AQA GCSE Physics 2-7
Nuclear Physics
GCSE Physics pages 198 to 209
March 25th 2011
AQA GCSE Specification
NUCLEAR DECAY
12.9 What happens to radioactive substances
when they decay?
Using skills, knowledge and understanding of how science works:
• to explain how the Rutherford and Marsden scattering experiment
led to the .plum pudding. model of the atom being replaced by the
nuclear model.
Skills, knowledge and understanding of how science works set in the
context of:
• The relative masses and relative electric charges of protons,
neutrons and electrons.
• In an atom the number of electrons is equal to the number of
protons in the nucleus. The atom has no net electrical charge.
• Atoms may lose or gain electrons to form charged particles called
ions.
• All atoms of a particular element have the same number of
protons.
• Atoms of different elements have different numbers of protons.
• Atoms of the same element which have different numbers of
neutrons are called isotopes.
• The total number of protons and neutrons in an atom is called its
mass number.
• The effect of alpha and beta decay on radioactive nuclei.
• The origins of background radiation.
NUCLEAR FISSION & FUSION
12.10 What are nuclear fission and nuclear fusion?
Using skills, knowledge and understanding of how science works:
• to sketch a labelled diagram to illustrate how a chain reaction may
occur.
Skills, knowledge and understanding of how science works set in the
context of:
• There are two fissionable substances in common use in nuclear
reactors, uranium 235 and plutonium 239.
• Nuclear fission is the splitting of an atomic nucleus.
• For fission to occur the uranium 235 or plutonium 239 nucleus
must first absorb a neutron.
• The nucleus undergoing fission splits into two smaller nuclei and 2
or 3 neutrons and energy is released.
• The neutrons may go on to start a chain reaction.
• Nuclear fusion is the joining of two atomic nuclei to form a larger
one.
• Nuclear fusion is the process by which energy is released in stars.
Atomic structure (Revision of Module 1)
An atom consists of a small
central nucleus composed
of protons and neutrons
surrounded by electrons.
An atom will always have
the same number of
electrons as protons.
A Lithium atom
protons
neutrons
electrons
Atomic and mass number
The atomic number (Z)
of an atom is equal to
the number of protons
in its nucleus.
The mass number (A)
of an atom is equal to
the number of protons
plus neutrons in its
nucleus.
protons = 3
neutrons = 4
electrons = 3
This Lithium atom has:
atomic number = 3
mass number = 7
Properties of protons, neutrons and electrons
Position in
the atom
Relative
mass
Relative
electric
charge
PROTON
nucleus
1
+1
NEUTRON
nucleus
1
0
ELECTRON
outside
nucleus
0.005
-1
Ions
An atom becomes an ion when it loses or gains
one or more electrons.
Lithium atom
electrons
Positive
Lithium ion
protons
Negative
Lithium ion
neutrons
Isotopes (Revision)
The atoms of an element always have the same number
of protons.
Isotopes are atoms of the same element with different
numbers of neutrons.
The three isotopes of hydrogen
hydrogen 1
hydrogen 2
(deuterium)
neutrons
hydrogen 3
(tritium)
Note: The number after ‘hydrogen’ is the mass number of the isotope.
Nuclear notation
An isotope of carbon consists of 6 protons and 8 neutrons.
This can be written as:
carbon 14
Number of protons
PLUS neutrons
(Mass number)
OR:
Number of protons
(Atomic number)
14
C
6
Chemical
symbol
Question 1
An isotope of uranium (chemical symbol U)
consists of 92 protons and 143 neutrons. Give
the two different ways of notating this isotope.
The mass number of the Uranium isotope:
= 92 + 143 = 235
uranium 235
AND
235
U
92
Question 2
Determine the number of protons and neutrons in
the isotopes notated below:
(a) 13
7
(c) 197
79
N
protons = 7
neutrons = 6
Au
p = 79
n = 118
(b) 60
27
Co
(d) 239
94
Pu
p = 27
n = 33
p = 94
n = 145
Note: Apart from the smallest atoms, most nuclei
have more neutrons than protons.
Alpha decay
Alpha particles consist of two protons plus two
neutrons.
They are emitted by some of the isotopes of the
heaviest elements.
Example: The decay of Uranium 238
238
92
U
234
90
Th +
4
2
α
Uranium 238 decays to Thorium 234 plus an alpha particle.
Notes:
1. The mass and atomic numbers must balance on each side
of the equation: (238 = 234 + 4 AND 92 = 90 +2)
2. The alpha particle can also be notated as:
4
2
He
Question
Show the equation for Plutonium 239 (Pu)
decaying by alpha emission to Uranium (atomic
number 92).
239
94
Pu
235
92
U
+
4
2
α
Beta decay
Beta particles consist of
high speed electrons.
They are emitted by
isotopes that have too many
neutrons.
One of these neutrons
decays into a proton and an
electron. The proton
remains in the nucleus but
the electron is emitted as
the beta particle.
Example: The decay of Carbon 14
14
6
C
14
7
N
+
0
-1
-
β
Carbon 14 decays to Nitrogen 14 plus a beta particle.
Notes:
1. The beta particle, being negatively charged, has an
effective atomic number of minus one.
2. The beta particle can also be notated as:
0
-1
e
Question
Show the equation for Sodium 25 (Na), atomic
number 11, decaying by beta emission to
Magnesium (Mg).
25
11
25
Na
12
Mg +
0
-1
-
β
Gamma decay
Gamma decay is the emission of electromagnetic radiation
from an unstable nucleus
Gamma radiation often occurs after a nucleus has emitted
an alpha or beta particle.
Example: Cobalt 90
90
27
Co
90
27
Co +
0
γ
0
Cobalt 90 with excess ENERGY decays to
Cobalt 90 with less ENERGY plus gamma radiation.
Changing elements
Both alpha and beta decay cause the an isotope to change
atomic number and therefore element. Alpha decay also
causes a change in mass number.
Decay type
Atomic number
Mass number
alpha
DOWN by 2
DOWN by 4
beta
UP by 1
NO CHANGE
gamma
NO CHANGE
NO CHANGE
Complete the decay equations below:
(a)
59
26
59
Fe
224
(b)
88
(c)
Ra
16
7
27
220
86
16
N
Co +
8
0
-1
Rn +
O +
0
-1
-
β
4
α
2
-
β
Write equations showing how Lead 202 could
decay into Gold. (This cannot happen in reality!)
Element Sym
Z
Platinum
Pt
78
Gold
Au
79
Mercury
Hg
80
202
198
4
Hg +
Pb
82
80
2
198
194
4
Hg
Pt
80
78
194
Thallium
Tl
81
Lead
Pb
82
194
Bismuth
Bi
83
78
Pt
α
2
0
Au
79
+
α
β
+
-
-1
There are other correct solutions
Background radiation
Background radiation is ionising
radiation from space (cosmic
rays), devices such as X-ray
tubes, and from radioactive
isotopes in the environment (for
example radon gas from rocks in
the ground).
Most of this radiation occurs
naturally but a small amount is
due to nuclear weapon testing
and nuclear power stations.
Background
radiation
pie-chart
Choose appropriate words to fill in the gaps below:
When an unstable nucleus emits an alpha particle its atomic
two
four
number falls by _______
and its mass number by ______.
neutrons
Beta particles are emitted by nuclei with too many ________.
one
In this case the atomic number increases by ______
while the
mass
________
number remains unchanged.
Background radiation is mainly due to natural sources of
ionising radiation such as from ________
radon
_________
gas that seeps
out from rocks in the ground.
WORD SELECTION:
four one ionising two neutrons mass radon
Nuclear reactions
Notes questions from pages 198 & 199
1.
2.
3.
4.
5.
6.
7.
8.
9.
10.
Copy the table near the top of page 198.
Define what is meant by: (a) an ion; (b) atomic number; (c) mass
number; (d) isotopes.
Copy all parts of Figure 1 on page 198.
Copy and answer question (a) on page 198.
By giving an example equation in each case describe the
processes of: (a) alpha decay and (b) beta decay.
Compare gamma emission with alpha and beta decay.
Copy and answer questions (b) and (c) on page 199.
What is background radiation? Copy the pie chart on page 199.
Copy the ‘Key points’ table on page 199.
Answer the summary questions on page 199.
Nuclear reactions
ANSWERS
In text questions:
(a) 92p, 143n
(b) 23890Th = 90p + 138n;
224 Ra = 88p + 136n
88
(c) 4019K = 19p + 21n;
40 Ca = 20p +20n
20
Summary
questions:
1. (a) 6p + 6n
(b) 27p + 33n
(c) 92p + 143n
2. (a) 92p + 146n
(b) 90p + 144n
(c) 91p + 143n
The Plum Pudding Atomic Model
Before about 1910
many scientists
believed that an atom
consisted of:
Positively charged
matter spread out like a
pudding embedded by
negatively charged
electrons (like plums in
a pudding).
The ‘Plum Pudding’ Model
Rutherford’s Atomic Model
In 1909 Ernest Rutherford suggested that an atom
consists of a a tiny positively charged nucleus
surrounded by negatively charged electrons.
Lord Rutherford
1871 - 1937
Geiger & Marsden’s alpha particle
scattering experiment
In 1909 Hans Geiger
and Ernest Marsden
performed an
experiment using alpha
particles to determine
which of the two
models was the better
in describing the
structure of an atom.
Geiger and Marsden
The apparatus
2
5
4
3
1
What was observed
alpha
source
thin metal foil
1. Virtually all of the alpha particles went straight through the metal foil.
2. A few alpha particles were deflected through a small angle.
3. About 1 in 10 000 were deflected backwards.
How the results can be explained
atom
1. Deflections occur because there is
a force between the charged
nucleus and the positively charged
alpha particles.
2. Most of the alpha particles do not
go near enough to the nucleus to
be deflected.
3. Backwards deflections occur when
the alpha particles make near
head on collisions with the
positively charged nucleus.
nucleus (highly enlarged)
How their results supported
Rutherford’s atomic model
1. The relatively small number of
deflections indicates that most of
the atom is empty space with only
a very small nucleus.
2. The backward deflections can
only occur if the nucleus is
positively charged and contains
most of the atom’s mass.
3. The ‘plum pudding’ model would
not produce backward
deflections.
Choose appropriate words to fill in the gaps below:
Rutherford an atom consists of a tiny,
According to __________
positively
nucleus
___________
charged __________
surrounded by a cloud of
________
negative electrons. The nucleus also contains most of the
______
mass of an atom.
alpha particle scattering
This model was supported by the ______
experiment in 1909. In this experiment most alpha particles
straight through a thin metal foil with only about 1
passed ________
backwards
in 10000 being deflected _________.
WORD SELECTION:
Rutherford mass backwards negative
straight positively alpha nucleus
The discovery of the nucleus
Notes questions from pages 200 & 201
1.
2.
3.
4.
5.
(a) With a copy made of Figure 1 describe the
experiment performed by Hans Geiger and Ernest
Marsden that supported Rutherford’s model of the
atom. (b) What were the results of the above
experiment? (c) Explain how these results supported
Rutherford’s model. Draw Figure 2 on page 201 as part
of your answer.
What was the plum pudding model?
Copy and answer questions (a) and (b) on pages 200
and 201.
Copy the Key Points on page 201.
Answer the summary questions on page 201.
The discovery of the nucleus
ANSWERS
In text questions:
(a) It had to hit
something much
heavier.
(b) Rutherford’s
model would
have been
incorrect.
Summary questions:
1. (a) Charge
(b) Diameter
(c) Mass
2. (a) Path B
(b) A is wrong because it is
attracted by the nucleus; C is
wrong because it is unaffected
by the nucleus; D is wrong
because it is repelled by the
nucleus through too great an
angle.
Nuclear fission
Nuclear fission is the splitting
of an atomic nucleus.
Nuclear fission can be used as
an energy source in a nuclear
reactor.
There are two fissionable
substances in common use in
nuclear reactors, uranium 235
and plutonium 239.
Chain reaction
The fission of a nucleus of
Uranium 235 can be initiated
by a neutron.
When this nucleus splits
further neutrons are
produced.
neutron
These neutrons in turn can
cause more nuclei to split.
An avalanche effect, called a
‘chain reaction’ can then
occur.
A chain reaction
Nuclear fission reactor
5
2
6
1
3&4
7
Nuclear reactor parts
1. Fuel rods
These contain U235 or Pu239. They
become very hot due to nuclear
fission.
4. Coolant
This transfers the heat energy of the
fuel rods to the heat exchanger.
Coolant be water, carbon dioxide gas
or liquid sodium.
2. Control rods
Made of boron, when placed inbetween the fuel rods these absorb
neutrons and so reduce the rate of
fission. Their depth is adjusted to
maintain a constant rate of fission.
5. Heat exchanger
Here water is converted into high
pressure steam using the heat energy
of the coolant.
3. Moderator
This surrounds the fuel rods and
slows neutrons down to make further
fission more likely. The moderator
can be water or graphite.
6. Reactor core
This is a thick steal vessel designed to
withstand the very high pressure and
temperature in the core.
7. Concrete shield
This absorbs the radiation coming
from the nuclear reactor.
Choose appropriate words to fill in the gaps below:
splitting up of the nucleus of an atom
Nuclear fission is the _________
energy and neutrons are also
into two smaller nuclei. ________
usually emitted.
reactors use Uranium _____
235 or Plutonium _____to
239
Nuclear ________
fission
produce energy by nuclear ________.
A controlled chain
control rods which
reaction is maintained by the use of _______
neutrons produced.
absorb some of the _________
atomic bomb is the consequence of an uncontrolled
An _______
chain reaction.
WORD SELECTION:
reactors energy 239 atomic splitting
neutrons 235 fission control
Nuclear fission
Notes questions from pages 202 & 203
1.
2.
3.
4.
5.
6.
7.
8.
9.
Define what is meant by nuclear fission.
With aid of a copy of Figure 1, explain what is meant by a chain
reaction.
Explain the purpose of fission neutrons in a chain reaction.
Copy and answer question (a) on page 202.
What isotopes are used for fission reactions?
Copy Figure 3 on page 203 explain how a nuclear fission reactor
produces steam to drive the turbines of a power station. Your
account should include the purpose of the moderator, control rods
and coolant.
Copy and answer question (b) on page 203.
Copy the Key Points on page 203.
Answer the summary questions on page 203.
Nuclear fission
ANSWERS
In text questions:
(a) The chain reaction
would go out of
control and the
reactor would
explode.
(b) The chain reaction
would go out of
control and the
reactor would
explode.
Summary questions:
1. (a) Nucleus,
uranium-235,
plutonium-239.
(b) Uranium-238,
plutonium-239.
2. 1A, 2C, 3B, 4A
Nuclear fusion
Nuclear fusion is the joining of two atomic
nuclei to form a larger one.
Hydrogen nuclei undergo fusion in stars to
make helium nuclei
Energy from fusion
Nuclear fusion is the
process by which
energy is released in
the Sun and other stars.
It is also the energy
source of the hydrogen
bomb.
Nuclear fusion reactors
Scientists are currently
working to make nuclear
fusion reactors.
The fuel for fusion reactors
is the isotope hydrogen 2
(deuterium) which is found
in sea water.
An experimental fusion reactor in
Seatle USA
Nuclear fusion
Notes questions from pages 204 & 205
1.
2.
3.
4.
5.
6.
Explain what is meant by nuclear fusion.
Outline the fusion reactions that take place inside the
Sun.
What would be the advantages of using a nuclear
fusion reactor over other methods of generating
electricity?
Copy and answer questions (a) and (b) on pages 204
and 205.
Copy the Key Points on page 205.
Answer the summary questions on page 205.
Nuclear fusion
ANSWERS
In text questions:
(a) Helium-3 nucleus
(b) It goes out of
control, the plasma
would touch the
walls and go cold.
Summary questions:
1. (a) Small, larger (b) Stable
2. (a) So the nuclei have enough
kinetic energy to overcome
the force of repulsion between
them and fuse.
(b) The energy output would
be less than the energy input
so it would not produce any
energy overall.
Virtual Physics Laboratory Simulations
NOTE: Links work only in school
Alpha Scattering.exe
Geiger Muller Tube
Nuclear Stability With stability curve
Nuclear Reactions Fission & Fusion with binding
energy
Online Simulations
Atoms, ions & isotopes (GCSE) - Powerpoint
presentation by KT
Build an atom - eChalk
Atomic Structure Quiz - by KT - Microsoft WORD
Hidden Pairs Game on Atomic Structure - by KT Microsoft WORD
Fifty-Fifty Game on What particles are positive - by KT Microsoft WORD
Rutherford Scattering - PhET - How did Rutherford figure
out the structure of the atomic nucleus without looking at
it? Simulate the famous experiment in which he
disproved the Plum Pudding model of the atom by
observing alpha particles bouncing off atoms and
determining that they must have a small core.
Rutherford Scattering Experiment Thomson Model of
evenly distributed charge and Nuclear Model - Michael
Fowler
Types of Radiation - S-Cool section on types of
radiations including an animation of absorption and a
couple of decay equations to fill in on screen.
Alpha Decay - PhET - Watch alpha particles escape from
a Polonium nucleus, causing radioactive alpha decay.
See how random decay times relate to the half life.
Beta Decay - PhET - Watch beta decay occur for a
collection of nuclei or for an individual nucleus
Decay series - Fendt
Andy Darvill's Radioactivity Pages
Understanding Radiation - National Radiological
Protection Board - Useful starting point to get at useful
areas of the site.
Nuclear Fission - PhET - Start a chain reaction, or
introduce non-radioactive isotopes to prevent one.
Control energy production in a nuclear reactor!
Nuclear Fission - Powerpoint presentation by Richard
Miller of 5SJW (2005)
Nuclear Fission - Powerpoint presentation that includes a
link to the 'mousetrap' demonstration
Power Station Animation - eChalk
Managing a Nuclear Power Plant Simulation - by Henrik
Eriksson
BBC AQA GCSE Bitesize Revision:
Atoms, isotopes & radioactivity - Core Science
Structure of an atom
Isotopes
Evidence for atomic structure
Alpha, beta & gamma radiation
Detecting radiation
Radioactive decay equations
Natural sources of background radiation
Artificial radiation
Nuclear fission
Nuclear fusion
Nuclear energy issues
Notes questions from pages 206 & 207
1. No questions
How Science Works
ANSWERS
(a) Random error. The differences show no pattern and
radioactive decay is a random process.
(b) The background radiation levels stay more or less
constant in these time scales and so will affect each
reading equally.
(c) It must be used sensibly and stored safely to ensure
that as little as possible gets into the environment.
(d) Examples:
(i) What did the people die from?
(ii) How many people were in the survey?
(iii) Do all prostate cancer patients get this treatment?
(iv) Was there a control group?
(v) How many would have died anyway?