Nuclear Flashcards
What was the Thomson Model of the atom
Thomson model of the atom, also known as the ‘plum pudding’ model. This model said that atoms were made up of a globule of positive charge, with negatively charged electrons sprinkled in it
Explain the Rutherford’s Scattering experiment
A stream of alpha particles from a radioactive source was fired at very thin gold foil.
When alpha particles from a radioactive source strike a fluorescent screen, a tiny visible flash of light is produced. Geiger and Marsden recorded these flashes, and counted the number of alpha particles scattered at different angles.
If the Thomson model was right, all the flashes should have been seen within a small angle of the beam. This wasn’t what they saw.
See pg 160 for diagram
What were the observations for the alpha particle scattering experiment
Geiger and Marsden observed that most alpha particles went straight through the foil, but a few scattered at angles greater than 90°, sending them back the way they came.
What were Rutherfords Conclusions from the experiments?
- Most of the fast, charged alpha particles went straight through the foil. So the atom is mainly empty space.
- Some of the alpha particles were deflected through large angles, so the centre of the atom must have a large, positive charge to repel them. Rutherford named this the nucleus.
- Very few particles were deflected by angles greater than 90 degrees, so the nucleus must be tiny and most of the mass must be in the nucleus (dense), since the fast alpha particles (with high momentum) are deflected by the nucleus.
So most of the mass and the positive charge in an atom must be contained within a tiny, central nucleus.
What is the proton number, what it symbol
The proton number is sometimes called the atomic number, and has the symbol Z.
Z is just the number of protons in the nucleus.
What defines and element?
The proton number (no two elements have the same)
What is the nucleon number
The nucleon number is also called the mass number, and has the symbol A.
It tells you how many protons and neutrons are in the nucleus.
What is an isotope
Atoms with the same number of protons but different numbers of neutrons are called isotopes.
How does changing the number of neutrons affect an atom’s properties?
Changing the number of neutrons doesn’t affect the atom’s chemical properties.
The number of neutrons affects the stability of the nucleus though.
Unstable nuclei may be radioactive.
What is the strong nuclear force ?
To hold the nucleus together, the strong nuclear force must be an attractive force that overcomes the electrostatic force. (the repulsive force between the positive charges of the protons)
How does strong nuclear force work with distance?
Experiments have shown that the strong nuclear force between nucleons has a short range. It can only hold nucleons together when they are separated by up to a few femtometres - the size of a nucleus.
The strength of the strong nuclear force between nucleons quickly falls beyond this distance.
How does the size of the strong nuclear force vary with the interaction of different nucleons
Experiments also show that the strong nuclear force works equally between all nucleons. This means that the size of the force is the same whether proton-proton, neutron-neutron or proton-neutron.
What happen to the nuclear force at small separations
At very small separations, the strong nuclear force must be repulsive - otherwise there would be nothing to stop it crushing the nucleus to a point.
What does a graph of strong nuclear force against electrostatic force look like
What are the Axis
See CGP page 164
Y axis : repulsion / attraction
X axis: nucleus separation
What cause nucleus instability?
- too many neutrons
- too many nucleons in total (too heavy)
- too few neutrons
- too much energy in the nucleus
What happens during nucleus decay
The nucleus decays by releasing energy and/or particles (nuclear radiation), until it reaches a stable form - this is called radioactive decay.
How do you predict when a specific particle will undergo radioactive decay
An individual radioactive decay is spontaneous and random
- it can’t be predicted.
Is it possible to predict the decaŷ of a large number of nuclei
Although you can’t predict the decay of an individual nucleus, if you take a very large number of nuclei, their overall behaviour shows a pattern.
6)
Any sample of a particular isotope (p.161) has the same rate of decay, i.e. the same proportion of nuclei will decay in a given time
What is nuclear fission
Heavy nuclei (e.g. uranium), are unstable. Some can randomly split into two smaller nuclei (and sometimes several neutrons) — this is called nuclear fission.
What does spontaneous or induced mean in terms of nuclear fission
This process is called spontaneous if it just happens by itself, or induced if we encourage it to happen.
Do flash cards on the different types of radiation
..
Why is energy released during nuclear fission?
Energy is released during nuclear fission because the new, smaller nuclei have a higher binding energy per nucleon and a lower total mass.
Are larger nuclei more or less likely to undergo fission than smaller nuclei and why
More likely because larger nuclei are less stable
Why are there only a certain number of elements, explain using nuclear spontaneous nuclear fission
This means that spontaneous fission limits the number of nucleons that a nucleus can contain
- in other words, it limits the number of possible elements.
How can fission be induced
Fission can be induced by making a neutron enter a 235U nucleus, causing it to become very unstable.
What is a name for a neutron that cab be absorbed by the nucleus
A low energy neutron is called a thermal neutron. Only low energy neutrons can be captured in this way by the nucleus
What is the fuel used for nuclear fission in a reactor
Nuclear reactors use rods of uranium that are rich in 235U as ‘fuel’ for fission reactions. (The rods also contain a lot of 238U, but that doesn’t undergo fission.)
How does a chain reaction occur in a nuclear reactor
These fission reactions produce more neutrons which then induce other nuclei to fission — this is called a chain reaction.
For a neutron to contribute to the chain reaction having been released what must happen to it and what causes this
The neutrons will only cause a chain reaction if they are slowed
cool water
down, which allows them to be captured by the uranium nuclei
- these slowed down neutrons are called thermal neutrons.
4) 235U fuel rods need to be placed in a moderator (for example, water) to
moderaton (water)
pump
slow down and/or absorb neutrons. You need to choose a moderator that will slow down some neutrons enough so they can cause further fission, keeping the reaction going at a steady rate.
What is critical mass?
You want the chain reaction to continue on its own at a steady rate, where one fission follows another.
The amount of ‘fuel’ you need to do this is called the critical mass — any less than the critical mass (sub-critical mass) and the reaction will just peter out. Nuclear reactors use a supercritical mass of fuel (where several new fissions normally follow each fission) and control the rate of fission using control rods.
What do control rods do?how are they used in an emergency?
Control rods control the chain reaction by limiting the number of neutrons in the reactor. They absorb neutrons so that the rate of fission is controlled. Control rods are made up of a material that absorbs neutrons (e.g. boron), and they can be inserted by varying amounts to control the reaction rate.
In an emergency, the reactor will be shut down automatically by the release of the control rods into the reactor, which will stop the reaction as quickly as possible.
What is coolant in a nuclear reactor used to do
Coolant is sent around the reactor to remove heat produced in the fission — often the coolant is the same water that is being used in the reactor as a moderator. The heat from the reactor can then be used to make steam for powering electricity-generating turbines.
What happens when fission is left unchecked with an example of how this is used
the chain reaction in a nuclear reactor is left to continue unchecked, large amounts of energy are released in a very short time.
Many new fissions will follow each fission, causing a runaway reaction which could lead to an explosion. This is what happens in a fission (atomic) bomb.
Pro and cons of Nuclear Fission Power Plants
Deciding whether or not to build a nuclear power station (and if so, where to build it) is a tricky business.
2)
Nuclear fission doesn’t produce carbon dioxide, unlike burning fossil fuels, so it doesn’t contribute to global warming (p.69). It also provides a continuous energy supply, unlike many renewable sources (e.g. wind/solar).
3)
4)
However, some of the waste products of nuclear fission are highly radioactive and difficult to handle and store.
When material is removed from the reactor, it is initially very hot, so it is placed in cooling ponds until the temperature falls to a safe level. The radioactive waste is then stored in sealed containers in specialist facilities until its activity has fallen sufficiently. This can take many years, and there’s a risk that material could escape from these containers. A leak of radioactive material could be harmful to the environment and local human populations both now and in the future, particularly if the material contaminated water supplies.
5)
Accidents or natural disasters pose a risk to nuclear reactors. In 2011 an earthquake and subsequent tsunami in Japan caused a meltdown at the Fukushima nuclear power plant. Over 100 000 people were evacuated from the area, and many tonnes of contaminated water leaked into the sea. The perceived risk of this kind of disaster leads many people to oppose the construction of nuclear power plants near their homes.
6)
Because of all of the necessary safety precautions, building and decommissioning nuclear power plants is very time-consuming and expensive.
What is nuclear fusion
Two light nuclei can combine to create a larger nucleus.
This is called nuclear fusion.
What type of fusion happens in the sun (with an equation)
In the Sun, hydrogen nuclei fuse to form helium:
See CGP for equation
What must nuclei overcome to undergo fusion
Nuclei can only fuse if they have enough energy to overcome the electrostatic (Coulomb) repulsion between them, and get close enough for the strong interaction to bind them.
What are the conditions required for fusion to occur
This means fusion reactions require much higher temperatures than fission, as well
as high pressures (or high densities). Under such conditions, generally only found inside stars, matter turns into a state called a plasma.
Why is energy released during nuclear fusion?
A lot of energy is released during nuclear fusion because the new, heavier nucleus has a much higher binding energy per nucleon (and so a lower total mass). The energy released helps to maintain the high temperatures needed for further fusion reactions.
Energetically which is better fusion or fission?
Although the energy released per reaction is generally lower in nuclear fusion than fission, the nuclei used in fusion have a lower mass, so a mole of the reactants in a fusion reaction weighs less than a mole of the reactants in a fission reaction. Gram for gram, fusion can release more energy than fission.
Do we currently have fusion reactors?
No, scientists are trying to develop fusion reactors so that we can generate nuclear electricity without the waste you get from fission reactors, but they haven’t yet succeeded in creating one that makes more electricity than it uses.
Not expiation booklet
P=p0e^ht
What is the equation that relates nuclear radius to nucleon number ?
R=r0A^1/3
R is radius of nucleus
r0 is constant equal to 1.4x10^-15 or 1.4fm
A is nucleon number
Generally what is the value of nuclear density
Roughly 10^17kgm^-3
(Could be worth knowing to check answer)
What does the fact nuclear density is higher than atomic density suggests
Most of an atoms mass is in its nucleus
The nucleus is small compared to the atom
An atom must contain a lot of empty space
What are hadrons
Particles that feel the strong nuclear force
what is fundamental particle
fundamental particle is a subatomic particle that is not composed of other particles.
Are hadrons are fundamental particle
No - they are made up of quarks
What are some example of hadrons
Protons and neutrons are hadrons. This is why they can make atomic nuclei — the nucleus of an atom is made up from protons and neutrons held together by the strong nuclear force
there are other hadrons that you don’t get in normal matter, like sigmas and mesons
