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1.10 Atomic structure
What is an atom made of?
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Key terms
Element – A substance which
contains just one type of atom.
An image showing atoms on the surface of a metal, produced by a method called scanning
tunnelling microscopy.
About 200 years ago, when scientists first realised that all matter was made up of atoms,
they thought that atoms were the smallest particles. But now we know that atoms are
made up of even simpler sub-atomic particles – protons, neutrons and electrons.
D
Atomic number – The number
of protons in the nucleus of an
atom.
All matter is made up of particles called atoms. Atoms are very, very small. It is not
possible to see an atom even under the most powerful light microscope. However,
scientists and engineers have developed ways of producing images of atoms using
other methods.
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All matter in the Universe
is made up of atoms. But
how much do we know
about atoms? Discuss with
a partner whether these
statements are true or
false:
• Atoms cannot be split up
into anything smaller.
• Atoms are solid objects
and nothing can pass
through them.
• There are millions of
different kinds of atoms.
• No-one has ever seen an
atom.
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Get started
Mass number – The total
number of protons and
neutrons in the nucleus of an
atom.
At the centre of an atom is a positive nucleus. If an atom were the size of The O2
arena, the nucleus would be the size of a ping-pong ball.
The nucleus is surrounded by electrons, arranged into layers or shells. Sometimes
these electron shells are called energy levels.
nucleus, containing
protons and neutrons
Did you know?
Because the nucleus is so
small, most of the atom is
actually empty space.
When scientists tried firing
sub-atomic particles at a thin
gold film, they found that most
of them passed straight through.
Only a tiny number of the
sub-atomic particles hit the
nuclei of the gold atoms and
bounced back.
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p
n
n
p
n
electron shells,
containing electrons
p
n
Key
n
neutron
p
proton
electron
An atom of lithium contains a nucleus surrounded by electron shells (energy levels).
BTEC First Principles of Applied Science
Principles of Science
UNIT 1
The sub-atomic particles
Each of the sub-atomic particles has different properties, as shown in this table.
Particle
Relative charge
Relative mass
Where in the atom is it
found?
Proton
+1
1
In the nucleus
Neutron
0
1
In the nucleus
Electron
−1
Almost 0 (actually
0.0005)
In shells surrounding the
nucleus
Link
Lesson 1.12 discusses the
arrangement of elements in
the periodic table.
Atomic number and mass number
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So far scientists have identified 118 different types of
atom. This means there are 118 different elements.
mass number
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Li
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Each element has a fixed number of protons in the
nucleus. For example, all atoms of lithium have 3
protons in the nucleus. This is the atomic number for
lithium. There are no other elements with an atomic
number of 3.
Activity A
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atomic number
The nuclear symbol for the lithium
atom shows you the atomic
number and mass number of
the atom.
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The mass number of an atom is the total number of
protons and neutrons in the nucleus. Lithium has 4
neutrons, so its mass numbner is 7. The nuclear symbol
for an atom is its chemical symbol with the atomic
number and mass number.
Worked example
You can use the nuclear symbol for an atom to work out the number of protons
and neutrons in the nucleus. If a phosphorus atom has a nuclear symbol 31
15P, how
many protons and neutrons are there in its nucleus?
1 Using the nuclear symbols
shown for these atoms,
write down the numbers of
protons and neutrons in the
nucleus of each atom.
16
 8 O
23
11 Na
2 An atom of potassium
(symbol K) has 19 protons
and 20 neutrons. Write
down the nuclear symbol
for this atom.
Take it further
Neutrons have an important
role to play in the nucleus of
an atom. They prevent the
positive protons from repelling
each other and blowing the
nucleus apart.
Step 1 The atomic number is 15. This is the number of protons in the nucleus.
Step 2 The mass number is 31. This is the total number of protons and neutrons
in the nucleus.
Step 3 So, the number of neutrons 5 31 2 15 5 16.
Just checking
1 Copy and complete this sentence about the structure of atoms.
Atoms contain a nucleus with a ___________ charge, surrounded by
___________ in shells. The size of the nucleus is much __________ than the
overall size of the atom.
Lesson outcomes
You should know about:
• the structure of an atom
• the relative size of the
nucleus
• that atoms of a given
element have the same
number of protons in the
nucleus
• the meaning of the terms
‘atomic number’ and ‘mass
number’.
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1.11 Isotopes and relative atomic mass
Key terms
Two kinds of carbon atoms?
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Isotopes – Atoms of the
same element which have the
same number of protons but
different numbers of neutrons.
Almost all of the carbon atoms found in living things and in the rocks, seas and
atmosphere of our planet have the nuclear symbol 12
 6 C. This is called carbon-12. It is so
stable that the nuclei of these carbon atoms have not changed since the Solar System
was formed over 4 billion years ago.
Relative atomic mass (of
an element) – The mass of
an atom of that element
compared to a standard mass.
A tiny fraction of the carbon atoms on Earth have the nuclear symbol 14
 6 C. This is
known as carbon-14. The nuclei of carbon-14 atoms have two extra neutrons when
compared with carbon-12, so the two atoms have different mass numbers.
The two different carbon atoms are called isotopes of the element carbon. There is
also a third isotope of carbon with a mass number of 13. It has the symbol 13
 6 C.
Did you know?
Lesson 1.10 explains how
to work out the number of
protons and neutrons from the
nuclear symbol.
Carbon-14 is radioactive. This means that when a living organism dies, the
carbon-14 atoms it contains gradually decay into more stable atoms. Scientists
can use this fact to find the age of the remains of long-dead organisms. This is
called carbon dating.
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Write down the number of
protons and neutrons in the
following atoms:
• carbon-12
• carbon-14
Ötzi the ‘Iceman’ was found in a glacier in the Alps. Archaeologists used the
amount of carbon-14 remaining in his body to calculate that he died about 5000
years ago.
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Activity A
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Link
Take it further
Relative atomic mass is a way
of comparing the mass of
an atom to a standard mass.
In the past, all masses were
compared to a hydrogen
atom – but now they are all
compared to 1/12 of the mass
of a carbon-12 atom.
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Relative atomic mass
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Only a tiny fraction of carbon atoms have a mass number of 13, or of 14. All the rest
have a mass number of 12. So, the average mass of a carbon atom is 12.01. This is
called the relative atomic mass of carbon.
BTEC First Principles of Applied Science
UNIT 1
Principles of Science
Isotopes and relative atomic mass
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The most common isotope of chlorine is 35
17 Cl. But about a quarter of all chlorine atoms
37
are a different isotope: 17 Cl. Because this is quite a high proportion, the relative
atomic mass is not a whole number – it actually works out to be 35.5.
Cl
37
Cl
35
Cl
35
35
Cl
35
35
Cl
37
Cl
35
Cl
Cl
Exam tip
You may be required to use
data about the abundance
of isotopes to predict which
elements will have relative
atomic masses that are not
whole numbers.
You also need to be able to
calculate the relative atomic
mass from these data.
About three-quarters of all chlorine atoms are chlorine-35 isotopes. The rest are chlorine-37.
There are many other elements like chlorine, where the relative atomic mass is not a whole
number because of the presence of different isotopes. The table shows three examples.
Relative atomic mass
Lithium
6.9
Magnesium
24.3
Copper
63.5
Isotopes present and
percentage abundance
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Element
6
3 Li
7
3 Li
(7.6%)
(92.4%)
(79%)
(10%)
(11%)
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12 Mg
25
12 Mg
26
12 Mg
(70%)
(30%)
D
63
29 Cu
65
29 Cu
Worked example
Activity B
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Boron has two isotopes: 10
 5 B (20%) and  5 B (80%). What is the relative atomic mass
of boron?
Step 1 First, multiply the mass number of each isotope by its percentage
abundance.
Step 2 Then, add all these values together.
Step 3 Finally, divide by 100 to get the average.
The element thallium has two
isotopes:
203
 81 TI
205
(30%)      81 TI (70%)
Use this information to
calculate the relative atomic
mass of thallium.
So, for boron, the relative atomic mass 5 (10 3 20) 1 (11 3 80) 5 1080 5 10.8
100
100
Just checking
1 Look at these nuclear symbols for four atoms, A–D:
14
16
15
14
 7 A
 8 B
 7 C
 6 D
(a) How many of these atoms have eight neutrons in their nucleus?
(b) Which two of these atoms are isotopes of each other? Explain your answer.
2 Bromine has two isotopes: 50% of bromine atoms have a mass number of 81
and 50% of bromine atoms have a mass number of 79. What is the relative
atomic mass of bromine? Explain your answer.
Lesson outcomes
You should know about:
• relative atomic mass and
isotopes
• how to calculate relative
atomic mass from
percentage abundances.
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