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P2.5.1 – Atomic Structure
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The structure of the atom
ELECTRON –
negative, mass
nearly nothing
NEUTRON –
neutral, same
mass as
proton (“1”)
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The nucleus is around 10,000
times smaller then the atom!
PROTON –
positive, same
mass as
neutron (“1”)
Atoms always have the same number of
protons and electrons so they are
neutral overall. They can gain or lose
electrons to form ions.
Structure of the atom
A hundred years ago people thought
that the atom looked like a “plum
pudding” – a sphere of positive
charge with negatively charged
electrons spread through it…
Ernest Rutherford, British scientist:
I did an experiment (with my colleagues
Geiger and Marsden) that proved this
idea was wrong. I called it the
“Scattering Experiment”
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The Rutherford Scattering Experiment
Alpha
particles
(positive
charge, part
of helium
atom)
Most particles passed
through, 1/8000 were
deflected by more than
900
Conclusion – atom is made up of a small, positively
charged nucleus surrounded by electrons orbiting
in a “cloud”.
Thin gold
foil
The structure of the atom
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Particle
Proton
Relative Mass
1
Relative Charge
+1
Neutron
Electron
1
1/2000 (i.e. 0)
0
-1
MASS NUMBER = number of
protons + number of neutrons
SYMBOL
PROTON NUMBER = number of
protons (obviously)
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Mass and atomic number revision
How many protons, neutrons and electrons?
Isotopes
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An isotope is an atom with a different number of neutrons:
Notice that the mass number is different. How many
neutrons does each isotope have?
Each isotope has 8 protons – if it didn’t then it just
wouldn’t be oxygen any more.
A “radioisotope” is simply an isotope that is radioactive –
e.g. carbon 14, which is used in carbon dating.
P2.5.2 – Atoms and Radiation
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Introduction to Radioactivity
Some substances are classed as “radioactive” – this means that
they are unstable and continuously give out radiation at
random intervals:
Radiation
The nucleus is more stable after emitting some radiation – this
is called “radioactive decay”. This process is NOT affected by
temperature or other physical conditions.
Ionisation
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Radiation is dangerous because it “ionises” atoms – in other
words, it turns them into ions by “knocking off” electrons:
Alpha radiation is the most ionising (basically, because it’s the
biggest). Ionisation causes cells in living tissue to mutate,
usually causing cancer.
Background Radiation
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13% are
man-made
Radon gas
Food
Cosmic rays
Gamma rays
Medical
Nuclear power
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Background Radiation by Location
In 1986 an explosion occurred at the Chernobyl nuclear power
plant. Here is a “radiation map” showing the background
radiation immediately after the event:
Other “risky” areas could be mining underground, being in a
plane, working in an x-ray department etc
Types of radiation
Unstable
nucleus
New
nucleus
Alpha
particle
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1) Alpha () – an atom decays into a new
atom and emits an alpha particle (2
protons and 2 ______ – the nucleus of a
______ atom)
2) Beta () – an atom decays into a new
atom by changing a neutron into a
_______ and electron. The fast moving,
Beta high energy electron is called a _____
particle particle.
Unstable
nucleus
New
nucleus
Unstable
nucleus
New
nucleus
3) Gamma – after  or  decay surplus
______ is sometimes emitted. This is
called gamma radiation and has a very
high ______ with short wavelength.
The atom is not changed.
Gamma
radiation
Words – frequency, proton,
energy, neutrons, helium, beta
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Changes in Mass and Proton Number
Alpha decay:
241
Am
95
237
Np
93
+
4
+
0
2
α
Beta decay:
90
Sr
38
90
Y
39
β
-1
Blocking Radiation
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Each type of radiation can be blocked by different materials:



Sheet of
paper (or
6cm of air
will do)
Few mm of
aluminium
Few cm of
lead
Summary
Property
Charge
Mass
Penetration
ability
Range in air
What is it?
Ionising ability
Alpha
Beta
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Gamma
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Deflection by Electric Fields
Alpha and beta particles
have a charge:
+
2 protons, 2 neutrons,
therefore charge = +2
+
1 electron, therefore
charge = -1
-
Because of this charge, they will be deflected by electric
fields:
+
1) Why did they move in opposite directions?
2) Which particle had the more curved- path
and why?
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Deflection by Magnetic Fields
Recall:
+
+
2 protons, 2 neutrons,
therefore charge = +2
-
1 electron, therefore
charge = -1
Because of this charge, they will also be deflected by
magnetic fields:
1) Why did they move in opposite directions?
2) Which particle had the more curved path
Region of magnetic field
and why?
Uses of radioactivity 1
Sterilising medical instruments
Gamma rays can be used to kill and sterilise
germs without the need for heating. The
same technique can be used to kill microbes
in food so that it lasts longer.
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Uses of radioactivity 2 - Tracers
A tracer is a small amount of radioactive material used to
detect things, e.g. a leak in a pipe:
Gamma
source
The radiation from the radioactive source is picked up above
the ground, enabling the leak in the pipe to be detected.
Tracers can also be used in medicine to detect
tumours:
For medicinal tracers, you would probably use a beta source
with a short half life – why?
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Uses of radioactivity 3 – Smoke Detectors
Smoke detectors
Alpha
emitter
+ve electrode
-ve electrode
Alarm
Ionised air particles
If smoke enters here a
current no longer flows
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Uses of Radioactivity 4 - Treating Cancer
High energy gamma radiation can be used to kill cancerous
cells. However, care must be taken in order to enure that the
gamma radiation does not affect normal tissue as well.
Radioactive iodine can be used to treat thyroid cancer. Iodine
is needed by the thyroid so it naturally collects there.
Radioactive iodine will then give out beta radiation and kill
cancerous cells.
Dangers of radioactivity
Alpha
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Radiation will ionise atoms in living
cells – this can damage them and
cause cancer or leukaemia.
Beta
Gamma
OUTSIDE the body  and  are
more dangerous as  radiation
is blocked by the skin.
INSIDE the body an  source
causes the most damage
because it is the most ionising.
Half life
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The decay of radioisotopes can be used to measure the
material’s age. The HALF-LIFE of an atom is the time
taken for HALF of the radioisotopes in a sample to decay…
= radioisotope
At start
there are 16
radioisotopes
After 1 half
life half have
decayed
(that’s 8)
= new atom formed
After 2 half
lives another
half have
decayed (12
altogether)
After 3 half
lives another
2 have
decayed (14
altogether)
A radioactive decay graph
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Count
1 half
life
1 half
life
1 half
life
Time
Dating materials using half-lives
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Question: Uranium decays into lead. The half life of uranium is
4,000,000,000 years. A sample of radioactive rock contains 7 times as
much lead as it does uranium. Calculate the age of the sample.
Answer: The sample was originally completely uranium…
1 half life
later…
1 half life
later…
1 half life
later…
8
8
4
8
2
8
1
…of the
sample was
uranium
Now only 4/8 of
the uranium
remains – the
other 4/8 is lead
Now only 2/8 of
uranium remains
– the other 6/8
is lead
Now only 1/8 of
uranium remains
– the other 7/8
is lead
8
So it must have taken 3 half lives for the sample to decay until only 1/8
remained (which means that there is 7 times as much lead). Each half
life is 4,000,000,000 years so the sample is 12,000,000,000 years old.
An exam question…
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Potassium decays into argon. The half life of potassium is
1.3 billion years. A sample of rock from Mars is found to
contain three argon atoms for every atom of potassium.
How old is the rock?
(3 marks)
The rock must be 2 half lives old – 2.6 billion years
P2.6.1 – Nuclear Fission
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Nuclear fission
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More
neutrons
Neutron
Uranium or
plutonium
nucleus
Unstable
nucleus
New nuclei
(e.g. barium
and krypton)
Chain reactions
Each fission reaction releases
neutrons that are used in
further reactions.
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Nuclear power stations
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Nuclear power stations use the energy from each reaction to
heat water and use the steam to drive turbines:
P2.6.2 – Nuclear Fusion
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Nuclear Fusion in stars
Proton
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Neutron
Nuclear fusion happens in stars but it’s not possible to use it
in power stations yet as it needs temperatures of around
10,000,000OC
The Life Cycle of a Star
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Stage 1: Nebulae
A nebulae is a collection of dust, gas and rock.
Some examples of nebulae…
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Dark nebula
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Emission nebula
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Reflection nebula
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Planetary nebula
(This nebula is smaller and will only form a planet)
Stage 2: Protostar
Gravity will slowly pull these
particles together…
As they move inwards
their gravitational
potential energy is
converted into heat and
a PROTOSTAR is formed
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Stage 3: Main Sequence
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In a main sequence star the forces
of attraction pulling the particles
inwards are _________ by forces
acting outwards due to the huge
__________ inside the star.
Stars are basically ________ reactors that use _______ as a
fuel. During its main sequence a star will release energy by
converting hydrogen and helium (light elements) into
_________ elements and this is why the universe now
contains a number of heavier elements.
Our sun is an example of a main sequence star –
it’s in the middle of a 10 billion year life span
Words – heavier, balanced, hydrogen, nuclear, temperatures
Stage 4: Red Giant
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Eventually the hydrogen and helium will run out. When
this happens the star will become colder and redder and
start to swell…
If the star is relatively small
(like our sun) the star will
become a RED GIANT
If the star is big (at
least 4 times the size of
our sun) it will become a
RED SUPERGIANT
Stage 5: The Death
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What happens at this point depends on the size of the
star…
1) For SMALL stars the red giant will collapse under its
own gravity and form a very dense white dwarf:
Red giant
White dwarf
Black dwarf
2) If the star was a RED
SUPERGIANT it will shrink and
then EXPLODE, releasing massive
amounts of energy, dust and gas.
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This explosion is
called a
SUPERNOVA
Before
After
The dust and gas on the outside
of the supernova are thrown
away by the explosion and the
remaining core turns into a
NEUTRON STAR.
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If the star is big
enough it could
become a BLACK
HOLE instead.
Stage 6: Second generation stars
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The dust and gas thrown out by a supernova can be used to
form a new star…
Our sun is believed to be a “______ ______ star” – this is
because it contains some __________ elements along with
hydrogen and ________. These heavier elements would have
been the products of a previous star that have been thrown
out by a ________. These heavier elements are also found on
planets, indicating that they might have been made from
remains of previous _______ as well.
Words – helium, heavier, second generation, stars, supernova
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The Life Cycle of a Star summary
Protostar
SMALL
stars
BIG
stars
Main sequence
Red giant
Red super giant
White dwarf
Supernova
Black dwarf
Neutron star
Basically, it all depends on the size of
the star!
Black hole