radiation, nuclear fusion and nuclear fission l.q: can i explain how these processes releases masses...
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Radiation, nuclear fusion and nuclear fission
L.Q: Can I explain how these processes releases masses of energy?
What you need to know
The atomDraw a labelled diagram of the atom showing the nucleus and labelling protons, neutrons and electrons.
nucleus
neutron proton
electron
Where are subatomic particles found?Protons, neutrons and electrons are NOT evenly distributed in an atom.
The electrons are spread out around the edge of the atom. They orbit the nucleus in layers called shells.
The protons and neutrons exist in a dense core at the centre of the atom. This is called the nucleus.
What we used to think…
It was believed that atoms were:
1. Spheres of positive charge.
2. With negative charges spread through it.
This resembled a plum-pudding, so it was called the ‘Plum –pudding’ model.
This was wrong!
How did we discover current ideas about the structure of the atom?
Rutherford’s team:
Ernest Rutherford and his team of scientists performed a famous experiment in Manchester:
They fired some alpha particles at a piece of thin gold foil (only a few atoms thick):
If the ‘Plum Pudding’ model of the atom was correct, the alpha particles should pass straight through and only be slightly
deflected.
This did not happen.
What Rutherford’s team observed……..
1. Most of the alpha particles went straight through the foil.
2. Some alpha particles were deflected through large angles.
3. A very few alpha particles were reflected straight back.
Rutherford’s conclusions
Observation ConclusionMost alpha particles went straight through the foil.
A few were deflected through large angles.
A very few were reflected
straight back.
Atoms are mostly space.
The nucleus is very small compared to the size of the atom and it contains most of the mass and all the positive charge.
Does the nucleus make sense?
• How are all those protons held together when they have the same charge?
• Strong force between protons and neutrons that is stronger than the electrostatic repulsion between protons
• Hold protons and neutrons together in a dense core nucleus
• Called the strong force
Calculate the number of protons, electrons and neutrons shown below -
12
C 6
13
C 6
14
C 6
These are all the element carbon, what is the difference between them?They have different numbers of neutrons.What do we call atoms of the same element with different numbers of neutrons?IsotopesWhat do we call isotopes that are unstable and emit radiation to become more stable?Radioisotopes
Isotope
• Element that exists in different forms – isotopes have the same atomic number but different mass numbers
• Different number of neutrons• Isotopes unstable – give off radiation
Radiation
• Form of energy given off by unstable nuclei• Ionising • Atoms that give off radiation are said to be
radioactive• Most objects give off some type of radiation –
even us!!• Energy is released and some mass is lost from
the release of a particle• So the nuclei becomes a new element
Background radiation
Background radiation is the radiation all around us.
Rocks Air
Building materials
Outer space
Food
Physical and chemical processes
• These do not change radioactivity of a sample• E.g. Freezing a sample or reacting it with
another chemical cannot change radioactivity
Why are some substances radioactive?
• Radioactive atoms have an unstable nucleus
• It becomes stable by losing a particle
• This is known as radioactive decay
Types of radiation
• What is emitted by the nucleus?• Alpha radiation (particle)• Beta radiation (particle)• Gamma radiation (EM wave)
Alpha decayType of decay:
What is emitted?
Description of decay:
Example of decay:
Effect on M and A:
Alpha particle (helium nuclei)
238 234 4
U Th + + energy 92 90 2
2 neutrons and 2 protons are emitted from the nucleus.
M decreases by 4, A decreases by 2
(M-4, A-2)
Beta decayType of decay:
What is emitted?
Description of decay:
Example of decay:
Effect on M and A:
High energy electron
14 14 0
C N + + energy 6 7 -1
A neutron in the nucleus decays into a proton and a high energy electron which is emitted.
M stays the same, A increases by 1
(M=, A+1)
Gamma decayType of decay:
What is emitted?
Description of decay:
Effect on A and Z:
High energy electromagnetic radiation.
Nucleus changes shape into a more stable shape. Gamma radiation emitted
as a result.
A stays the same, Z stays the same
(A=, Z=)
Thin mica Thin aluminiumstops BETA
Thick leadreduces GAMMA
Skin or paper stops ALPHA
The penetration power of the three types of radiation.
STRONG FORCE!!
• Electrostatic forces between protons cause them to be repelled....but....
• A stronger force keeps the protons and neutrons concentrated in the nucleus
• This is a called the strong force• This force only works when the protons and
neutrons are very close together.• If they move too far apart the strong force
disappears
Nuclear fuel
• Fuel which releases energy through changes in the nucleus
• The energy released from a nuclear material is much greater than a chemical reaction containing a similar mass of material
Nuclear fusion
• Two nuclei collide• Two nuclei fuse to form a larger nuclei, releasing
lots of energy• Two H nuclei form He• Must get close enough to overcome the repulsion
of electrostatic attraction• And allow the strong force to hold them together• Need energy for that – carried out at high
temperatures/pressure
E = mc2
• Mass can be converted into energy• E = energy• M = mass• C = speed of light in a vacuum (3x108 m/s)• Mass is lost when fission/fusion occurs
Nuclear Power - fission
When a nucleus decays it gives out heat energy.
In a nuclear power station, the uranium-235 atoms decay and give out energy and neutrons.
Each time a uranium atom splits it produces 2 or 3 neutrons (depending on the reaction). These go on to hit other uranium atoms, which causes them to decay. A chain reaction is set up where more and more energy is released. In a nuclear reactor the process is carefully controlled so that neutrons are absorbed harmlessly and the energy released is controlled.
In a nuclear bomb the reaction is not controlled, and the bomb explodes!
When a nucleus decays it gives out heat energy.
In a nuclear power station, the uranium-235 atoms decay and give out energy and neutrons.
Each time a uranium atom splits it produces 2 or 3 neutrons (depending on the reaction). These go on to hit other uranium atoms, which causes them to decay. A chain reaction is set up where more and more energy is released. In a nuclear reactor the process is carefully controlled so that neutrons are absorbed harmlessly and the energy released is controlled.
In a nuclear bomb the reaction is not controlled, and the bomb explodes!
Nuclear fission
• Uranium/plutonium• Neutron splits a large and unstable nucleus into two smaller
parts, roughly equal in size• This will release more neutrons and cause more
neighbouring nuclei to break down• Generates an uncontrolled chain reaction of nuclear fission• In nuclear power stations the reaction is controlled• Neutron release is controlled so reaction occurs at a
controlled pace• If not could cause a nuclear explosion
Nuclear Power - fission
Fast neutron from previous decay cause the Uranium nucleus to split.
Nuclear Power
Kr
Ba
n
n
n
n
Fis
sion
Uranium
In the reaction above a neutron from a previous decay can lead to more and more decays.
This is called a chain reaction.
More
decays
How is this controlled• Uranium is very radioactive and therefore the fission reaction needs
to be controlled to prevent an explosion• Nuclear fission of uranium occurs when the nucleus absorbs a
neutron• The nucleus splits into two smaller nuclei and releases some
neutrons• Each of these neutrons can then be absorbed by another uranium
nucleus• This is an example of a chain reaction where each fission causes more
fissions to occur• Each fission releases lots of energy• Fuel rods become very hot so can heat coolant which then turns into
steam which turn turbines
• Control rods containing Boron are put in between the uranium rods, to absorb some of the neutrons
• This keeps the rate of nuclear fission constant• So there is a steady release of energy, not
uncontrolled• Coolant is used to take away the heat
produced by nuclear fission
Nuclear fission
• Splitting of an atom called “chain reaction” (controlled)
• A slow moving neutron gets absorbed by uranium (U-235) or plutonium (Pu - 239
• When one atom of U-235 splits 2 or 3 neutrons are given off
• Each neutron goes on to split another 3 atoms, so another 9 neutrons are released, then 27 etc
• If there isn’t a large enough amount of U-235 no reaction will occur
• The minimum amount of uranium needed is called the critical mass
Fission vs fusion
• Fission:• Nucleus absorbs neutron• Unstable • Splits large nucleus into
smaller nuclei• Releases more neutrons• That split more large nuclei• Starts a chain reaction• Uranium/plutonium• Mass lost as energy
(E=mc2)
• Fusion:• Small nuclei collide• High temp/pressure• Two nuclei fuse together• Larger nuclei formed• E.g. Hydrogen – helium• Mass lost as energy
(E=mc2)• More energy released
compared with fission