AQA P2 2.5 What happens when radioactive substances decay, and the uses and dangers of their emissions

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  • Created by: Rchilds
  • Created on: 06-06-17 21:40
Describe the basic structure of an atom
small central nucleus composed of protons and neutrons surrounded by electrons
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What are the relative masses and relative electric charges of protons, neutrons and electrons?
charges: p=+1, n=0, e=-1 masses: p=1, n=1, e=0 (very small)
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What is the relative size of the nucleus compared to the size of the atom?
The nucleus is about 100,000 times smaller than the atom (if nucleus was a football, atom would be a football stadium.... most of the atom is empty space!)
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Why is an atom neutral overall?
It has the same number of protons as electrons so the positive charges are perfectly cancelled out by the negative charges
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What does ionisation mean?
When an atom loses or gain electrons so become a charged ion
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What decides what type of element an atom is?
how many protons it has e.g. helium ALWAYS has 2 protons, carbon ALWAYS has 6 protons etc
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What are isotopes?
Two atoms of the same element that have the same number of protons but a different number of neutrons (so same atomic number but different mass number).
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What is radioactive decay?
When the nucleus of an atom is unstable, it will break down and release radiation. This can be an alpha particle, beta particle or gamma ray.
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What can control radioactive decay?
Nothing. It is completely natural, random and spontaneous.
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What happens to radioactive decay if you heat up the radioactive atom?
Nothing. It is completely natural, random and spontaneous so you can't change, predict or control it.
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What is the plum pudding model?
An old (incorrect) model of an atom where the whole atom was a sphere of uniform positive charge with small negative electrons dotted through it
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What has not been discovered yet when scientists thought the atom looked like the plum pudding model?
they had discovered the nucleus, protons (although they knew there was positive charge, just not that it was split into several protons) or neutrons. Electrons had been discovered but not the electron shells
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Which experiment discovered the nucleus?
Rutherford and Marsden's alpha scattering experiment
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Describe the set up of Rutherford and Marsden's alpha scattering experiment
An alpha source was placed in lead shielding, in a vacuum, and the alpha particles released were fired and thin, gold foil. The paths of the alpha particles was studied by detecting where the alpha particles went using a flash of light on a screen
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Why did the alpha scattering experiment prove the plum pudding model wrong?
The plum pudding model couldn't explain the results. It only predicted that alpha particles would go thorough and only be deflected by small angles but the experiment found that the alphas were also defected by larger angles and deflected backwards.
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Why was there a flash of light?
The screen was covered in a chemical. When an alpha particle hits the screen it causes a flash of light (kinetic energy of alpha particle turns into light energy). 1 flash = 1 alpha particle so they could locate them.
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Why was a vacuum used?
Because alpha particles cannot travel very far in air (only a few cm) as they collide with the air particles, this would have interfered with the experiment
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Why was lead shielding used?
To absorb alpha particles going in the wrong direction. A small hole was left so only a narrow beam of alpha particles could escape, directed at the gold foil.
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What 3 observations were made that led to the conclusion that there must be a nucleus? (2 cards)
(1) most of the alpha particles went straight through the gold THEREFORE most of the atom is empty space. (2) Some alpha particles were deflected at large angles THEREFORE the nucleus must have a large positive charge that is ...
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conclusions continued...
...repelling the positive alpha particle. (3) A small number of the alpha particles were deflected backwards THEREFORE the nucleus must be very small, but also very dense (have most of the mass of the atom in)
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What 2 factors cause some alpha particles to be defected more than others?
They must have experienced a larger repulsive force because they were closer to the nucleus OR they must have experienced the force for longer because they were moving slower (less kinetic energy)
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What is an alpha particle?
2 protons and 2 neutrons - the same as a helium nucleus
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When will an nucleus decay and give out an alpha particle?
When it is unstable because it is too big
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When will an nucleus decay and give out a beta particle?
When it is unstable because it has too many neutrons (compared to it's protons) therefore a neutron breaks down and turns into a proton and an electron (the electron is the beta particle)
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What is a beta particle?
A fast moving electron that is fired out of the nucleus
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When will a nucleus give out gamma radiation?
When it is unstable because it has given out alpha or beta but still has too much energy
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What is gamma radiation?
An electromagnetic wave with a very high frequency
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For radioactive decay, how do you balance a nucleus equation? (note: they look similar to chemical equations but include the mass and atomic numbers)
Look at the mass numbers first, the sum of the mass numbers on the left of the arrow must equal the sum of the mass numbers on the right side because mass is conserved. The same rule also applies for the atomic numbers (because charge is conserved)
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What is the mass and (atomic) charge number of an alpha particle?
mass = 4, charge = +2 (same as helium nucleus)
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What is the mass and (atomic) charge number of a beta particle?
mass = 0, charge = -1 (same as an electron)
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What is the mass and (atomic) charge number of a gamma ray?
mass = 0, charge = 0 (because it's a wave)
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Compare the ionisation powers of alpha, beta and gamma
alpha = highly ionising (because of its high mass and charge), beta = middle, gamma = least ionising
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Compare the penertration powers of alpha, beta and gamma
alpha = least penetrating (high mass and charge means it's more likely to collide with other particles and lose its energy), beta = middle, gamma = most penertrating
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How far can alpha, beta and gamma travel in air?
alpha = approx. 5cm; beta = approx, 1m; gamma = several kilometers
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What materials can stop alpha, beta and gamma?
alpha = paper, beta = approx 2mm of aluminium, gamma = thick lead (at least 10cm will reduce it, not stop it completely)
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How are alpha, beta and gamma effected by electric and magnetic fields?
alpha is deflected slightly, beta is deflected more (because they have charge), gamma is not effected at all (because it has no charge)
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How is the deflection of alpha and beta in magnetic/electric fields different?
They are deflected in opposite directions because they have opposite charges. Alpha is deflected less than beta because its mass is so high (so the force has less effect)
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Why are alpha, beta and gamma dangerous to humans?
Because they are IONISING. If they ionise your CELLS, they can KILL or MUTATE the cell which may lead to CANCER
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How are alpha, beta and gamma sources used safely?
Users should LIMIT THEIR EXPOSURE TIME, wear protective clothing, never touch the sources directly e.g. use tongs, stand behind metal shields if possible, all sources should be clearly labelled
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What does half life mean?
The average time it takes for the activity of a radioactive sample to halve
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What is activity and what is it's unit?
It is how many decays happen per second and is measured in Becquerels (Bq)
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How does the activity of a radioactive sample change over time?
It decreases (exponentially)
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How do you find half life from an activity vs. time graph?
Look at the starting activity, halve it and look at the time for that activity. (double check by choosing another activity, halving it looking at the time DIFFERENCE between the two activities). Half life should stay the same for a given isotope
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What should you consider when choosing a radioactive source for a particular job?
1) How ionising and/or penetrating it needed to be (hence choose alpha beta or gamma, if more than one would work, then choose the least ionising one as it's safer) (2) How long it's half life needs to be
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What type of radioactive source should be used in a smoke alarm and why?
Alpha source with a long half life - smoke alarms need to ionise the air and alpha is the most ionising, it only has a short range and can be easily blocked so won't affect people in the room, long half life so it doesn't need replacing (cheaper)
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What type of radioactive source should be used in a thickness gauge (e.g. for paper or aluminium) and why?
Beta with a long half life - the amount of beta that gets through depends on the thickness so it can be monitored, alpha would be stopped completely, all of gamma would get through. Long half life so machine doesn't need to be replaced (cheaper)
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What type of radioactive source should be used to detect leaks in underground water pipes and why?
Gamma source with a short half life - gamma so it can penetrate the ground and be detected, short half life so it is out of the water supply quickly before people drink/bathe in it
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What type of radioactive source should be used in radiotherapy to kill cancerous cells and why?
Gamma source with a long half life (the source is in the machine releasing gamma towards the patient, not inside the patient) long HL so it doesn't need replacing (cheaper), gamma so it can penetrate the body and reach the tumour
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What type of radioactive source should be to kill bacteria in food (irradiation of food) and on medical equipment (sterilisation) and why?
Gamma source with long half life. Long half life so it doesn't need replacing (source is in a machine firing gamma, not in the food/medical equipment so they do not become radioactive), gamma so it can penetrate through packaging
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What is a medical tracer?
A radioactive gamma source is injected into or ingested (eaten) by the patient and travels in the blood stream. Tumours need a good blood supply so an external detector can be used to trace where the blood is going and map the inside of the body
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What type of radioactive source should be used as medical tracers and why?
gamma source, short HL. Tracer is injected/ingested so short HL means it will be out of the body quickly. Gamma so it can penetrate out of the body to a detector, and is least ionising so safest for patient
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What is background radiation and is it dangerous?
It is radiation (usually gamma) that is all around us from natural and man-made sources. It is at a low level of radiation so is safe.
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What are the sources of background radiation?
Natural: cosmic rays from space, rocks, radon gas, all living things. Man-made: nuclear weapons tests, nuclear accidents, hospital waste, nuclear power station waste
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Why do different parts of the country have different levels of background radiation?
They might be on different rock types, some rocks contain more radioactive sources than others. They might be nearer to man made sources.
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Why do airline pilots and astronauts receive more background radiation than the average person?
The atmosphere protects us from many cosmic rays, they have less atmosphere between them and space so receive more cosmic rays (but it's still so low that it's safe compared to normal dose)
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Why are all living things slightly radioactive?
All living things have carbon in them. Normal (stable) carbon has a mass number of 12, but there is an isotope with two extra neutrons (carbon-14) that is radioactive. (this is used in carbon dating)
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Where does radon gas come from?
Uranium is naturally found in rocks, when it decays ones of the products is radon gas which is radioactive and gradually seeps out of the rocks
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What detectors can detect radiation (alpha, beta, gamma)
Geiger-Muller tube and photographic film
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If you are trying to measure the activity of a sample, how should you deal with background radiation?
Take the source out of the room, measure the background radiation in the place where you want to do your experiment several times and find an average. Bring the source back and do the experiment. Subtract the background reading from your result.
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Other cards in this set

Card 2

Front

What are the relative masses and relative electric charges of protons, neutrons and electrons?

Back

charges: p=+1, n=0, e=-1 masses: p=1, n=1, e=0 (very small)

Card 3

Front

What is the relative size of the nucleus compared to the size of the atom?

Back

Preview of the front of card 3

Card 4

Front

Why is an atom neutral overall?

Back

Preview of the front of card 4

Card 5

Front

What does ionisation mean?

Back

Preview of the front of card 5
View more cards

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