How was Rutherford’s experiment set up? (3)
Alpha source opposite gold foil (approximately 400 atoms thick)
Zinc sulphide detector set at different angles
Done in a vacuum to stop air molecules from deflecting the alpha particles
What did the results of Rutherford’s experiment indicate about the structure of an atom? (3)
Most particles passed through undeviated - atoms are mostly empty space
1 in 2000 were deflected through small angles - nucleus is positively charged
1 in 8000 rebounded - small, dense concentrated region of positive charge
What is the approximate radius of a nucleus?
10^-15
What is the approximate radius of an atom?
10^-10
Define proton number
The number of protons/electrons in an atom
Define nucleon number
The number of protons and neutrons in the nucleus
Define isotope (2)
A nucleus of the same element with the same number of protons but a different number of neutrons
Isotopes all undergo the same chemical reactions
What is the strong nuclear force? (3)
The force that allows the protons to stay together despite the electrostatic repulsion
Repulsive below 0.5fm
Attractive to about 3fm
How does the density of nuclei compare to the density of atoms?
Nuclei are very small, concentrated regions of mass so it is extremely dense compared to ordinary matter
What are antiparticles? (2)
Every particle has a corresponding antiparticle with the same mass and opposite charge
When a particle and antiparticle meet they annihilate, producing a high-energy pair of photons
What forces act on hadrons? (2)
Strong nuclear force
Weak nuclear force
What forces act on leptons?
Weak nuclear force
What are some examples of hadrons? (2)
Protons
Neutrons
What are some examples of leptons? (2)
Electrons
Neutrinos
Fundamental forces - strong nuclear (4)
Experienced by nucleons
10^-15 m
Binds nucleus
Exchange particle - gluon
Fundamental forces - electromagnetic (4)
Experienced by static and moving charged particles
Infinite range
Binds atoms
Exchange particle - photon
Fundamental forces - weak nuclear (3)
Responsible for beta-decay
10^-18 m
Exchange particle - W+/- and Z bosons
Fundamental forces - gravitational (2)
Experienced by all particles with mass
Binds solar system
What is a fundamental particle?
A particle that has no internal structure meaning that it can’t be divided
What are baryons?
Hadrons made up of three quarks
What are mesons?
Hadrons made up of two quarks (quark and anti-quark)
Charge on quarks - up
+ 2/3 e
Charge on quarks - down
- 1/3 e
Charge on quarks - charm
+ 2/3 e
Charge on quarks - strange
- 1/3 e
Charge on quarks - top
+ 2/3 e
Charge on quarks - bottom
- 1/3 e
What is the quark composition of a proton?
up, up, down
What is the quark composition of a neutron?
up, down, down
Baryon number - up, charm, top
+ 1/3
Baryon number - down, strange, bottom
+ 1/3
Lepton number - electron, tauon, muon
+ 1
Lepton number - neutrinos
- 1
Strangeness - strange quark
- 1
What is beta-minus decay?
neutron > proton + electron + anti-electron neutrino
What is beta-plus decay?
proton > neutron + positron + electron neutrino
What are the conservation laws in emission equations? (4)
Charge
Nucleon and proton number
Baryon number
Lepton number
What is alpha radiation? (4)
A helium nucleus
Charge = + 2e
Very ionising
Very short range - absorbed by thin sheet of paper
What is beta radiation? (4)
Fast-moving electrons
Charge = -e
Less ionising than alpha radiation
Range of about 1 metre - absorbed by 1-3 mm of aluminium
What is gamma radiation? (4)`
High-energy photons
Travel at the speed of light, carrying no charge
Least ionising
Long range - absorbed by a few centimetres of lead
How can the absorption of different forms of radiation be investigated? (2)
Set up a radioactive source opposite a GM tube, with an absorber between them
Measure the background radiation first in order to produce a corrected count rate after the experiment
When does gamma decay take place?
When a nucleus has surplus energy following an alpha or beta emission
Why is radioactive decay random? (2)
It is impossible to predict when a particular nucleus in the sample will decay
Each nucleus in the sample has the same chance of decaying per unit time
Why is radioactive decay spontaneous? (2)
It is not affected by the presence of other nuclei in the sample
It is not affected by external factors like pressure
Define half-life
The average time it takes for half the number of active nuclei in the sample to decay
Define activity
The rate at which nuclei decay
What does activity depend on? (2)
The number of undecayed nuclei present
The half-life of the isotope
What is the decay constant?
The probability of decay of an individual nucleus per unit time
How can half-life be measured experimentally? (3)
Determine the background count rate
Use a GM tube to record a 10 second count every half-minute
Plot a graph of corrected count rate against time to find half-life
What is carbon dating? (2)
The ratio of radioactive carbon-14 from the atmosphere to carbon-12 is measured in an artefact
The half-life of carbon-14 is used to calculate the age of the artefact
What is the half-life of carbon-14?
5700 years
What are the limitations of carbon dating? (2)
Assumes ratio of C-12:C-14 has remained constant over time whereas CO2 emissions, solar flares and testing nuclear bombs may have caused it to change
The activity of C-14 is very small and comparable to the background count rate
Why does carbon dating not work on rocks?
They do not contain carbon
Carbon-14 would have a half life that is too short to date rocks
What is used to date rocks?
Rubidium-87 because it has a half life of 49 billion years
What is mass?
A form of energy
How does mass change in radioactive decay? (2)
The mass of the daughter nuclei must be less than the mass of the parent nuclei as energy has also been released
The decrease in mass is equivalent to the energy released
Define mass defect
The difference between the mass of the completely separated nucleons and the mass of the nucleus
What causes the mass defect?
Work is done to separate the nucleons in a nucleus so the total mass of the separated nucleons must be greater than the mass of the nucleus
Define binding energy
The minimum energy required to completely separate a nucleus into its constituent nucleons
What does the graph of binding energy per nucleon against nucleon number show? (5)
For nuclei below iron, binding energy increases with nucleon number
For nuclei above iron, binding energy decreases with nucleon number
Iron is the most stable nucleus
Fusion takes place up to iron
Fission takes place in nuclei above iron
What is the process of induced fission? (3)
Uranium-235 undergoes fission when it absorbs a slow neutron, forming U-236
U-236 decays into two daughter nuclei (often barium and krypton) and releasing three neutrons
The change in mass causes energy to be released
Why does fission cause a chain reaction?
Three neutrons are released which can go on to react cause fission in other nuclei
Structure of a fission reactor - fuel rods
Spaced evenly within steel reactor core
Structure of a fission reactor - coolant
Removes thermal energy produced within the fissile fuel
Structure of a fission reactor - moderator (2)
Slows down fast neutrons by causing a transfer of KE through collisions
Uses cheap and readily available materials - usually `H-2 as this causes a larger change in KE
Structure of a fission reactor - control rods (3)
Absorb neutrons to control the chain reaction
Raised or lowered in the core to ensure that one neutron survives each reaction
Usually boron or cadmium
What is the environmental impact of nuclear fission? (3)
U-238 readily absorbs neutrons, quickly decaying into plutonium-239
Plutonium-239 is extremely toxic, radioactive and has a very long half-life
High level waste must be buried deep underground on secure sites to avoid food and water supplies
Why are high temperatures needed for nuclear fusion?
The nuclei need to be moving fast enough to overcome the electrostatic repulsion between them
Why is high pressure needed for nuclear fusion?
It moves the nuclei close enough to fuse
How would a nuclear fusion reactor be constructed? (3)
Superconducting coils - provide high current and strong magnetic field with zero resistivity at temperatures just above 0K
Lithium blanket - source of tritium fuel, used to remove heat and slow down neutrons
Water is heated to produce steam and turn turbine
What are the advantages of fusion over fission? (2)
Safer
There is enough fuel to provide power for millions of years
What must be true about binding energy for a nuclear reaction to release energy? (2)
Binding energy must increase
This decreases the energy in the nucleus, showing that some has been released
