Physics - Topic 4 - Atomic structure
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- Created by: hannah.o
- Created on: 07-02-18 18:10
What was the plum pudding model?
Before 1914, some scientists thought that the atom was like a 'plum pudding' with positively charged matter evenly spread out, and electrons buried inside.
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What did Ernest Rutherford do?
Discovered alpha+beta radiation is made up of different types of particles. He realised alpha particles could be used to probe atom. He asked two of his workers, Geiger+Marsden, to investigate how a thin metal foil scatters a beam of alpha particles.
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What did the results of his alpha particle investigation show?
Most of the alpha particles passed straight through the metal foil. The number of alpha particles deflected per minute decreased as the angle of reflection increased. About 1 in 10,000 alpha particles were deflected by more than 90'.
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What did Rutherford deduce from these results?
There is a positively charged nucleus at the centre of every atom that is much smaller than the atom and is where most of the mass of the atom is located.
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What did Bohr say?
Electrons orbiting the nucleus do so at certain distances called energy levels. His theoretical calculation agreed with experimental data.
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What does the current model of the atom look like?
.
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What do all atoms of each element have?
A set number of protons (so each nucleus has a given positive charge). The number of protons in an atom is its atomic number.
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What does the mass number of an atom involve?
Number of protons + number of neutrons
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What are isotopes?
Isotopes of an element are atoms with the same number of protons but a different number of neutrons.
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What do unstable isotopes tend to do?
They tend to decay into other elements and give out radiation as they try to become more stable. This process is called radioactive decay.
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What do radioactive substances do?
They spit out one or more types of ionising radiation from their nucleus - alpha, beta, and gamma radiation. They can also release neutrons when they decay, as they rebalance their atomic and mass numbers.
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What is ionising radiation?
Ionising radiation is radiation that knocks electrons off atoms, creating positive ions. The ionising power of a radiation source is how easily it can do this.
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What is alpha radiation?
Alpha radiation is when an alpha particle is emitted from the nucleus. An alpha particle is two neutrons and two protons (like a helium nucleus).
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What do alpha particles do?
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 a sheet of paper. Because of their size, they are strongly ionising.
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Where is alpha radiation used?
Alpha radiation is 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 - meaning the current stops and the alarm sounds.
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What is a beta particle?
A beta particle is simply a fast-moving electron released by the nucleus. Beta particles have virtually no mass and a charge of -1.
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What do beta particles do?
They are quite ionising. They travel quite far into materials before colliding+have a range in air of a few ms. They are absorbed by sheet of aluminium (around 5mm). For every beta particle emitted, a neutron in the nucleus has turned into a proton.
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What are beta emitters used as?
Beta emitters are used to test the thickness of sheets of metal, as the particles are not immediately absorbed by the material like alpha radiation would be and do not travel as far as gamma rays.
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What are gamma rays?
Gamma rays are waves of electromagnetic radiation released by the nucleuss.
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What do gamma rays do?
They travel far into materials without being stopped and will travel a long distance through air. This means they are weakly ionising as they tend to pass through rather than collide with atoms. Eventually they hit something and do damage.
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What can gamma rays be absorbed by?
Thick sheets of lead or metres of concrete.
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What do nuclear equations show?
Radioactive decay by using element symbols.
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What form are nuclear equations written in?
atom before decay -> atom after decay + radiation emitted
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What is the golden rule to remember?
The total mass and atomic numbers must be equal on both sides.
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Uranium-238 -Alpha decay-> Thorium-234-> alpha particle. What is the nuclear equation for this decay?
.
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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 beta particle). The number of protons in the nucleus has increased by 1. This increases the positive charge of the nucleus.
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""
Because the nucleus has lost a neutron and gained a proton during beta decay, the mass of the nucleus doesn't change.
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What is a beta particle written as in nuclear equations?
e
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What can gamma rays get rid of?
Gamma rays are a way of getting rid of excess energy from a nucleus. This means that there is no change to the atomic mass or atomic number of the atom.
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How can radiation be measured?
Radiation given out by radioactive substances can be measured with a Geiger-Muller tube and counter, which records the count-rate - the number of radiation counts reaching it per second.
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Is radioactive decay predictable?
No because radioactive decay is entirely random. So you can't predict exactly which nucleus in a sample will decay next, or when any one of them will decay.
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What can you find out about the time of radioactive decay?
The time it tales for the amount of radiation emitted by a source to halve, this is known as the half-life. It can be used to make predicitions about radioactive sources, even though their decays are random.
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What can half-life be used to find?
The rate at which a source decays - its ACTIVITY. Activity is measured in becquerels, Bq (where 1 Bq is 1 decay per second).
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What happens each time a radioactive nucleus decays to become a stable nucleus?
The activity as a whole will decrease (older sources emit less radiation).
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How long do isotopes take to decay?
For some isotopes it takes just a few hours before nearly all the unstable nuclei have decayed, whilst others last for millions of years.
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What is the problem with trying to measure the time it takes for radioactive decay to happen?
The problem is that the activity never reaches 0, which is why we have to use the idea of half-life to measure how quickly the activity drops off.
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What is the half-life?
The half-life is the time taken for the number of radioactive nuclei in an isotope to halve.
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What does a short half-life mean?
It means the activity falls quickly, because the nuclei are very unstable and rapidly decay. Sources with a short half-life are dangerous because of the high amount of radiation they emit at the start, but they quickly become safe.
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What does a long half-life mean?
It means the activity falls more slowly because most of the nuclei don't decay for a long time-source just sits there, releasing small amounts of radiation for a long time. This can be dangerous because nearby areas are exposed to radiation for years
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What is the shape of a half-life graph?
.
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How is the half-life found from the graph?
The half-life is found from the graph by finding 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?
Background radiation is the low-level radiation that's around us all the time. You should always measure and subtract the background radiation from your results, to avoid systematic errors.
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Where does background radiation come from? (1)
Radioactivity of naturally occurring unstable isotopes which are all around us - in the air, in food, in building materials and in the rocks under our feet.
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Where does background radiation come from? (2)
Radiation from space, which is known as cosmic rays. These come mostly from the Sun. Luckily, the Earth's atmosphere protects us from much of this radiation.
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Where does background radiation come from? (3)
Radiation due to human activity, e.g. fallout from nuclear explosions or nuclear waste. But this represents a tiny proportion of the total background radiation.
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What does the radiation dose tell us?
The risk of the harm to body tissues due to exposure to radiation. It's measured in sieverts (Sv).
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What is irradiation?
Exposure to radiation
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What happens to objects near a radioactive source?
They are irradiated by it.
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What are some ways of reducing the effects of irradiation?
Keeping sources in lead-lined boxes, standing behind barriers or being in a different room and using remote-controlled arms.
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What is an object called if unwanted radioactive atoms get onto or into an object?
Contaminated
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What could happen to those contaminating atoms?
They might decay, releasing radiation which could cause you harm.
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Why is contamination dangerous?
Because radioactive particles could get inside your body.
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What should be used when handling radioactive sources?
Gloves and tongs, to avoid particles getting stuck to your skin or under your nails. Some industrial workers wear protective suits to stop them breathing in particles.
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What makes the seriousness of irradiation and contamination differ?
The radiation type.
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Which type of radiation is more dangerous outside the body?
Beta and gamma sources are the most dangerous outside the body because beta and gamma can penetrate the body and get to the delicate organs. Alpha is less dangerous because it can't penetrate the skin and is easily blocked by a small air gap.
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Which type of radiation is more dangerous inside the body?
Alpha sources are the most dangerous because they do all their damage in a very localised area. So contamination, rather than irradiation, is the major concern when working with alpha particles.
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Beta sources are less damaging inside the body, as radiation is absorbed over a wider area, and some passes out of the body. Gamma sources are the least dangerous inside the body, as they mostly pass straight out - they have the lowest ionising power
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What are the risks to using radiation?
Radiation can enter living cells + ionise atoms and molecules with them. This can lead to tissue damage. Lower doses tend to cause minor damage without killing the cells. This can give rise to mutant cells which divide uncontrollably. This is cancer.
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Higher doses tend to kill cells completely, causing radiation sikness (leading to vomiting, tiredness and hair loss) if a lot of cells all get blatted at once.
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How can certain radioactive isotopes be used?
They can be injected into people and their progress around the body can be followed using 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 these certain radioactive isotope?
Iodine-123, which is absorbed by the thyroid gland just like normal iodine-127, but it gives out the radiation which can be detected to indicate whether the thyroid gland is taking in iodine as it should.
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What are the isotopes taken into the body?
Usually gamma (never alpha), so that the radiation passes out of the body without causing much ionisation. They should have a short half-life so the radioactivity inside the patient quickly disappears.
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What can gamma rays be used for treating?
Cancer, since high doses of ionising radiation will kill all living cells.
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What happens when treating cancer (radiotherapy)?
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 (usually beta-emitters) can also be put next to or inside tumours.
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What is a disadvantage of radiotherapy?
A fair bit of damage is done to normal cells, which makes the patient feel very ill.
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What is perceived risk?
How risky a person thinks something is. It's not the same as the actual risk of the procedure and the perceived risk can vary from person to person.
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What is nuclear fission?
Nuclear fission is a type of nuclear reaction that is used to release energy from large and unstable atoms by splitting them into smaller atoms.
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What fission rarely happens?
Spontaneous. Usually, the nucleus has to absorb a neutron before it will split.
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What happens when the atom splits during nuclear fission?
It forms two new lighter elements that are roughly the same size. 2 or 3 neutrons are released. If any of these neutrons are moving slow enough to be absorbed by another nucleus, they can cause more fission - a chain reaction.
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Draw the fission reaction with U
.
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What happens to the energy not transferred to the kinetic energy stores?
It is carried away by gamma rays.
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What can this energy be used to do?
To heat water, making steam to turn turbines and generators.
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What is the amount of energy produced by fission in a nuclear reactor controlled by?
It is controlled by changing how quickly the chain reaction can occur. This is done using control rods, which are lowered and raised inside a nuclear reactor to absorb neutrons, slow down the chain reaction and control the amount of energy released.
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What can uncontolled chain reactions lead to?
They can quickly lead to lots of energy being released as an explosion - this is how nuclear weapons work.
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What is nuclear fusion?
It is the opposite of nuclear fission.
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What happens in nuclear fusion?
Two light nuclei collide at high speed and join (fuse) to create a larger, heavier nucleus. For example, hydrogen nuclei can fuse to produce a helium nucleus.
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What is different about the heavier nucleus produced?
It does not have as much mass as the two separate, light nuclei did. Some of the mass of the lighter nuclei is converted to energy. This energy is then released as radiation.
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What does fusion release?
A lot of energy - more than fission for a given mass of fuel
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Have scientists found any use of fusion?
Scientists haven't found a way of using fusion to generate electricity for us to use. The temperatures and pressures are so high that fusion reactors are really hard and expensive to build.
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Other cards in this set
Card 2
Front
What did Ernest Rutherford do?
Back
Discovered alpha+beta radiation is made up of different types of particles. He realised alpha particles could be used to probe atom. He asked two of his workers, Geiger+Marsden, to investigate how a thin metal foil scatters a beam of alpha particles.
Card 3
Front
What did the results of his alpha particle investigation show?
Back
Card 4
Front
What did Rutherford deduce from these results?
Back
Card 5
Front
What did Bohr say?
Back
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