1. Radiation, nuclear fusion and nuclear fission
L.Q: Can I explain how these processes releases masses of energy?

2. What you need to know

3. The atom nucleus electron neutron
Draw a labelled diagram of the atom showing the nucleus and labelling protons, neutrons and electrons.
nucleus
electron
proton
neutron

4. Where are subatomic particles found?
Protons, neutrons and electrons are NOT evenly distributed in an atom.
The protons and neutrons exist in a dense core at the centre of the atom. This is called the nucleus.
The electrons are spread out around the edge of the atom. They orbit the nucleus in layers called shells.

5. What we used to think… It was believed that atoms were:
Spheres of positive charge.
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?

6. 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.

7. What Rutherford’s team observed……..
Most of the alpha particles went straight through the foil.
Some alpha particles were deflected through large angles.
3. A very few alpha particles were reflected straight back.

8. Rutherford’s conclusions
Observation
Conclusion
Most 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.

9. 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

11. Calculate the number of protons, electrons and neutrons shown below -12 C 6 13 C 6 14 C 6
What do we call isotopes that are unstable and emit radiation to become more stable?
What do we call atoms of the same element with different numbers of neutrons?
These are all the element carbon, what is the difference between them?
They have different numbers of neutrons.
Radioisotopes
Isotopes

12. 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

14. 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

17. Background radiation
Background radiation is the radiation all around us.
Rocks
Air
Building materials
Outer space
Food

18. 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

19. Why are some substances radioactive?
Radioactive atoms have an unstable nucleus
It becomes stable by losing a particle
This is known as radioactive decay

20. Types of radiation What is emitted by the nucleus?
Alpha radiation (particle)
Beta radiation (particle)
Gamma radiation (EM wave)

21. Alpha decay Type of decay: What is emitted? Description of decay:
Example of decay:
Effect on M and A:
Alpha decay
Alpha particle (helium nuclei)
2 neutrons and 2 protons are emitted from the nucleus.
U  Th +  + energy
M decreases by 4, A decreases by 2
(M-4, A-2)

22. Beta decay Type of decay: What is emitted? Description of decay:
Example of decay:
Effect on M and A:
Beta decay
High energy electron
A neutron in the nucleus decays into a proton and a high energy electron which is emitted.
C  N +  + energy
M stays the same, A increases by 1
(M=, A+1)

23. Gamma decay Type of decay: What is emitted? Description of decay:
Effect on A and Z:
Gamma decay
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=)

26. The penetration power of the three types of radiation.  
Skin or paper
stops ALPHA
Thin aluminium
stops BETA
Thick lead
reduces GAMMA
Thin mica

27. 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

28. 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

29. 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

30. 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

31. 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!

32. 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

33. Nuclear Power - fission
Fast neutron from previous decay cause the Uranium nucleus to split.

34. Nuclear Power Kr n
Fission
More
decays n n
Uranium n
In the reaction above a neutron from a previous decay can lead to more and more decays. Ba
This is called a chain reaction.

35. 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

36. 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

37. 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

41. 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