PAL (IGCSE) – PHYSICS
Section 5 Atomic Physics
Atomic Physics
PAL (IGCSE) Single Science
Revision Book - Section 5
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Section 5 Atomic Physics
Syllabus Content_______________________________
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Section 5 Atomic Physics
Syllabus Details________________________________
5.1 Radioactivity
5.1 (a) Detection of radioactivity
Core
• Show awareness of the existence of background radiation
Background radiation



Radiation is all around us
Some of the main sources of background radiation are…
o Rocks – e.g. granite
o Outer space
o Building materials
o Radon gas in the air
Radon gas is a problem when it collects in houses
• Describe the detection of α-particles, β –particles and γ -rays (β + are not included: β
particles will be taken to refer to β –)
Geiger – Muller Tube
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Radiation enters the tube
The radiation ionises the argon gas
The positive ions go to the cathode and the electrons go to the anode
This causes a tiny current to flow
The current is amplified and detected on a counter
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Photographic Film

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Photographic film can be used to detect radiation
Radiation makes photographic film turn black
This only lets you know if there was radiation after you develop the film
5.1 (b) Characteristics of the three kinds of emission
Core
• State that radioactive emissions occur randomly over space and time
Radioactive emission:


This occurs randomly over space and time
You can not predict which nucleus will decay and when
• State, for radioactive emissions:
– their nature
– their relative ionising effects
– their relative penetrating abilities
Property
Alpha
Beta
Gamma
Nature
Helium nuclei
(2 protons and 2
neutrons)
High
Low (paper)
High energy
electron from the
nucleus
Medium
Medium (5mm
Al)
-ve
Electromagnetic
radiation
~Ionising power
Penetration
(absorbed by)
Charge
+2ve
Low
High (Thick lead)
None
• Describe their deflection in electric fields and magnetic fields
Emission
Effect of electric field
Effect of magnetic field
Alpha particle
Deflected towards negative
parallel to field
Deflected towards positive
parallel to field
No effect
Deflected perpendicular to
the magnetic field
Deflected perpendicular to
the magnetic field
No effect
Beta particle
Gamma radiation
• Interpret their relative ionizing effects



Alpha particles have a strong ionizing effect as they have 2 positive charges per
particle
Beta particle have a medium ionizing effect as they have 1 negative charge per
particle
Gamma rays are weakly ionizing as they have no charge
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5.1 (c) Radioactive decay
Core
• State the meaning of radioactive decay, using equations (involving words or symbols) to
represent changes in the composition of the nucleus when particles are emitted
Alpha decay….
A
Z
Example….
X  ZA42Y  24He
4
U  234
90Th  2 He
238
92
In alpha decay…



An alpha particle is lost from the nucleus of an atom
The number of protons reduces by 2
The number of neutrons reduces by 2
o The nucleon number reduces by 4
o The proton number reduces by 2
Beta decay…
A
Z
Example….
X Z A1Y  10
C 147N  10
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In Beta decay…


A high energy electron is lost from the nucleus
One Neutron changes into a proton
o The proton number increases by 1
o The nucleon number does not change
Gamma decay…
A
Z
X *  ZA X  00 
Example….
60
28
60
Ni*  28
Ni  00
In Gamma decay…
 The atom goes from a high energy state to a low energy state
 There is no change to the proton number or the nucleon number
5.1 (d) Half-life
Core
• Use the term half-life in simple calculations, which might involve information in tables or
decay curves
Half life…


The time taken for half of a particular radioactive isotope to decay
Example…
o If the half time of Carbon 14 is 5600 years
o After 5600 years 1/2 the atoms have not decayed
o After 11200 (2 x 5600) ¼ of the atoms have not decayed
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Half life of a nuclide = time taken for half the number of
particles present in the sample to decay
Number of nuclides able to decay
N0
Decay curve
N1/2
N1/4
T1/2
T1/2
time
Half Life


The activity of a radioactive isotope decreases with time
This type of curve is called an exponential curve
5.1 (e) Safety precautions
Core
• Describe how radioactive materials are handled, used and stored in a safe way
SAFE USAGE AND STORAGE OF RADIOACTIVE MATERIALS….
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
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Radioactive materials produce emissions that are highly ionizing
These emissions can ionize the skin and organs of humans, so causing birth
defects, cancer and burns
These materials must be handled so that these emission do are not incident on
the people working with them
o Storage – Often under water as the water absorbs the emissions well
o Handling – Within fume cupboards and often using robots
o Usage – Within closed environments (with thick highly absorbing walls)
o Waste disposal – Still a big issue but increasingly stored deep
underground
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5.2 The nuclear atom
5.2 (a) Atomic model
Core
• Describe the structure of an atom in terms of a nucleus and electrons
10-14m
-ve Electron Cloud
+ve Protons
Neutrons
Nucleus
10-10m
Supplement
• Describe how the scattering of α-particles by thin metal foils provides evidence for the
nuclear atom
Gold Foil (10 -8m thick)
Source of
alpha particles
Beam of
alpha particles
Most pass
straight through
Some are deviated
through large angles
~1:8000
repelled back
Geiger and Marsden’s Experiment
~1:8000
repelled back
+ve nucleus
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Gold Atom
Some are deviated
through large angles
(the +ve alpha particles are
deviated by the +ve nucleus)
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Observation
Conclusion
Most particles pass straight through the gold
leaf
~1:8000 are repelled back
Some are repelled through large angles
Gold is predominantly empty space
Gold has small positively charged nuclei
The nucleus has a large positive charge
5.2 (b) Nucleus
Core
• Describe the composition of the nucleus in terms of protons and neutrons
• Use the term proton number Z
• Use the term nucleon number A
Use the term nuclide and use the nuclide notation X
NOTATION
Nucleon number
A
Z
Chemical Symbol
X
Proton number
EXAMPLE
12 nucleons
6 protons
12
6
C
Carbon
5.2 (c) Isotopes
Supplement
• Use the term isotope
ISOTOPE: Nuclides with the same number of protons but different number of neutrons
• Give and explain examples of practical applications of isotopes

Carbon dating – Using the decay of carbon 14 the age of old organic
materials can be calculated
o The percentage of carbon 14 in the air is stable and known
o The percentage of carbon 14 in trees is kept constant by
photosynthesis
o When trees or plants are cut down and stop photosynthesising
the carbon 14 in them decays
o By calculating the percentage of carbon 14 present in old
organic artefacts (wooden tools, etc), and knowing the half life
of carbon 14, the age of the artefacts can be calculated
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