly, vibrate around fixed positions
Particles are free to move because of spaces between them
They are free to move
Forces between particles become less, particles able to move more freely, eventually melts and then boils
More spaces open up, volume increases, eventually boils
Particles vibrate even faster, pressure increases if gas is enclosed
Forces between particles increase as particles get closer to each other
Particles lose energy, attractive forces increase and volume of liquid decreases
Particles lose energy, attractive forces increase and pressure decreases if gas is enclosed
Water behaves differently from what is expected. Water expands on freezing. If this were not the case, then ice would sink, causing rivers, dams and oceans to freeze over when conditions get to freezing point. The result would be that nothing could live below the ice as we know happens now. The diagram alongside shows a three dimensional representation of water molecules when in the solid phase. They form a hexagonal shape as shown alongside 7 . (See page 103 in
Chemical Systems)
Covalently bound molecules will be gases, liquids or lowmelting point solids. The higher the melting or boiling point, the greater the strength of intermolecular forces holding the molecules together. Note too that the smaller a covalent molecule, the lower its melting and boiling points.
A giant structure (ionic, metallic or covalent) will be a high-melting point solid.
6

Activity 10 Applications of Kinetic Molecular Theory
1. Given the following information 8
CH
C
C
C
C
C
4
8
16
20
25
4
H
H
HYDROCARBON
Formula
H
H
H
10
18
34
42
52
Name
Methane
Butane
Octane
Hexadecane
Eicosane n-Pentacosane
MELTING POINT
( °C)
–182
–138
–57
19
37
54
BOILING POINT
( °C)
–161
0
126
288
344
402 a) Which of the substances are gases at room temperature? b) Which of the substances are liquids at room temperature? c) Which of the substances are solids at room temperature? d) Do you notice any trend between melting and boiling points and the size of the hydrocarbon? e) C
25
H
52
is one of the main ingredients in the candles we buy but yet C
20
H
42
is not.
Using kinetic molecular theory, can you provide any reasonable explanation for this?
2. What relationship do you think there is between the size of a molecule and the strength of the intermolecular forces holding the molecules together?
3. What relationship is there between viscosity and the strength of the intermolecular forces between particles?
Revision Questions – Matter and Materials
1. Why is it important for the country to be able to export so much of its mineral wealth?
2. Give another name for metalloid.
3. What is the chemical name of the precipitate formed from the test for carbon dioxide?
4. In terms of valence and conduction bands, why are all metals conductors of electricity?
5. In terms of magnetism, what are domains?
6. Name the two allotropes of carbon.
7. In the ionic bond involving sodium carbonate, identify the positive and negative ions.
7

8. Apart from his work on chemistry, into what other field of science did Newton make a vital contribution?
9. Which scientist is responsible for the discovery of the a) electron? b) proton? c) neutron?
10. Draw an Aufbau diagram for each of the following: a) Mg 2+ b) F c) F d) Ne
References:
1 Clugston M, Flemming R et al, 2002, Chemistry , Oxford, Cape Town, page 282
2 Emsley J, 2001, Nature’s Building Blocks , Oxford University Press, Oxford, page 338
3 http://www.webelements.com/webelements/compounds/text/Na/Cl1Na1-7647145.html
4 http://www.webelements.com/webelements/elements/text/C/allotropes.html
5 http://www.teachmetuition.co.uk/Chemistry/Chemicalstructureandbonding/Structures%20Test.htm
6 http://www.chemguide.co.uk/atoms/structures/whichsort.html#top
7 http://www.lsbu.ac.uk/water/ice1h.html
8 http://www.ausetute.com.au/usehydrc.html
8