Physics Topic 4 - Atomic Structure

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What did John Dalton believe about the atom? (1804)
He believed that atoms were tiny spheres than couldn't be broken down into anything smaller, and they made up matter. Each element was made up of a different type of atom.
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How did J.J Thomson change this?
He discovered electrons and therefore suggested that atoms were spheres of positive charge with tiny negative electrons stuck in them (plum pudding model)
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What happened after the alpha particle experiment in 1909?
Rutherford and Marsden realised that most of the mass of the atom must be concentrated in the centre in a tiny nucleus, which has a positive charge. The rest of the atom must have been empty space, a cloud of negative electrons- the nuclear model.
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Who else contributed to the nuclear model as we know it?
Neils Bohr said that electrons orbited the nucleus at certain distances called energy levels. James Chadwick proved the existence of the neutron in 1932.
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What is the radius of an atom?
1 x 10 ^-10 m
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What is an isotope?
An atom with the same number of protons but a different number of neutrons (and therefore the same atomic number but a different mass number).
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What happens to unstable isotopes?
They decay into other elements and give out radiation as they try to become more stable- this is called radioactive decay.
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What are the three types of ionising radiation that radioactive substances give out?
Alpha, beta and gamma radiation.
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What else can isotopes do when they decay?
Release neutrons as they rebalance their atomic and mass numbers.
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What does ionising radiation do and what is ionising power?
It knocks electrons off atoms, creating positive ions. The ionising power is how easily the source can do this.
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When does alpha radiation occur?
This is when an alpha particle (two neutrons and two protons) is emitted from the nucleus.
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What are the properties of alpha particles?
They don't penetrate very far into materials and are stopped quickly- they can only travel a few cm in air and are absorbed by paper. They are strongly ionising because of their size.
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Where is alpha radiation used?
In smoke detectors- it ionises air particles causing a current to flow. If there is smoke in the air, it binds to the ions so the current stops and the alarm sounds.
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What are beta particles?
They are fast moving electrons released by the nucleus, with virtually no mass and a charge of -1. For every beta particle emitted, a neutron in the nucleus has turned into a proton.
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What are the properties of beta particles?
They are moderately ionising and penetrate moderately far into materials before colliding. They have a range in air of a few metres and are absorbed by a sheet of aluminium (5mm).
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Where are beta emitters used?
To test the thickness of sheets of metal, as they are not immediately absorbed and do not penetrate too far.
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What are gamma rays?
They are waves of electromagnetic radiation released by the nucleus.
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What are the properties of gamma rays?
They penetrate far into materials without being stopped and will travel a long distance through air. They are weakly ionising because they tend to pass through rather than collide with atoms. They can be absorbed by thick sheets of lead or concrete.
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What are nuclear equations and how are they written?
They show radioactive decay by using element symbols. They are written as: atom before decay --> atom after decay + radiation emitted
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What happens to an atom when it emits an alpha particle?
Its atomic number reduces by 2 and its mass number reduces by 4. The charge of the nucleus decreases.
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How is an alpha particle written in nuclear equations and give an example.
It can written as a Helium nucleus. U --> Th + He (238 --> 234 + 4, and 92 --> 90 + 2)
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What happens when beta decay occurs?
A neutron in the nucleus turns into a proton and releases a fast-moving electron. The number of protons in the nucleus has increased by 1, increasing the positive charge of the nucleus. The mass doesn't change.
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Give an example of a nuclear equation for beta decay.
C (14) --> N (14) + e (-1) (14 --> 14 + 0, and 6 --> 7 - 1)
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What happens when gamma radiation occurs?
They get rid of excess energy from a nucleus and therefore don't change the atomic mass or atomic number of the atom.
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How can radiation be measured?
With a Geiger-Muller tube and counter, which records the count-rate.
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Can radioactive decay be predicted?
No, it is entirely random. You can't predict which nucleus in a sample will decay next or when any of them will decay.
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What happens each time a radioactive nucleus decays to become a stable nucleus?
The activity as a whole will decrease, so older sources emit less radiation.
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What is the problem with trying to measure radioactive decay?
It never reaches zero, so we have to use half-life to measure how quickly the activity drops off.
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What is half-life?
This is the time taken for the number of radioactive nuclei in an isotope to halve.
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What can half-life be used for?
To find the rate at which a source decays- its activity. Activity is measured in becquerels (Bq), where 1 Bq=1 decay per second.
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What does a short half-life mean?
It means that the activity falls quickly, because the nuclei are very unstable and rapidly decay. These sources are dangerous because of the high amount of radiation initially but they quickly become safe.
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How do you find the half life from a graph?
Find the time interval on the bottom axis corresponding to a halving of the activity on the vertical axis.
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What is background radiation?
This is the low-level radiation that's around us all the time. You should always measure it and subtract it from your results.
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Where does it come from?
Natural occurring unstable isotopes (e.g. in food, materials, rocks), space (e.g. cosmic rays from the sun), human activity (e.g. nuclear explosions/waste)
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What does radiation dose tell you and what is it measured in?
It tells you the risk of harm to body tissues due to radiation, and is measured in Sieverts (SV) or often milisieverts (mSv)
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What is exposure to radiation called?
Irradiation- we are always being irradiated by background radiation sources. It doesn't make something radioactive.
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How can you reduce the effects of irradiation?
Keeping sources in lead-lined boxes, standing behind barriers, being in a different room, sing remote controlled arms.
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What is contamination?
This is when unwanted radioactive atoms get onto or into an object. These contaminating atoms might decay, releasing radiation and causing harm.
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How can you avoid contamination?
Use gloves and tongs, wear protective suits.
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Why are beta and gamma sources the most dangerous outside the body?
This is because they can penetrate the body and reach the organs, whereas alpha radiation can't penetrate the skin and is easily blocked by a small air gap.
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Why are alpha sources the most dangerous inside the body?
The is because they do all their damage in a very localised area, and have the strongest ionising power.
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What are the risks of using radiation?
They can enter living cells and ionise atoms, leading to tissue damage. In lower doses, it can damage cells and cause them to mutate (cancer). Higher doses may cause radiation sickness (vomiting, tiredness, hair loss) as cells are killed completely.
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How is gamma radiation used in medical tracers?
Sources can be injected/swallowed by patients and their progress around the body can be followed used an external detector. A computer converts the reading to a display showing where the strongest reading is coming from.
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What is an example of this?
Iodine- 123 which is absorbed by the thyroid gland, just like iodine -127, but gives out radiation which can be detected to indicate whether the thyroid gland is taking in iodine as it should.
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How can radiation be used to treat cancers?
Gamma rays are directed carefully and at just the right dosage to kill the cancer cells without damaging too many normal cells. Radiation emitting implants can also be put next to or inside tumours.
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What is nuclear fission?
A type of nuclear reaction that is used to release energy from large and unstable atoms (e.g. uranium or plutonium) by splitting them into smaller atoms.
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What usually has to happen before nuclear fission takes place?
The nucleus has to absorb a neutron. When the atom splits it forms two new lighter elements that are roughly the same size, with some energy in their kinetic stores.
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What else happens when an atom splits?
Two or three neutrons are released. If these are moving slow enough, they may be absorbed by another nucleus and cause more fission to occur- a chain reaction.
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What happens to the kinetic energy not transferred to the products and how can it be used?
It is carried away by gamma rays. The energy can then be used to heat water, making steam to turn turbines and generators.
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How is the amount of energy produced by fission in a nuclear reactor controlled?
By changing how quickly the chain reaction occurs. This is done using control rods, which are lowered and raised inside a nuclear reactor to absorb neutrons, slow down chain reactions and control the energy released.
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What happens when chain reactions aren't controlled?
Lots of energy is quickly released as an explosion- this is how nuclear weapons work.
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What is nuclear fusion?
This is where two light nuclei collide at high speed and fuse to create a larger, heavier nucleus. For example, hydrogen nuclei can fuse to produce a helium nucleus.
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Why happens to the mass lost by the two operate light nuclei?
It is converted to energy, which is then released as radiation. The heavier nucleus then has less mass than the separate nuclei did.
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Fusion releases lots of energy- why do we not use it to generate energy for us to use?
The temperatures and pressures needed for fusion are so high that fusion reactors are really hard and expensive to build.
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Card 2

Front

How did J.J Thomson change this?

Back

He discovered electrons and therefore suggested that atoms were spheres of positive charge with tiny negative electrons stuck in them (plum pudding model)

Card 3

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What happened after the alpha particle experiment in 1909?

Back

Preview of the front of card 3

Card 4

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Who else contributed to the nuclear model as we know it?

Back

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Card 5

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What is the radius of an atom?

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