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 Overview
Edexcel Topic 6

Radioactivity
Part 1
• Atoms
• Types of radiation
• Background radiation
Part 2
• Radioactive decay and half life
• Uses and dangers of
radioactivity
Part 3
• Nuclear reactions
LearnIT!
KnowIT!
Part 1

• Atoms
• Types of radiation
• Background radiation
 Structure of the atom - subatomic particles
• An atom has a positively charged nucleus containing protons and neutrons,
surrounded by negatively charged electrons in shells.
• The nuclear radius much smaller than that of the atom and with almost all mass
of an atom is concentrated in the nucleus.
• An atom contains equal numbers of protons and electrons.
• Atoms have no overall electrical charge because the number of positive
protons equals the number of negative electrons.
number of protons = atomic number
• All atoms of an element have the same number of protons in the nucleus. This
number is unique to that element.

Mass Charge Location

Proton 1 + (positive) nucleus
Neutron 1 no charge nucleus
Electron 1/1835 negligible - (negative) shells
 Mass number and atomic number

All atoms of a particular element have the same number of protons.

The number of protons in an element is called its atomic number.
Neutrons
Protons The total number of protons
Neutrons and neutrons in an atom is
Electrons called its mass number.
Oxygen has a mass number of
16. If it has 8 protons it must
therefore have 8 neutrons to
Protons make a mass number of 16.
On the periodic table,
oxygen is shown as Electrons
having an atomic Atoms are electrically neutral
number of eight, so there must be the same
therefore 8 protons. number of electrons (-) as
protons (+); 8 electrons.

Oxygen has: 8 protons, (16 – 8) = 8 neutrons, and 8 electrons

 Structure of an atom
• An atom has a positively charged nucleus containing protons and neutrons, surrounded
by negatively charged electrons in shells.
• The nuclear radius much smaller than that of the atom and with almost all mass of an
atom is concentrated in the nucleus.

Electrons are arranged in orbits or Atom of Carbon 12

energy levels around the nucleus.
12 . 9 x 1013 atoms
Energy levels can hold a maximum
of: 6
C in this dot of ink

2 e- in the first level Nucleus made up of

8 e- in the second level nucleons, these can
8 e- in the third level be protons or
Electrons change orbit when there neutrons
is absorption or emission of
electromagnetic radiation. Protons: charge +1
Neutrons: charge 0
The radius of the nucleus is less than
1/10 000 the radius of the atom – the Radius of an The nucleus holds
atom is 99.9999999% empty space! atom 1 x 10 -10m 99% of the mass of
the atom
 Mass number, atomic number and isotopes

Isotopes are elements with different atomic masses (nucleon number).

The number of protons can not change or it would not be the same
element so an isotope is an element with different numbers of neutrons.
Carbon 12 Carbon 13 Carbon 14
12 13 14

6
C 6
C 6
C

6 protons 6 protons 6 protons

6 neutrons 7 neutrons 8 neutrons

The isotopes have the same number of protons and the same number of
electrons. Only the number of neutrons changes in an isotope.
 Mass number, atomic number and isotopes

Atoms can form ions if they gain or lose electrons.

Atoms do this so they have full outer energy levels.
Beryllium can lose 2 electrons
from its outer energy level to
become an ion.
If Beryllium
loses 2 e-
it now has:

4 protons 4+
2 electrons 2-
2+

Berylium 2+

Atoms can lose (-) electrons to become positive (+) ions

or gain (-) electrons to become negative (-) ions.
 Radioactive decay and nuclear radiation
The nuclei of some atoms are unstable. To become more stable these
nuclei give out radiation. This process is called radioactive decay.

Unstable Stable
atom atom
Radioactive decay
Activity = rate at
which a source of Count-rate = number
unstable nuclei of decays recorded
decays, measured each second by a
in becquerels (Bq). detector
Radioactive rock

Radioactivity can be
detected by using Different radioactive isotopes
Geiger–Müller tube or decay at different rates and emit
photographic film. different types of radiation.
Geiger–Müller tube
 Radioactive decay and nuclear radiation
There are three types of radioactive decay, alpha, beta (β– beta minus and β+
positron) and gamma. All come from an unstable nucleus of an atom.
In the examples below, only the nucleus is shown. This is a random process
Alpha decay (symbol α or ) consist of 2 protons and 2
neutrons emitted from the nucleus. They have a positive
charge as they contain 2 (+) protons.

β– (beta minus) decay , consist of an electron emitted from

_ the nucleus. This results from a neutron splitting into a
e-
proton and an electron.
β+ (positron) decay , consist of an positron emitted from
+ e+ the nucleus. This results from a proton splitting into a
neutron and an positron.
(A positron is has the same mass as an electron).

Gamma rays (symbol γ) are electromagnetic radiation

emitted from the nucleus. Gamma radiation has no mass
and no electrical charge.
 Radioactive decay and nuclear radiation

Properties of alpha, beta and gamma radiation.

Alpha, beta and gamma radiation can
penetrate different materials due to
their differing nature.
Alpha – easily stopped by a few
sheets of paper.
Beta – penetrates paper but stopped
by a thin sheet of aluminium.
Gamma – only stopped by thick lead
or several metres of concrete.

Range in Ionising
All three types of radiation Name Symbol Speed
air power
cause ionisation of other
Alpha α Slowest 6 - 8 cm High
atoms. If these atoms are
in living cells it can cause Beta β Medium 1–2m Medium
damage which could lead 300 -
to cancer. Gamma γ Fastest Low
500 m
 Background radiation

Background radiation is the constant , low level radiation in the environment.

This can be natural radiation from rocks, building materials, cosmic rays etc.
Radioactive pollution from nuclear testing, nuclear power and
industrial/medical waste also contributes to background radiation.
Sources of radioactive
exposure and contamination.

Radiation dose
is measured in:
sieverts (Sv)

1 Sv = 1000
millisieverts

Everyone receives background radiation but people who work or live in locations
with high levels of radiation receive additional doses of radiation.
Some nuclear workers, medical staff, military and industrial workers may have
higher doses due to working with radioactive sources.
QuestionIT!
Part 1

• Atoms
• Types of radiation
• Background radiation
 Part 1 - QuestionIT

1. The diameter of an atom is about 0.000 000 000 2 m. What is this distance in
standard form?

2. What is the nucleus of an atom composed of?

3. Describe what happens when an electron drops to a lower energy level in an

atom.

4. An atom of sodium is represented by:

Use this information to determine the number of protons, neutrons and

electrons in an atom of sodium.

5. What is the electrical charge attached to:

a neutron
an electron
a proton
 Part 1 - QuestionIT

6. What is the mass number and the atomic number for fluorine?

7. Beryllium has the chemical symbol:

9
4 Be
Use this information to draw a representation of an atom of beryllium.

8. A different isotope of beryllium has an extra neutron. Give the chemical

symbol of this new isotope of beryllium.
 Part 1 - QuestionIT
9. The radioactive element Uranium has two common isotopes.
236 238
92 U and 92 U
Complete the table to show the number of protons, neutrons and electrons in
each isotope.
Isotope Protons Neutrons Electrons
236
92 U
238
92 U

10.Sodium can lose its outer electron to have a full outer energy level. What will the
atom now become?
 Part 1 - QuestionIT

11. Which part of an atom is involved with radioactive decay?

12. Explain the meaning of the term activity, as applied to

radioactive materials and state the units of activity.

13. What is meant by the term “count rate”?

14. Copy and complete the table to show the nature of alpha, beta
and gamma radiations.
Electrical
Radiation Symbol Composition
charge
Beta β
Electromagnetic
Gamma
wave
Alpha +2
 Part 1 - QuestionIT

15. A piece of radioactive rock shows a reading of 350 counts/min.

When covered in aluminium foil, this drops down to 4 counts/min.
Explain which type of radiation this rock is emitting.

16. Radioactive emissions are often described as ionising radiations.

What does this mean?

17. Smoke detectors use americium-241 which is an alpha emitter.

Explain why an alpha source is used in these detectors.

18. Why is an alpha particle often described as a helium nuclei?

AnswerIT!
Part 1

• Atoms
• Types of radiation
• Background radiation
 Part 1 - AnswerIT

1. The diameter of an atom is about 0.000 000 000 2 m. What is this distance in
standard form?
2 x 10 -10 m
2. What is the nucleus of an atom composed of?
Protons and neutrons (except Hydrogen which has no neutrons).
3. Describe what happens when an electron drops to a lower energy level in an
atom.
It releases a photon of electromagnetic radiation.
4. An atom of sodium is represented by:

5. Use this information to determine the number of protons, neutrons and

electrons in an atom of sodium.
Protons = 11 Neutrons = 12 Electrons = 11
6. What is the electrical charge attached to:
a neutron Neutral
an electron Negative
a proton Positive
 Part 1 - AnswerIT

6. What is the mass number and the atomic number for fluorine?
Mass number = 19; atomic number = 9

7. Beryllium has the chemical symbol:

9
4 Be
Use this information to draw a representation of an atom of beryllium.

8. A different isotope of beryllium has an extra neutron. Give the chemical

symbol of this new isotope of beryllium.
 Part 1 - AnswerIT
9. The radioactive element Uranium has two common isotopes.
236 238
92 U and 92 U
Complete the table to show the number of protons, neutrons and electrons in
each isotope.
Isotope Protons Neutrons Electrons
236
92 U 92 144 92
238
92 U 92 146 92

10. Sodium can lose its outer electron to have a full outer energy level. What will
the atom now become?
An ion with a charge of 1+
 Part 1 - AnswerIT

11. Which part of am atom is involved with radioactive decay?

The nucleus only.
12. Explain the meaning of the term activity as applied to
radioactive materials and state the units of activity.
The rate at which a source of unstable nuclei decays. Units Bq.
13. What is meant by the term “count rate”?
The number of radioactive decays recorded in a given time.
14. Copy and complete the table to show the nature of alpha, beta
and gamma radiations.
Electrical
Radiation Symbol Composition
charge
Beta β an electron -1
Electromagnetic
Gamma γ wave
0

2 protons and 2 +2
Alpha α neutrons
 Part 1 - AnswerIT

15. A piece of radioactive rock shows a reading of 350 counts/min.

When covered in aluminium foil, this drops down to 4 counts/min.
Explain which type of radiation this rock is emitting.
Could be alpha or beta as both would be stopped by the foil and
gamma would not be stopped by the foil.
16. Radioactive emissions are often described as ionising radiations. What does
this mean?
The emissions knock off electrons from atoms which then become ions.
17. Smoke detectors use americium-241 which is an alpha emitter. Explain why an
alpha source is used in these detectors.
Alpha particles are easily stopped by smoke.
They do not travel far in air so are safe for the user.
18. Why is an alpha particle often described as a helium nuclei?
It contains 2 protons and 2 neutrons, the same as the nucleus of a helium
atom.
LearnIT!
KnowIT!
Part 2

• Radioactive decay and

half life
• Uses and dangers of
radioactivity
 Nuclear equations
Nuclear equations show the changes to an atom when it emits radiation.

Nucleus loses 2 protons and

Alpha emission

2 neutrons.
Atomic number will reduce
by 2 and atomic mass by 4.

Nucleus loses an electron

which is produced when a
Beta emission

neutron turns into a proton.

So mass stays the same but
atomic number of the
product increases by one.
Gamma emission

No particles are emitted so

there is no change to the
nucleus. Atomic mass and
atomic number stay the
same.
 Half life and the random nature of radioactive decay

Radioactive decay is a random process so the likelihood of a decay taking place is a

probability problem. For this reason, the half-life of an isotope is given rather than
saying how long it will take to fully decay.
The half-life of a radioactive isotope is the time it takes for the number of nuclei
of the isotope in the sample to halve, or the time it takes for the count rate from a
sample containing the isotope to fall to half its initial level.

The net decline of the isotope

is the fraction remaining after
a number of half lives.
E.g. 100 50 25
After 2 half lives net decline is
75/100 = 3/4
 Different half lives of isotopes

Radioactive isotopes have an enormous range of half-lives.

Examples of the range of half-lives Half-life and hazard (Physics only)

of radioactive materials Radioactive isotopes with a short half-
life often give high doses of radiation in a
short period of time so are often
dangerous.
Long half-life isotopes are low dose
hazards but they are around for a very
long time. Uranium-238 is the main fuel
producer for the nuclear industry but is
so slow at emitting radiation it is often
considered quite safe by scientists.
Products of the nuclear industry such as
Iodine-131 are much more dangerous as
they emit radiation at a much faster rate
and are soluble so they get into the food
chain much more easily.
 Half life and the random nature of radioactive decay

Calculating the half life of a radioactive isotope.

If you know the start and finish count 1
rate and the time taken, you can Radioactive sample of Fermium - 252
calculate the half life.
Example:
The count rate of an isotope is 1008 Bq.
This falls to a count rate of 126 over a
period of 21 days.

1008 504 252 126

1 2 3
3 half lives for count rate to fall to 126.
200 counts / min at the beginning.
These 3 half lives took 21 days so each 100 counts/min occurred after 1.2 hours.
half life took 7 days. 50 counts/min occurred after 2.4 hours.
It always takes 1.2 hours for the count rate
Half life if this isotope = 7 days to halve.
Half life of Fermium - 252 = 1.2 hours.
 Uses of radioactivity (Physics only)

Smoke Alarms
• Contains a source of alpha particles
• There is an electrical circuit with a gap between 2 charged plates
• Air in gap is constantly ionised therefore constant electric current
• When smoke get in the alpha particles are absorbed and stops the current drops =and the
alarm sounds
Irradiating food
• Bacteria cause food to decay or make us ill
• Gamma rays kill bacteria
• Makes food safer and longer lasting
• Does not make food radioactive
• Foods like Fruit, cereals and shellfish are irradiated
Sterilisation of equipment
• To kill microorganisms surgical instruments need to be sterilised
• Heat is usually used to sterilise surgical instruments but cannot be
used on certain plastics
• They are irradiated with Gamma rays instead
 Uses of radioactivity (Physics only)

Tracers in the environment

• Added to water to monitor pollution or leaking pipes underground
• A Geiger–Müller tube follows the pipe to detect leaks

Checking thickness of paper or aluminium foil

• Use a detector to measure the rate Beta particles passing through the paper or foil
• The higher the beta count the thinner paper or foil
Diagnosis of Cancer
• Gamma rays used
• A tracer solution injected into body that collects in cancerous cells
• Gamma camera used to detect cancerous cells
• Gamma rays pass through the body so easily detected from the outside

Treatment of Cancer
• Radiotherapy ionising radiation to cancerous cells, can use
metal implants, be injected or swallowed
• Gamma rays used as beams aimed from different positions
to target and kill cancerous cells
 Dangers of ionising radiation

Radioactive materials are hazardous to life. Nuclear radiation can ionise

(add or remove electrons) substances within the human body. This can
change the way cells behave, damage DNA or destroy human cells.
Body part Effect of ionising radiation
Hair Hair loss
Skin Can cause burns or lead to skin cancers
Reproductive organs High doses can cause sterility or mutations
in offspring
Thyroid Exposure to radioactive iodine can destroy
the cells in the thyroid or cause cancers
Bone marrow Can cause leukaemia or other blood cancers

Rapidly dividing cells like cells that produce hair or those in the
reproductive organs are most susceptible to ionising radiation.
 Radioactive irradiation and contamination

Irradiation is when an object or person Contamination is when a radioactive

is exposed to radiation. Protection source is in contact with an object or
from irradiation means stopping the person. The radioactive substance
radiation from reaching you. rather than the emissions are present.

The object remains radioactive until

Medical dressings are often irradiated the contamination is removed or
but present no danger to the user. decays naturally.
Radioactive materials are hazardous, so certain precautions can be taken to reduce
the risk when using radioactive sources. These include:
• wear protective clothing to prevent the body becoming contaminated should
radioactive isotopes leak out
• limit the dose and monitor exposure using detector badges, etc
 Uses of nuclear radiation (Physics only)

Nuclear radiations are used as tracers in Radiation therapy is used to treat illnesses
the body to explore possible injury or such as cancer. Cancer cells are living cells
disease of internal organs. and so are killed off by relatively high
A radioactive isotope is either injected or doses of gamma rays.
ingested into the body, given time to
circulate and accumulate in damaged
areas. Then the emissions radiating out
of the body are detected.

A camera such as a gamma detector or a Here, the gamma rays are directed from the
PET scanner detects any accumulation of outside. The high dose required to kill the
the tracer. Tracers have to be beta or cancer cells will also kill healthy cells. The
gamma emitters as alpha does not technique uses a 3 dimensional set of
penetrate the body. The tracer must also gamma ray guns all focussed on the cancer
have a very short half-life to minimise cells.
dosage. This is why they need to be This kills the cancer cells while minimising
produced nearby. the damage healthy cells.
QuestionIT!
Part 2

• Radioactive decay and

half life
• Uses and dangers of radioactivity
 Part 2 – QuestionIT
1. Complete the nuclear equation for the beta decay of carbon

2. Uranium-235 undergoes an alpha decay to produce thorium-231. (atomic number of

Uranium is 92). Complete the nuclear equation for this process.

3. When iodine 131 decays, there is no mass change in the nucleus and no new
products formed. What type of radioactive emission is this?

4. Explain what is meant by the term “half life”.

5. A radioactive sample reduces its count rate from 240 counts/min to 30

counts/min over a period of 60 hours what is its half life?
 Part 2 – QuestionIT

6. Use the decay curve below to work out the half-life of the isotope.

7. Calculate the net decline of the above isotope expressed as a ratio, during
radioactive emission after 3 half-lives.
 Part 2 – QuestionIT

8. Explain the difference between radioactive irradiation and

radioactive contamination.

9. Copy and complete the table below to suggest one way of

preventing exposure to irradiation and contamination by
radioactive materials.

Type of exposure Method of preventing exposure

Irradiation

Contamination
 Part 2 – QuestionIT

10. Radium - 226 is an alpha emitter with a half-life of 1600 years. Explain how
the way this material is stored is influenced by these properties.

11. The diagram shows how three separate gamma

beams are used to treat a cancer tumour.
Why is this preferred to using one powerful
beam?

12. Alpha emitting radioisotopes cannot be used as tracers in the body to

explore injured or diseased organs. Why?
AnswerIT!
Part 2

• Radioactive decay and

half life
• Uses and dangers of radioactivity
 Part 2 – AnswerIT
1. Complete the nuclear equation for the beta decay of carbon

2. Uranium-235 undergoes an alpha decay to produce thorium-231. (atomic

number of Uranium is 92). Complete the nuclear equation for this process.
231
235
92 U 90
Th
3. When iodine 131 decays, there is no mass change in the nucleus and no new
products formed. What type of radioactive emission is this?
Gamma emission
4. Explain what is meant by the term “half life”. The time it takes a radioactive sample
to lose half its radioactivity (as measured by count rate).
5. A radioactive sample reduces its count rate from 240 counts/min to 30
counts/min over a period of 60 hours what is its half life?
240 120 60 30
1 2 3
Three half lives in 60 hours = 20 hour half life
 Part 2 – AnswerIT

6. Use the decay curve below to work out the half-life of the isotope.

80 = 0 day; 40 = 2 days. Difference = 2 days. Half-life = 2 days

7. Calculate the net decline of the above isotope expressed as a ratio, during
radioactive emission after 3 half-lives.

Counts/ min reduce from 80 to 10 in 3 half-lives.

Decline is 70/80 or 7/8ths
 Part 2 – AnswerIT

8. Explain the difference between radioactive irradiation and

radioactive contamination.
Irradiation is exposure to emissions from radioactive materials
that are not in contact with an object. Contamination is when
radioactive materials are in contact with the object.
9. Copy and complete the table below to suggest one way of
preventing exposure to irradiation and contamination by
radioactive materials.
Type of exposure Method of preventing exposure
Wear protective clothing, e.g., lead apron, to shield
from radiation.
Irradiation
Move away from the radiation.
Shield the radiation with appropriate material.
Avoid contact with radioactive materials.
Contamination Prevent radioactive materials being released into the
environment.
 Part 2– AnswerIT

10. Radium - 226 is an alpha emitter with a half-life of 1600 years. Explain how
the way this material is stored is influenced by these properties.
Any sealed container will prevent radiation escaping as alpha particles are
not very penetrating. Radioactive material will need to be placed in
permanent storage, buried underground, as it will be radioactive for a very
long time.
11. The diagram shows how three separate gamma
beams are used to treat a cancer tumour.
Why is this preferred to using one powerful
beam?
Single beam will damage both healthy or cancer
cells but all three beams are focussed on the tumour so these cells receive
a triple dose of radiation to kill them and reduces harm to healthy cells.
12. Alpha emitting radioisotopes cannot be used as tracers in the body to
explore injured or diseased organs. Why?
Alpha particles are highly ionising so they will cause damage to bodily cells.
They are also easily absorbed by body tissue so they will not escape the
body to be detected.
LearnIT!
KnowIT!
Part 3

• Nuclear reactions
(physics only)
 Nuclear fission (Physics only)

Nuclear fission is the splitting of large, unstable atoms into two or

more atoms along with the release of energy.
Here, a nucleus of Uranium - 235 Two smaller elements are
is bombarded with a neutron to form produced in this fission process
Uranium 236 which is unstable. (Barium and Krypton).
Unstable
nucleus

ENERGY
All the fission products
have kinetic energy
Fission means splitting.
Uranium or Plutonium are
often used in nuclear reactors These neutrons may go
to produce heat as their on to split further
nuclei are easy to split. Uranium atoms.
 Nuclear fission (Physics only)

Nuclear fission releases more neutrons which can lead to further fission reactions.
If this is uncontrolled a chain reaction can occur which will release vast amounts of energy.
Neutrons
U235

U235
U235
Neutron U235 Each fission stage
takes less than a
millionth of a second!

U235
U235

U235
If the chain reaction is not controlled a vast amount of energy is released
almost instantly. This is how a nuclear weapon works.
 Nuclear fission (Physics only)

Thermal energy from the chain reaction is used in the generation of electricity in a
nuclear power station. The water is heated to steam this then turns a turbine and
the turbine turns a generator to produce electricity.

In a nuclear reactor,
some of the neutrons
are absorbed by boron
rods to control the
reaction and hence
control the amount of
energy released.
There is also a graphite
core called a moderator
it slows the neutrons
down so that they are
more likely to be
absorbed into a nearby
fuel rod.
 Nuclear fusion (Physics only)

Nuclear fusion is the joining of two small (light) nuclei to form a larger nucleus.
When two small nuclei join to form a larger nucleus, a small amount of mass is
changed into a large amount of energy.

Fusion reactions take place in

stars to release vast amounts of
energy.
Here, two types of hydrogen
nuclei, deuterium and tritium,
fuse to form helium and release a
neutron plus energy.

As the nuclei of atoms are positive when they come near to each other they
repel because they have the same charge. For this reason fusion can only occur
at very high temperatures and pressures. This makes fusion impractical due to
the difficulty of building the reactor and the costs involved.
QuestionIT!
Part 3

• Nuclear reactions
(physics only)
 Part 3 – QuestionIT

1. Which particle is needed to begin the fission of a large,

unstable nuclei?

2. During the fission of uranium, two smaller nuclei are produced

and what else?

3. Copy and complete the diagram below to show the chain

reaction of a sample of uranium
 Part 3 – QuestionIT

4. Explain what is meant by a controlled chain reaction.

5. What is nuclear fusion?

6. Where does nuclear fusion take place on a large scale?

7. Draw a diagram to show the process of fusion between

deuterium and tritium to produce energy.
AnswerIT!
Part 3

• Nuclear reactions
(physics only)
 Part 3 – AnswerIT

1. Which particle is needed to begin the fission of a large,

unstable nuclei?
A neutron
2. During the fission of uranium, two smaller nuclei are produced
and what else?
A number of neutrons and large amounts of energy.
3. Copy and complete the diagram below to show the chain
reaction of a sample of uranium
 Part 3 – AnswerIT

4. Explain what is meant by a controlled chain reaction.

Nuclear fission reaction where some of the neutrons produced
in the reaction are absorbed to prevent the reaction running
out control.
5. What is nuclear fusion?
The joining of two small nuclei to form a single larger nucleus.
6. Where does nuclear fusion take place on a large scale?
In stars (sun)
7. Draw a diagram to show the process of fusion between
deuterium and tritium to produce energy.