Download Unit 10: Structure and Bonding

IGCSE CHEMISTRY
Unit 10: Structure and Bonding
Part 1: Kinetic Particle Theory
States of Matter
Matter is defined as anything that occupies space & has mass.
Matter can exist as a solid, liquid, gas.
 In solids, the particles are closely packed in orderly manner.
 In liquids, the particles are not as closely packed as solids and are
arranged in disorderly manner.
 In gases, the particles are distributed very far apart in random
manner.
 In solids, the particles vibrate about their fixed positions.
 In liquids, the particles move about sliding and rolling past one
other in random motion.
 In gases, the particles move about freely at high speeds and in
random motion.
Task: Fill in the blanks with ‘definite’ or ‘not definite’.
Physical state
Volume
Shape
Example
Solid
Ice
Liquid
Water
Gas
Water vapor
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Changes of state
Task: Use your knowledge to fill in the gaps below
Melting
When a solid is heated, its particles gain more _______ and vibrate
more. Due to the increase in vibrations, the solid expands. At _______
point, the particles vibrate so much that they break away from their
positions. It is at this point that a solid becomes a liquid.
Boiling
When a liquid is supplied with heat, its particles gain more energy and
therefore move around ________. This increase in movement causes the
liquid to ______. At _______ point the liquid particles gain enough
energy to overcome the ______ holding them together - these particles
break away from one another and the liquid now becomes a ____.
Evaporating
Not all particles in a liquid contain the same amount of energy - some have
more! This is why evaporation of a liquid can take place below its boiling
point. This is called ___________ and explains why puddles dry up on
sunny days.
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Condensing
When you cool a gas, the particles lose energy. This loss in energy causes
the particles to move more _______ as they move closer together. When
the particles no longer have sufficient energy to move away as they bump
into one another the gas becomes a _______.
Solidifying
When a liquid is _______, the particles once more lose energy, they
_______ less as they slow down. Eventually they stop moving, except for
vibrations in fixed positions. A solid has now formed.
Task: Use your knowledge from this lesson to fill in the table below.
Properties
Shape
Diagram showing the
arrangement of particles
Forces of attraction
between particles
(intermolecular forces)
Motion of particles
Energy content
Volume
Density
Compressibility
Solid
Liquid
Gas
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Fluidity
Heating Curve
Summaries what is happening at each stage within the heating curve in
the space below:
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IGCSE CHEMISTRY
Task: Use the vocabulary you have used so far to complete the crossword
References
IGCSE Chemistry pages 1-5
Learning Objectives
Candidates should be able to:
.
 Describe the states of matter and explain their conversion in terms of
the kinetic particle theory
IGCSE CHEMISTRY
Unit 10: Structure and Bonding
Part 2: Diffusion
Starter Task: Match the keyword with the definition
PARTICLE
The 3 ways in which matter can be found.
STATES OF MATTER
To get bigger – it does this when heated.
SOLID
When something is heavy for its volume.
LIQUID
A very small unit that all 3 states of matter
are made from.
GAS
To get smaller - it does this when cooled.
COMPRESS
A material with particles that quite close
together, but free to move.
DENSE
To squash into a smaller space.
EXPAND
A material with particles far apart, moving
quickly and in all directions.
CONTRACT
A material with particles that are packed
closely together. The particles vibrate but do
not move from place to place.
Diffusion
Diffusion is the random movement of particles from a place of high
concentration to a place of low concentration.
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How Does Diffusion Work?
In liquids diffusion occurs a lot more slowly because there are not as
many gaps in between the particles.
The diffusion of gases- The reaction between ammonia and hydrogen
chloride
Gases will diffuse at different rates. If one piece of cotton wool is
soaked in concentrated ammonia solution and another is soaked in
concentrated hydrochloric acid and these are put at either end of a dry
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glass tube, then after a few minutes a white cloud of ammonium chloride
appears. This shows the position at which the white clouds meet and
react.
The white cloud forms in the position shown because the ammonia
particles are lighter than the hydrogen chloride particles (released from
the hydrochloric acid) and so move faster.
References
IGCSE Chemistry pages 1-5
Learning Objectives
Candidates should be able to:
.
IGCSE CHEMISTRY
Unit 10: Structure and Bonding
Part 3: The Atom
An atom is defined as the smallest part of an element that can exist
as a stable entity.
The sub-atomic particles
Just over a century ago, scientists believed that atoms were solid
indestructible particles – just like tiny snooker balls. As a result of
experiments carried out by e.g. J.J. Thompson, Geiger and Marsden, and
Rutherford among others scientists now believe that all atoms are
composed of three important sub-atomic particles called protons,
neutrons and electrons.
Task 1: Can you complete the table below?
relative mass
relative charge
Proton
Neutron
electron
The nucleus
The nucleus is at the centre of the atom and contains the protons and
neutrons. Protons and neutrons are collectively known as nucleons.
Virtually all the mass of the atom is concentrated in the nucleus, because
the electrons weigh so little.
Working out the numbers of protons and neutrons
No of protons = ATOMIC NUMBER of the atom
The atomic number is also given the more descriptive name of proton
number.
No of protons + no of neutrons = MASS NUMBER of the atom
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The mass number is also called the nucleon number.
This information can be given simply in the form:
How many protons and neutrons has this atom got? The atomic number
counts the number of protons (9); the mass number counts protons +
neutrons (19). If there are 9 protons, there must be 10 neutrons for the
total to add up to 19.
The atomic number is tied to the position of the element in the Periodic
Table and therefore the number of protons defines what sort of element
you are talking about. So if an atom has 8 protons (atomic number = 8), it
must be oxygen. If an atom has 12 protons (atomic number = 12), it must
be magnesium.
Similarly, every chlorine atom (atomic number = 17) has 17 protons; every
uranium atom (atomic number = 92) has 92 protons.
Working out the number of electrons
Atoms are electrically neutral, and the positive charge of the protons is
balanced by the negative charge of the electrons. It follows that in a
neutral atom:
no of electrons = no of protons
So, if an oxygen atom (atomic number = 8) has 8 protons, it must also
have 8 electrons; if a chlorine atom (atomic number = 17) has 17 protons,
it must also have 17 electrons.
Task 2
1.
Can you calculate the number of subatomic particles present in the
following species:
Element
Symbol
Z
Sodium
6
12
84
A
No.
Protons
No.
Neutrons
23
12
12
210
No.
Electrons
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Chlorine
Chlorine
17
17
35
37
Drawing Electronic configurations
 Electrons are not evenly spread.
 The exist in energy levels known as shells.
 The arrangement of electrons in these shells is often called the
electron configuration.
RULES:
Electrons exist in shells around the central nucleus,
o
o
o
o
The 1st shell can contain a maximum of 2 electrons
(electrons 1-2)
The 2nd shell can contain a maximum of 8 electrons
(electrons 3-10)
The 3rd shell also has a maximum of 8 electrons
(electrons 11-18)
The 19th and 20th electrons go into the 4th shell,
(required limit of GCSE knowledge).
 Remember the total electrons to be arranged equals the
atomic/proton number for a neutral atom.
 If you know the atomic (proton) number, you know it equals
the number of electrons in a neutral atom, you then apply the
rules to work out the electron arrangement (configuration).
 For elements 1 to 20 the electron
arrangements/configurations are written out in the following
manner:
IGCSE CHEMISTRY
 Note that each number represents the number of electrons in a
particular shell, dots or commas are used to separate the numbers of
electrons in each shell. They are written out in order of increasing
average distance from the positive nucleus which holds these negative
electrons in their energy levels (shells). The electron configurations
are summarised below with reference to the periods of the periodic
table and in order of increasing atomic number.
o
o
o
o
Period 1 - elements 1 to 2 (2 elements)
 the electron arrangement is written out simply as 1 or
2
Period 2 - elements 3 to 10 (8 elements)
 electron arrangements of 2.1 to 2.8 (since 1st shell is
full with 2 electrons i.e. the first number)
Period 3 - elements 11 to 18 (8 elements)
 denoted by 2.8.1 to 2.8.8 (1st,2nd full shells with
2,8 electrons)
Period 4 - first two elements 19 to 20
 written out as 2.8.8.1 and 2.8.8.2 (1st,2nd,3rd full
shells with 2,8,8 electrons)
IGCSE CHEMISTRY
Working Out Electron Arrangements
 How many electrons do the element atoms have? (This will equal
the atomic number).
 Keeping track of the total used, feed them into the shells working
outwards until you have used them all up.
Basic atomic structure- Questions
1) a) What three particles are atoms made from?
(3)
...................................................................................................................................
b) Which particles are in the nucleus?
(2)
...................................................................................................................................
c) Explain why atoms are neutral even though they contain positive
and negative particles.
(2)
...................................................................................................................................
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2) a) Define the atomic (or proton) number of an atom.
(1)
...................................................................................................................................
b) Define the mass number of an atom.
(1)
...................................................................................................................................
...................................................................................................................................
c) Using the mass number and atomic number of an atom:
i) How do you work out the number of neutrons in an atom?
(1)
...................................................................................................................................
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ii) How do you work out the number of electrons in an atom?(1)
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iii) How do you work out the number of protons in an atom? (1)
...................................................................................................................................
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3)
Complete the table below about the structure of atoms.
Atom
Atomic
Mass
number number
40
No. of
No.of
Ar
27
18
Al
9
13
4
19
5
1
No. of
protons neutrons electrons
0
(21)
IGCSE CHEMISTRY
4) Draw diagrams showing the electronic structure of the elements within
the table
References
IGCSE Chemistry pages 32-39
Learning Objectives
Candidates should be able to:
 State the relative charges and approximate relative masses of
protons, neutrons and electrons.
 Define proton number and nucleon number.
 Use proton number and the simple structure of atoms to explain the
basis of the Periodic table (see syllabus section 9), with special
reference to the elements of proton number 1 to 20.
IGCSE CHEMISTRY
Unit 10: Structure and Bonding
Part 4: Isotopes
Isotopes
Isotopes are defined as atoms with…
the same atomic number but different mass number
OR
the same number of protons but different numbers of neutrons.
The properties of isotopes
The chemical properties of isotopes are identical, this is because
isotopes of the same element have the same number of outer electrons.
It is the outer electrons that determine the reactivity.
The physical properties such as density, melting and boiling points can
differ slightly.
Theory
Relative atomic masses measured by chemical methods rarely produce
whole numbers but they should do (allowing for the low relative mass of
the electron).
However, it has been found using a special technique (known as mass
spectrometry) that atoms of the same element can have different masses
due to the variation in the number of neutrons in the nucleus. The
relative atomic mass (Ar) is the average mass of an atom of an element,
taking account of its natural isotopes and their percentage abundance.
IGCSE CHEMISTRY
ISOTOPES - CALCULATIONS
There are two common isotopes of chlorine. Calculate the average relative
atomic mass of chlorine atoms
There are two common isotopes of chlorine. Calculate the average relative
atomic mass of chlorine atoms:
Task 1: Bromine contains 50.5% bromine-79 and 49.5% bromine-81.
Calculate the relative atomic mass of natural bromine showing your
working.
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.
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Radioactive and Non radioactive isotopes
Do NOT assume the word isotope means the atom it is radioactive,
this depends on the stability of the nucleus i.e. unstable atoms
(radioactive) might be referred to as radioisotopes.
Many isotopes are extremely stable in the nuclear sense and NOT
radioactive i.e. most of the atoms that make up you and the world
around you!
Uses of Isotopes
Medical Uses:
 In a suitable chemical form, the radioisotope is injected into the body
and its 'movement' can be followed.
 Time is allowed for the radioactive tracer to spread and its progress
tracked with a detector outside the body.
 The patient can be placed next to a 'detection screen' that shows
where the radioactive tracer is.
 The effective function of organs like the liver and digestion system
can be checked.
 Similarly, a patient can breathe in air with a gaseous gamma emitter in
it, and the effectiveness and structure of the lungs can be checked.
 The half-life must be relatively short so it does not linger in the body
increasing the harmful effects of cell damage.
Industrial Uses:
Carbon Dating


When the 'carbon containing' material is in a living organism there
is a constant interchange of carbon with the environment as food
or carbon dioxide. This means the carbon-14 % remains constant.
When the organism is dead the exchange stops and the carbon-14
content of the material begins to fall as it radioactively decays.
Compared to when it was 'alive', if an object has 1/2 of the
expected carbon-14 it must be 5700 years old. If it only has 1/4 (1/2
of a 1/2) of the expected carbon-14 left, the object it must be
IGCSE CHEMISTRY
11400 years old (5 700 + 5 700). If only 1/8 (1/2 of 1/4) left it is
17100 years old (11 400 + 5700) etc. etc.
Research Task
Use the internet to research the use of isotopes within industry. Make
detailed notes of your findings, you will need them to revise for your
exams! The list below may help you begin your research.
Iridium-192
Used in gamma radiography to locate flaws in metal components.
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Krypton-85,
Strontium-90,
Used for industrial gauging.
Thallium-204
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Nickel-63
Used in light sensors in cameras and plasma display, also electronic
discharge prevention and in electron capture detectors for thickness
gauges.
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americium-241
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IGCSE CHEMISTRY
Uranium-235
The radio isotope Uranium-235 is used in nuclear power stations to produced
energy. There are many positive and negative aspects to this type of energy
production.
HOMEWORK TASK- Write about some of the advantages and disadvantages of
using Uranium-235 as a source of energy, possible issues for discussion include
the long term nature of nuclear energy (sustainable long after coal and oil run
out) and environmental considerations such as the disposal of radioactive waste
(make it simple but clear, 150-200 words, ON A4 LINED PAPER)
References
IGCSE Chemistry pages 34-35
Learning Objectives
Candidates should be able to:
 Define the term isotope
 State the two types of isotopes as being radioactive and non-radioactive.
 State one medical and one industrial use of radioactive isotopes.
 Describe radioactive isotopes, such as
235
U as a source of energy.
IGCSE CHEMISTRY
Starter- Use what you have learnt so far in this module to complete the
past exam question below.
An unknown element is found to exist as the following two isotopes:
24
13
(a)
X
28
13
X
Complete the following table:
number of protons
24
13
X
28
13
X
number of neutrons
number of electrons
13
13
15
[2]
(b)
Describe what is meant by the term ‘isotopes’.
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......................................................................................................................................................................... [2]
(c)
In experiments, both isotopes were found to show the same chemical behaviour.
Use the information in the table to explain why the chemical behaviour of both isotopes
is the same.
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..............................................................................................................................................................................
......................................................................................................................................................................... [2]
(d)
A naturally occurring sample was found to contain 25% of X-24 and 75% of X-28.
Calculate the relative atomic mass of the element X.
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......................................................................................................................................................................... [3]
IGCSE CHEMISTRY
Unit 10: Structure and Bonding
Part 5: Elements, Mixtures, Compounds
Some elements, such as the noble gases, exist as
individual atoms.
Often, elements consist of two or more atoms joined
together. We call these molecules.
If more than one type of atom is in a molecule, we call
it a compound.
This is a mixture of elements made up of atoms,
elements made up of molecules, and compounds.
In a mixture, the particles are not joined together.
Task
1)
Decide whether each of the following substances is an element
or a compound.
H2
Br2
Co
HF
H2S
S8
C6H12O6
(8)
CO
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2)
Look at the diagrams below and decide whether each one
represents the particles in an element, a compound or a mixture.
Different colour atoms represent atoms of different elements.
(6)
1
3)
2
3
4
5
6
The diagram below represents the particles in air.
a) Is air an element, a mixture or a compound?
(1)
b) i) Give the formula of each of the substances shown in the air.
ii) Give the formula of each of the substances shown that are
elements.
iii)Give the formula of each of the substances shown that are
compounds.
(10)
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N
N
N
N
N
N
N
N
N
N
Ar
H O
H
O
O
N
N
N
N
O
O
N
N
N
N
O
O
C
O V
O
N
N
N
N
Task
In the table below, name and describe the separation technique that could
be used to separate each mixture
Mixture
1. Oxygen from liquid air
2. Red blood cells from plasma
3. Petrol and kerosene from crude oil
4. Coffee grains from coffee solution
5. Pieces of steel from engine oil
6. Amino acids from fruit juice solution
7. Ethanol and water
8. Pure water from sea water
Method
IGCSE CHEMISTRY
References
IGCSE Chemistry pages 11-20
Learning Objectives
Candidates should be able to:
 Describe the differences between elements, mixtures and compounds,
and between metals and non-metals.
 Become familiar with some methods that can be used to separate
mixtures
IGCSE CHEMISTRY
Unit 10: Structure and Bonding
Part 6: Metallic Bonding and Alloys
Metals are a highly valuable group of materials, used for hundreds of
products and produced in huge quantities.
Task- In the space below, discuss with the person next to you and fill in
the table below
Property
Metals
Appearance
Shiny
Melting
Non-Metals
and
boiling point
Density
Strength
Malleability
Ductility
Heat Conductivity
Electrical
Conductivity
Why do metals have these particular properties?
To understand why metals have certain properties we have to have a close
look at the structure of a metal.
IGCSE CHEMISTRY
Metallic Structure
The atoms in a pure metal are in tightlypacked layers, which form a regular lattice
structure.
The outer electrons of the metal atoms
separate from the atoms and create a ‘sea of
electrons’.
These electrons are delocalized and so are free to move through the
whole structure.
The metal atoms become positively charged ions and are attracted to
the sea of electrons. This attraction is called metallic bonding.
Why do metals conduct heat and electricity?
Metals are good conductors of electricity and heat because the free
electrons within the structure are able to carry charge or heat energy
through the metal structure.
Why are metals malleable? (Can be hit into a different shape)
When metals are hit the layers of ions are able to slide over each other.
The free electrons act as a lubricant between the layers of ions.
This also explains why metals are DUCTYLE, can be drawn into a wire.
IGCSE CHEMISTRY
Alloys
An alloy is a mixture of a metal with at least one other element.
The final alloy may have very different properties to the original metal.
Examples of Alloys
Steel
Steel is a common example of an alloy. It contains iron mixed with carbon
and other elements. Adding other elements to a metal changes its
structure and so changes its properties.
The atoms in pure iron are arranged in denselypacked layers. These layers can slide over each
other. This makes pure iron a very soft material.
The atoms of other elements are different sizes.
When other elements are added to iron, their
atoms distort the regular structure of the iron
atoms.
It is more difficult for the layers of iron atoms in
steel to slide over each other and so this alloy is
stronger than pure iron.
Type of steel
Iron alloyed with:
Properties
low carbon steel
about 0.25 per cent
easily shaped
carbon
car body panels
high carbon steel
up to 2.5 per cent
hard
carbon
cutting tools
stainless steel
chromium and nickel
cutlery
sinks
resistant to corrosion
Typical use
and
IGCSE CHEMISTRY
Brass
Brass
is
an
alloy
of
copper
(70%)
and
zinc
(30%).
Brass is harder and cheaper than copper and is used for musical
instruments, corrosion resistant screws, fixtures and ornamental objects.
Exam Style Question
Various parts of the helicopter below are made out of metal, because metals are strong
and malleable.
(a)
Describe, with the aid of a diagram, the metallic bonding present in metals.
[4]
(b)
Use your knowledge of the structure of metals to explain why:
(i) they are strong
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........................................................................................................................................................... [2]
(ii)
they are malleable
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........................................................................................................................................................... [1]
IGCSE CHEMISTRY
(c)
As the blades of the helicopter rotate, heat produced in the joints is conducted
away along the blades.
Explain how metals conduct heat.
................................................................................................................................................................
........................................................................................................................................................... [2]
(d)
The properties of some of the metals used in the helicopter have been altered
by mixing them with other metals. What name is given to this type of mixture?
........................................................................................................................................................... [1]
References
IGCSE Chemistry pages 11-20
Learning Objectives
Candidates should be able to:
 Describe and explain the structure and properties of a metal
 Describe alloys, such as brass, as a mixture of a metal with other
elements.
 Identify representations of alloys from diagrams of their structures.
IGCSE CHEMISTRY
Unit 10: Structure and Bonding
Part 7: Ionic Bonding
Ionic bonds are formed by one atom transferring electrons to another
atom to form ions.
Elements consist of neutral atoms or molecules, the electrical
neutrality is because the number of positive protons equals the number
of negative electrons.
Ions are atoms, or groups of atoms, which have lost or gained electrons to
have a net electrical charge overall .
The atom losing electrons forms a positive ion (a cation) and is usually a
metal. The overall charge on the ion is positive due to excess positive
nuclear charge (protons do NOT change in chemical reactions).
The atom gaining electrons forms a negative ion (an anion) and is usually
a non-metallic element. The overall charge on the ion is negative because
of the gain, and therefore excess, of negative electrons.
The examples below combining a metal from Groups 1 (Alkali Metals), 2 or
3, with a non-metal from Group 6 or Group 7 (The Halogens). The electron
structures are shown in () or []. Only the outer electrons of the original
atoms, and where they end up in the ions, are shown in the dot and cross
(ox) diagrams
Ionic bonding is not directional like covalent bonding, in the sense that
the force of attraction between the positive ions and the negative ions
act in every direction around the ions.
Example 1: A Group 1 metal + a Group 7 non-metal e.g. sodium +
chlorine ==> sodium chloride NaCl or ionic formula Na+Cl- In terms of
electron arrangement, the sodium donates its outer electron to a chlorine
atom forming a single positive sodium ion and a single negative chloride
ion. The atoms have become stable ions, because electronically, sodium
becomes like neon and chlorine like argon.
Na (2.8.1) + Cl (2.8.7) ==> Na+ (2.8) Cl- (2.8.8)
can be summarised electronically to give the stable 'noble gas' structures
as [2,8,1] + [2,8,7] ==> [2,8]+ [2,8,8]-
IGCSE CHEMISTRY
ONE
combines
with
ONE
to
form
The valencies of Na and Cl are both 1, that is, the numerical charge on
the ions. sodium fluoride NaF, potassium bromide KBr and lithium iodide
LiI etc. will all be electronically similar.
Note:
The charge on the sodium ion Na+ is +1 units (shown as just +) because
there is one more positive proton than there are negative electrons in the
sodium ion.
The charge on the chloride ion Cl- is -1 units (shown as just -) because
there is one more negative electron than there are positive protons in the
chloride ion.
Task: Use the example given above to help you, draw diagrams to
represent the following ionic compounds
Example 2: A Group 2 metal + a Group 7 non-metal e.g.
magnesium + chlorine ==> magnesium chloride
IGCSE CHEMISTRY
Example 3: A Group 3 metal + a Group 7 non-metal .
aluminium + fluorine ==> aluminium fluoride AlF3,
Example 4: A Group 1 metal + a Group 6 non-metal
potassium + oxygen ==> potassium oxide
Example 5: A Group 2 metal + a Group 6 non-metal
calcium + oxygen ==> calcium oxide
IGCSE CHEMISTRY
Example 6: A Group 3 metal + a Group 6 non-metal e.g. aluminium +
oxygen ==> aluminium oxide Al2O3 or ionic formula (Al3+)2(O2-)3 In terms
of electron arrangement, two aluminium atoms donate their three outer
electrons to three oxygen atoms. This results in two triple positive
aluminium ions to three double negative oxide ions. All the ions have the
stable electronic structure of neon 2.8. Valencies, Al 3 and O 2.
2Al (2.8.3) + 3O (2.6) ==> 2Al3+ (2.8) 3O2- (2.8)
can be summarised electronically as 2[2,8,3] + 3[2,6] ==> [2,8]3+2 [2,8]2-3
TWO
combine with THREE
to form
Note:
The charge on the aluminium ion Al3+ is +3 units (shown as 3+) because
there are three more positive protons than there are negative electrons
in the aluminium ion.
The charge on the oxide ion O2- is -2 units (shown as 2-) because there
are two more negative electrons than there are positive protons in the
oxide ion.
IGCSE CHEMISTRY
The properties of Ionic Compounds
 The diagram on the right is typical
of the giant ionic crystal structure
of
ionic
compounds
like
sodium
chloride and magnesium oxide.
 The
alternate
positive
and
negative ions in an ionic solid are
arranged in an orderly way in a giant
ionic lattice structure shown on the
left.
 The ionic bond is the strong electrical attraction between the
positive and negative ions next to each other in the lattice.
 The bonding extends throughout the crystal in all directions.
Melting and Boiling Point






Salts and metal oxides are typical ionic compounds.
This strong bonding force makes the structure hard (if brittle) and
have high melting and boiling points, so they are not very volatile!
A relatively large amount of energy is needed to melt or boil ionic
compounds..
The bigger the charges on the ions the stronger the bonding
attraction e.g. magnesium oxide Mg2+O2- has a higher melting point
than sodium chloride Na+Cl-.
Unlike covalent molecules, ALL ionic compounds are crystalline
solids at room temperature.
They are hard but brittle, when stressed the bonds are broken
along planes of ions which shear away. They are NOT malleable like
metals
IGCSE CHEMISTRY
Solubility

Many ionic compounds are soluble in water, but not all, so don't
make assumptions. Salts can dissolve in water because the ions can
separate and become surrounded by water molecules which weakly
bond to the ions. This reduces the attractive forces between the
ions, preventing the crystal structure to exist. Evaporating the
water from a salt solution will eventually allow the ionic crystal
lattice to reform.
Electrical Conductivity

The solid crystals DO NOT conduct electricity because the ions
are not free to move to carry an electric current. However, if the
ionic compound is melted or dissolved in water, the liquid will now
conduct electricity, as the ion particles are now free.
Task- Summaries the properties of an ionic substance in the table below,
give brief reasons why they have each propertie.
PROPERTY
.
DUE TO
IGCSE CHEMISTRY
Past Paper Style Question
(a)
of:
Use the Periodic Table to write down the electronic arrangement
(i) calcium
……………………………………………………………………………………………………………………………[1]
(ii) chlorine………………………………………………………………………………………………………………………………………(1)
(b)
If calcium is heated and put into a gas jar of chlorine a violent
reaction takes place and solid calcium chloride is formed. The
reaction proceeds due to the transfer of electrons.
(i) Use the electron arrangements and the idea of electron transfer
to explain how the reaction between atoms of calcium and chlorine
takes place.
(You may include a diagram in your answer)
………………………………………………………………………………………………………………………………………
………………………………………………………………………………………………………………………………………
………………………………………………………………………………………………………………………………………
………………………………………………………………………………………………………………………………………
………………………………………………………………………………………………………………………………………
[4]
(ii)
Write down the formulae of the ions that calcium and chlorine
form during the reaction.
IGCSE CHEMISTRY
.......................................................................................................................................................... [2]
(iii) Write down the formula of calcium chloride.
........................................................................................................................................................... [1]
(c)
Calcium chloride is a solid at room temperature and has a high
melting point.
Explain why calcium chloride has a high melting point.
................................................................................................................................................................
................................................................................................................................................................
.......................................................................................................................................................... [2]
References
Lesson Objectives:
IGCSE Chemistry pages 11-20
Unit 10: Structure and Bonding
Part 8: Covalent Bonding
Covalent Bonding - electron sharing in big or small molecules!
Covalent bonds are formed by atoms sharing electrons to form molecules.
This type of bond usually formed between two non-metallic elements.
The molecules might be that of an element i.e. one type of atom only OR
from different elements chemically combined to form a compound.
IGCSE CHEMISTRY
The covalent bonding is caused by the mutual electrical attraction
between the two positive nuclei of the two atoms of the bond, and the
negative electrons between them.
One single covalent bond is a sharing of 1 pair of electrons, two pairs of
shared electrons between the same two atoms gives a double bond and it
is possible for two atoms to share 3 pairs of electrons and give a triple
bond.
The bonding in Small Covalent Molecules
Example 1: two hydrogen atoms (1) form the molecule of the element
hydrogen H2
and
combine to form
where both atoms of helium
havef 2 (full) outer electrons around each atom's nucleus. Any covalent
bond is formed from the mutual attraction of two positive nuclei and
negative electrons between them. The nuclei would be a tiny dot in the
middle of where the H symbols are drawn! H valency is 1.
Task- Draw diagrams to represent the examples given below
Example 2: two chlorine atoms (2.8.7) form the molecule of the element
chlorine Cl2
Both atoms have a complete argon structure of 8 outer electrons around
each atom. All the other halogens would be similar e.g. F2, Br2 and I2 etc.
Valency of halogens like chlorine is 1 here.
Example 3: one atom of hydrogen (1) combines with one atom of chlorine
(2.8.7) to form the molecule of the compound hydrogen chloride HCl
IGCSE CHEMISTRY
In this molecule hydrogen is electronically like helium and chlorine like
argon. All the other hydrogen halides will be similar e.g. hydrogen fluoride
HF, hydrogen bromide HBr and hydrogen iodide HI.
Note: Hydrogen chloride gas is a true covalent substance consisting of
small HCl molecules. If the gas is dissolved in a hydrocarbon solvent like
hexane or methylbenzene it remains as HCl molecules and because there
are no ions present, the solution does not conduct electricity. However, if
hydrogen chloride gas is dissolved in water, things are very different and
the HCl molecules split into ions. Hydrochloric acid is formed which
consists of a solution of hydrogen ions (H+) and chloride ions (Cl-). The
solution then conducts electricity and passage of a d.c. current causes
electrolysis to take place forming hydrogen and chlorine.
Example 4: two atoms of hydrogen (1) combine with one atom of oxygen
(2.6) to form the molecule of the compound water H2O
The molecule can be shown as
with two hydrogen - oxygen
single covalent bonds (AS note: called a V or bent shape, the H-O-H bond
angle is 105o). Hydrogen sulphide will be similar, since sulphur (2.8.6) is in
the same Group 6 as oxygen. Valency of oxygen and sulphur is 2 here.
Example 5: three atoms of hydrogen (1) combine with one atom of
nitrogen (2.5) to form the molecule of the compound ammonia NH3
IGCSE CHEMISTRY
Molecule can be shown as
with three nitrogen - hydrogen
single covalent bonds (AS note: called a trigonal pyramid shape, the H-NH bond angle is 107o). PH3 will be similar since phosphorus (2.8.5) is in the
same Group 5 as nitrogen. Valency of nitrogen or phosphorus is 3 here.
Example 6: four atoms of hydrogen (1) combine with one atom of carbon
(2.4) to form the molecule of the compound methane CH4
Molecule can be shown as
with four carbon - hydrogen single
covalent bonds This is called a tetrahedral shape, the H-C-H bond angle
is 109o). SiH4 will be similar because silicon (2.8.4) is in the same group as
carbon.
Example 7: Two atoms of oxygen (2.6) combine to form the molecules of
the element oxygen O2.
The molecule has one O=O double covalent bond
. Oxygen valency 2.
Example 8: One atom of carbon (2.4) combines with two atoms of oxygen (2.6) to form the
compound carbon dioxide CO2.
IGCSE CHEMISTRY
The molecule can be shown as
with two carbon = oxygen
double covalent bonds (AS note: called a linear shape, the O=C=O bond
angle is 180o). Valencies of C and O are 4 and 2 respectively.
Example 9: Two atoms of carbon (2.4) combine with four atoms of
hydrogen (1) to form ethene C2H4.
The molecule can be shown as
with one carbon =
carbon double bond and four carbon - hydrogen single covalent bonds
(called a planar shape, its completely flat!, the H-C=C and H-C-H bond
angles are 120o). The valency of carbon is still 4.
Extension Exersise 10-13: Some more complex examples.. Ex. 10
nitrogen N2; Ex. 11 ethane C2H6; Ex. 12 chloromethane CH3Cl and Ex. 13
methanol CH3OH.
Typical properties of simple covalent substances - small molecules!
The electrical forces of attraction, that is the chemical bond,
between atoms in a molecule are usually very strong, so, most covalent
molecules do not change chemically on moderate heating.
e.g. although a covalent molecule like iodine, I2, is readily vapourised on
heating, it does NOT break up into iodine atoms I. The I-I covalent bond
is strong enough to withstand the heating and the purple vapour still
consists of the same I2 molecules as the dark coloured solid is made up
of.
So why the ease of vaporisation on heating?
IGCSE CHEMISTRY
The electrical attractive forces between individual molecules are weak, so
the bulk material is not very strong physically and there are also
consequences for the melting and boiling points.
These weak electrical attractions are known as intermolecular forces and
are readily weakened further on heating. The effect of absorbing heat
energy results in increased the thermal vibration of the molecules which
weakens the intermolecular forces. In liquids the increase in the average
particle kinetic energy makes it easier for molecules to overcome the
intermolecular forces and change into a gas or vapour. Consequently,
small covalent molecules tend to be volatile liquids with low boiling points,
so easily vapourised, or low melting point solids.
On heating the inter-molecular forces are easily overcome with the
increased kinetic energy of the particles giving the material a low melting
or boiling point and a relatively small amount of energy is needed to
effect these state changes.
This contrasts with the high melting points of giant covalent structures
(covered next lesson) with their strong 3D network.
Covalent structures are usually poor conductors of electricity because
there are no free electrons or ions in any state to carry electric charge.
Most small molecules will dissolve in some solvent to form a solution.
This again contrasts with giant covalent structures where the strong
bond network stops solvent molecules interacting with the particles
making up the material.
The properties of these simple small molecules should be compared and
contrasted with those molecules of a giant covalent nature (next section).
Task 2- Summaries the properties of small simple covalent compounds
below
Low melting
points
IGCSE CHEMISTRY
Gases
and
liquids with
low
boiling
points.
Electrical
conductivity
Solubility in
water
Task 3 – Additional questions page 67 IGCSE Chemistry. Questions 1,2
and 3
Lesson Objectives:
 Describe the formation of single covalent bonds in H2, Cl2, H2O, CH4 and
HCl as the sharing of pairs of electrons leading to the noble gas
configuration.
 Describe the electron arrangement in more complex covalent molecules
such as N2, C2H4, CH3OH and CO2.
IGCSE CHEMISTRY
Unit 10: Structure and Bonding
Part 9: Giant Covalent Structures
Covalent bonds act in a particular direction i.e. along the 'line' between
the two nuclei of the atoms bonded together in an individual bond, strong
structures can be formed, especially if the covalent bonds are arranged in
a strong three dimensional giant covalent lattice.
You have to be able to draw and describe The structure of the three
allotropes of carbon (diamond, graphite and fullerenes), silicon and silicon
dioxide (silica)
Diamond
Diamond is a form of ______in which each carbon atom is joined to four
other carbon atoms, forming a giant ______ structure, which is rigid.
A large amount of _______ is needed to break the many strong covalent
bonds
As a result, diamond is very _____and has a high _____ point.
It does not conduct electricity as it has no free electrons.
Graphite
Graphite is a form of carbon in which the carbon atoms form _______.
IGCSE CHEMISTRY
These ______ can slide over each other, so graphite is much ______
than diamond.
It is used in pencils, and as a_________. Each carbon atom in a layer is
joined to only ______ other carbon atoms.
Graphite conducts electricity as it has fee _________ between its
layers.
PLEASE NOTE: The layers are only held together by weak
intermolecular forces NOT by strong covalent bonds, This is why
graphite is soft.
Silica
Silica, which is found in sand, has a similar structure to diamond.
It is also hard and has a high melting point, but contains silicon and
oxygen atoms, instead of carbon atoms.
The fact that it is a semi-conductor makes it immensely useful in the
electronics industry: most transistors are made of silica.
Although it is a compound, it has a giant covalent structure with certain
similarities to diamond.
IGCSE CHEMISTRY
Buckminsterfullerene (NOT A GIANT COVALENT COMPOUND)
Structure of a buckminsterfullerene molecule - a large ball of 60 atoms
Buckminsterfullerene is yet another allotrope of carbon. It is actually not
a giant covalent structure, but a giant molecule in which the carbon atoms
form pentagons and hexagons - in a similar way to a leather football. It is
used in lubricants.
Task: Summarise the properties of diamond and graphite in the table
below.
PROPERTY
Appearance
Hardness
Conductivity
Melting point
Diamond
Graphite
IGCSE CHEMISTRY
Unit 10: Structure and Bonding
(a)
Part 10: Exam Style Questions
Oxygen and sulphur are in the same group of the periodic table.
Complete the table below to show the arrangement of electrons in oxygen and
sulphur atoms.
Atom
oxygen
sulphur
shell 1
shell 2
shell 3
[2]
(b) (i) Draw a diagram of an oxygen atom.
[2]
(ii) Use your diagram to describe two ways in which oxygen can achieve a full outside
shell of electrons.
..............................................................................................................................................................................
..............................................................................................................................................................................
......................................................................................................................................................................... [2]
(c)
Some of the oxygen in the atmosphere is in the form of ozone, O3. Scientists believe
that chlorine radicals are destroying ozone.
(i) What is the difference between a chlorine radical and a chlorine molecule?
..............................................................................................................................................................................
......................................................................................................................................................................... [2]
(ii) What substances are thought to be main sources of chlorine radicals in the
atmosphere?
......................................................................................................................................................................... [1]
TOTAL / 9
IGCSE CHEMISTRY
QUESTIONSHEET 2
High Demand Questions
(a)
There are eleven electrons in a sodium atom.
The amounts of energy required to remove each one have been measured and the
results shown
on a sketch graph.
180000
160000
140000
120000
Energy /kJmol-1 100000
80000
60000
40000
20000
0
1
2
3
4
5
6
7
8
9
10
11
Number of electrons
(i)
Why is the first electron in sodium the easiest to remove?
..............................................................................................................................................................................
......................................................................................................................................................................... [2]
(ii) Why do the energies for electrons 2 to 9 go up by relatively small amounts?
......................................................................................................................................................................... [1]
(iii) Why are the last two energies very high?
......................................................................................................................................................................... [1]
(b)
Draw a sketch graph to show the energies needed to remove the first four electrons in
aluminium.
Explain your answer.
......................................................................................................................................................................... [3]
TOTAL / 7
IGCSE CHEMISTRY
High Demand Questions
(a) (i)
QUESTIONSHEET 3
Explain what is meant by the numbers in the symbol
40
20
Ca .
..............................................................................................................................................................................
......................................................................................................................................................................... [2]
40
20
(ii) Use the numbers to state the number of protons, neutrons and electrons found in
Ca
protons
electrons
neutrons
[3]
(b)
Complete the table below to show the numbers of protons, neutrons and electrons
found in the ions shown.
ion
H
protons
neutrons
electrons
1
1
9
4
Be 2
56
26
Fe 3
127
53
79
34
I
Se 2
[8]
TOTAL / 13
IGCSE CHEMISTRY
High Demand Questions
Neon has two main isotopes,
(a)
20
10
Ne and
22
10
Ne
Draw diagrams of the atoms of each of the neon isotopes.
[3]
(b) Neon gas is made up of 90% of neon-20 and 10% of neon-22.
Calculate the exact relative atomic mass of neon.
..............................................................................................................................................................................
..............................................................................................................................................................................
..............................................................................................................................................................................
......................................................................................................................................................................... [3]
(c) How do the chemical properties of each isotope of neon compare with each other?
Explain your answer.
..............................................................................................................................................................................
..............................................................................................................................................................................
..............................................................................................................................................................................
......................................................................................................................................................................... [3]
TOTAL / 9
IGCSE CHEMISTRY
High Demand Questions
QUESTIONSHEET 5
The diagram below shows the electronic arrangement of an unknown substance.
(a) (i)
How many protons would there be if the unknown substance was a neutral atom?
......................................................................................................................................................................... [1]
(ii) What would the diagram represent if it was a neutral atom?
......................................................................................................................................................................... [1]
(b)(i) How many protons would there be if the substance was a 2+ ion?
......................................................................................................................................................................... [1]
(ii) What would the diagram represent if it was a 2+ ion?
......................................................................................................................................................................... [1]
(c) (i) In what group of the periodic table would the unknown substance be if the
represented a 1+ ion?
......................................................................................................................................................................... [1]
(ii) How many neutrons would the 1+ ion have if its relative atomic mass was 39?
......................................................................................................................................................................... [1]
(d) (i) In what group of the periodic table would the unknown substance be if it represented
a 1- ion?
......................................................................................................................................................................... [1]
(ii) What would the relative atomic mass of the 1- ion be if it had 20 neutrons?
......................................................................................................................................................................... [1]
TOTAL / 8
IGCSE CHEMISTRY
QUESTIONSHEET 6
High Demand Questions
An unknown element is found to exist as the following two isotopes:
24
13
(a)
28
13
X
X
Complete the following table:
number of protons
24
13
X
28
13
X
number of neutrons
number of electrons
13
13
15
[2]
(b)
Describe what is meant by the term ‘isotopes’.
..............................................................................................................................................................................
..............................................................................................................................................................................
......................................................................................................................................................................... [2]
(c)
In experiments, both isotopes were found to show the same chemical behaviour.
Use the information in the table to explain why the chemical behaviour of both isotopes
is the same.
..............................................................................................................................................................................
..............................................................................................................................................................................
......................................................................................................................................................................... [2]
(d)
A naturally occurring sample was found to contain 25% of X-24 and 75% of X-28.
Calculate the relative atomic mass of the element X.
..............................................................................................................................................................................
..............................................................................................................................................................................
......................................................................................................................................................................... [3]
(e)
Use your periodic table to identify X.
......................................................................................................................................................................... [1]
TOTAL / 9
IGCSE CHEMISTRY
QUESTIONSHEET 7
High Demand Questions
The diagrams below represent the electronic arrangement of different atoms and ions.
(a)
A
B
C
D
E
F
Which letter represents an atom of a Group I metal?
......................................................................................................................................................................... [1]
(b)
Which letter represents an atom of a Group VII non-metal?
......................................................................................................................................................................... [1]
(c)
Which letter represents an atom of a noble or inert gas?
......................................................................................................................................................................... [1]
(d) Which letter represents an atom which is not in the second period of the Periodic
Table?
......................................................................................................................................................................... [1]
(e)
Which letter represents an ion of a Group I metal?
......................................................................................................................................................................... [1]
(f)
Which two letters represent atoms which form an ionic compound with a formula
similar to Na2S?
......................................................................................................................................................................... [1]
(g) Which two letters represent atoms which form a molecule with a formula similar to
SiCl4?
......................................................................................................................................................................... [1]
TOTAL / 7
IGCSE CHEMISTRY
High Demand Questions
(a)
QUESTIONSHEET 8
Use the Periodic Table to write down the electronic arrangement of:
(i) calcium
......................................................................................................................................................................... [1]
(ii) chlorine
......................................................................................................................................................................... [1]
(b)
If calcium is heated and put into a gas jar of chlorine a violent reaction takes place
and solid calcium chloride is formed. The reaction proceeds due to the transfer of
electrons.
(i)
Use the electron arrangements and the idea of electron transfer to explain how the
reaction between atoms of calcium and chlorine takes place.
(You may include a diagram in your answer)
..............................................................................................................................................................................
..............................................................................................................................................................................
..............................................................................................................................................................................
......................................................................................................................................................................... [4]
(ii) Write down the formulae of the ions that calcium and chlorine form during the
reaction.
......................................................................................................................................................................... [2]
(iii)
Write down the formula of calcium chloride.
......................................................................................................................................................................... [1]
(c)
Calcium chloride is a solid at room temperature and has a high melting point.
Explain why calcium chloride has a high melting point.
..............................................................................................................................................................................
..............................................................................................................................................................................
......................................................................................................................................................................... [2]
IGCSE CHEMISTRY
TOTAL / 11
Explain why graphite conducts electricity.
..............................................................................................................................................................................
......................................................................................................................................................................... [2]
TOTAL / 7
IGCSE CHEMISTRY
Key Words
atom – The smallest particle that can exist on its own.
atomic number – The number of protons in the nucleus of an atom, also
known as the proton number.
electron – Negatively charged particle that orbits the nucleus of an atom.
element – A substance made up of only one type of atom.
isotopes – Different atoms of the same element. They have the same
number of protons and electrons, but a different number of neutrons.
atom – The smallest particle that can exist on its own.
atomic number – The number of protons in the nucleus of an atom, also
known as the proton number.
electron – Negatively charged particle that orbits the nucleus of an atom.
element – A substance made up of only one type of atom.
isotopes – Different atoms of the same element. They have the same
number of protons and electrons, but a different number of neutrons.