EDEXCEL IGCSE / CERTIFICATE IN PHYSICS 7-4
Particles
Edexcel IGCSE Physics pages 226 to 231
March 5th 2013
All content applies for Triple & Double Science
Edexcel Specification
Section 7: Radioactivity and particles
c) Particles
describe the results of Geiger and Marsden’s experiments with gold foil and
alpha particles
describe Rutherford’s nuclear model of the atom and how it accounts for the
results of Geiger and Marsden’s experiment and understand the factors
(charge and speed) which affect the deflection of alpha particles by a nucleus
understand that a nucleus of U-235 can be split (the process of fission) by
collision with a neutron, and that this process releases energy in the form of
kinetic energy of the fission products
understand that the fission of U-235 produces two daughter nuclei and a small
number of neutrons
understand that a chain reaction can be set up if the neutrons produced by
one fission strike other U-235 nuclei
understand the role played by the control rods and moderator when the fission
process is used as an energy source to generate electricity.
Models of the atom
In about 500 BC The Greek
philosopher Democritus
suggested that all matter was
made up of tiny, indivisible
particles. The word ‘atom’
meaning ‘cannot be cut or divided’
In 1803, Thomas Dalton
proposed that atoms were tiny
hard spheres.
He further proposed that an
element was made up of identical
atoms all have the same mass but
each different element was made
up of its own type of atom.
Thomas Dalton
1766-1844
Electrons and sub-atomic particles
In 1897 J.J. Thomson
discovered the electron.
He found that this was a
negatively charged particle
that was much smaller than
an atom.
This discovery showed that
atoms were not indivisible
but were made up of
smaller sub-atomic
particles.
J.J. Thomson
1856-1940
The Plum Pudding Atomic Model
Based on his discovery of
the electron J.J. Thomson
suggested that atoms
consisted of a ball of:
- positively charged matter
- embedded with negatively
charged electrons like the
plums in a pudding
This became known as the
plum pudding model of
the atom.
The ‘Plum Pudding’ Model
Geiger & Marsden’s alpha particle
scattering experiment
In 1909 Hans Geiger and
Ernest Marsden performed
an experiment that involved
firing alpha particles onto a
very thin foil of gold.
The results from this
experiment would show that
the plum pudding of the atom
was incorrect.
Geiger and Marsden
The experiment
lead container
containing an
alpha source
very thin
gold foil
θ
vacuum
evacuated
container
inner zinc
sulphide
coating
The apparatus is contained
in an evacuated container
otherwise the alpha
particles would not be able
to reach the gold foil and
the zinc sulphide coating.
The alpha particles cause
the zinc sulphide detector to
emit a flash of light.
Geiger and Marsden noted
the distribution of the angles
of deflection, θ of the alpha
particles.
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 a 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 a positively
charged nucleus.
nucleus (highly enlarged)
Rutherford’s Atomic Model
Based on the results of Geiger and Marsden’s experiment
Ernest Rutherford suggested that an atom consists of a a
tiny positively charged nucleus surrounded by negatively
charged electrons.
Lord Rutherford
1871 - 1937
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
Nuclear fission
Nuclear fission is the
splitting of an atomic
nucleus.
The process of nuclear
fission was discovered
in 1938 by Lise
Meitner, Otto Hahn and
Fritz Strassmann.
Lise Meitner
1878-1968
Otto Hahn
1879-1968
Fissile materials
A fissile material easily
undergoes fission.
The mostly commonly used fissile
material is uranium-235.
When this undergoes fission two
daughter nuclei are produced that
are called fission fragments.
The process also produces
neutrons, gamma radiation and
a considerable amount of kinetic
energy.
The fission of uranium-235
The fission of a nucleus uranium-235 can be initiated by
its absorption of a slow-moving neutron.
235
92
U +
1
0
236
n
92
U
Uranium-236 is very unstable and splits apart.
236
92
144
U
56
Ba +
89
36
1
0
0
0
Kr + 3 n + γ
fission fragments
barium-144 and krypton-89
Other fission examples
Other fission products can be produced. Their masses
usually have a roughly 3:2 ratio.
236
92
134
U
54
Xe +
90
38
1
0
0
0
1
0
0
0
Sr + 2 n + γ
fission products = xenon-134 and strontium-90
Plutonium-239 is also fissile.
239
94
137
Pu
55
Cs +
89
39
Y
+3 n + γ
fission products = caesium-137 and yttrium-89
Fission equation questions
Complete the equations below:
236
92
143
U
239
94
145
Pu
E
235
92
55
Cs +
56
Ba +
131
U
53
F
+
I
Iodine
91
B
37
C
92
D
38
0
0
0
1
0
0
0
1
0
0
0
Rb + 2A n + γ
Rubidium
Sr + 2 n + γ
91
39
1
Y
+3 n + γ
This last example combines the uranium-235 neutron absorption
with the fission of uranium-236
Chain reaction
The fission of a nucleus of
Uranium-235 can be initiated
by a slow-moving neutron.
When this nucleus splits
further neutrons are
produced.
These neutrons in turn can
cause more nuclei to split.
An avalanche effect, called a
‘chain reaction’ can then
occur.
Nuclear fission reactors
A nuclear reactor is used to
produce substantial and
controllable energy from nuclear
fission.
The thermal energy produced is
used to convert water to high
pressure steam.
This steam is used to drive
turbines which rotate generators
to produce electricity.
top view of a nuclear
reactor
Nuclear power plant
Structure
of a
reactor
What each part does
1. Uranium fuel rods
These contain enriched uranium. This consists
of uranium-238 with a higher proportion of
uranium-235 than is found in natural reserves
of uranium. The fuel rods become very hot and
dangerously radioactive due to nuclear fission.
2. Removable control rods
Made of boron or cadmium, 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.
3. Moderator (graphite core)
Made of graphite. This surrounds the fuel rods
and slows the neutrons down to make
absorption of neutrons by uranium-235 more
likely.
uranium fuel
rods
4. Coolant
This can be water, carbon dioxide
gas or liquid sodium. This transfers
the thermal energy of the fuel rods
to the heat exchanger.
5. Heat exchanger
Here water is converted into high
pressure steam using the heat
energy of the coolant.
6. Concrete shield
This absorbs the radiation coming
from the nuclear reactor.
reactor underneath
water coolant
Choose appropriate words to fill in the gaps below:
splitting up of the nucleus of an atom
Nuclear fission is the _________
gamma
into two smaller nuclei. Energy, neutrons and _________
radiation are also 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 gamma 239 atomic splitting
neutrons 235 fission control
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
Online Simulations
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:
Nuclear fission
Nuclear fusion
Particles
Notes questions from pages 226 to 231
1.
2.
3.
4.
5.
6.
7.
8.
Describe Geiger and Marsden’s alpha particle experiment (see
pages 227 and 228).
Explain how the alpha particle experiment supports Rutherford’s
nuclear model of the atom (see pages 227 and 228).
What is meant by (a) nuclear fission, (b) fission product and (c)
chain reaction. (see pages 228 and 229).
Give an example of an equation of the fission of uranium-235
(see page 229).
Draw a diagram showing the structure of a nuclear reactor (see
page 230).
Explain the function in a reactor of (a) control rods, (b) moderator
and (c) heat exchanger (see pages 229 and 230).
Answer the questions on page 231.
Verify that you can do all of the items listed in the end of chapter
checklist on page 231.