Topic 4 - Atomic Structure Flashcards
Which group of historic people were the first to think about atoms?
The greeks
What did Democritus think about matter? When? (2)
- Thought all matter was made up of identical lumps called ‘atomos’.
- 5th century BC
What did John Dalton think/do regarding the understanding of atoms? When? (3)
- He agreed with Democritus that matter is made up of tiny spheres that couldn’t be broken up.
- But thought each element was made up of different types of atoms.
- 1804
What did JJ Thomson discover? When? (2)
- Discovered particles called electrons that could be removed from atoms.
- Around 1905
What did JJ Thomson’s discovery mean for John Dalton’s theory? What did Thompson suggest instead? (2)
- Dalton’s theory was now proved incorrect.
- Thomson suggested that atoms were spheres of positive charge with tiny negative electrons stuck in them - ‘plum pudding model’.
What was Rutherford’s experiment? How did it work? (3)
- Alpha Particles Scattering Experiment.
- Fired beam of alpha particles at thin gold foil.
- A circular detector screen surrounds the gold foil and the alpha source, and is used to detect alpha particles deflected by an angle.
What was the expected outcome of the alpha particles scattering experiment?
Expected that the positively charged alpha particles would go straight through or be slightly deflected by electrons if plum pudding model was true.
What was the actual outcome of the alpha particle scattering experiment? (3)
- Most particles went straight through foil.
- Some deflected back more than expected and some came completely out.
- Showed that atoms must have small, positively charged nuclei at centre.
Why did the alpha particle scattering experiment show that atoms must have small, positively charged nuclei at their centre? (3)
- Most of atom = empty space as most alpha particles passed straight through the foil.
- Nucleus must have large positive charge as some positively charged alpha particles were repelled and deflected by a big angle.
- Nucleus must be small as very few alpha particles were deflected back.
How did Niels Bohr adapt the initial nuclear model, which had been derived from the alpha particle scattering experiment? Was it accepted? Why? (3)
- He concluded that electrons orbiting the nucleus can only do so at certain distances.
- These distances are called energy levels.
- Bohr’s theoretical calculations were found to agree with experimental data, so the model was accepted.
How did evidence from further experiments change Niels Bohr’s nuclear model?
Evidence changed the model to think of the positively charged nucleus as a group of particles (protons) which all had the same positive charge that added up to the overall charge of the nucleus.
What did James Chadwick do after the idea of the nucleus was accepted? When? (2)
- He proved the existence of the neutron, which explained the imbalance between atomic and mass numbers.
- 1932 - about 20 years after the idea of the nucleus was accepted.
What 3 types of particle does the atom contain, according to the nuclear model?
- Electrons => negatively charged
- Protons => positively charged
- Neutrons => neutral/no charge
Where is the nucleus? What is the mass of the nucleus? What does it contain? The size? (4)
- At centre of atom.
- Tiny but makes up most of the mass of the atom.
- Contains protons and neutrons - giving it an overall positive charge.
- Radius of nucleus is about 10,000 times smaller than the radius of the atom.
What gives the atom its overall size?
The negative electrons which move round the outside of the nucleus really fast.
What is the radius of an atom?
About 1 x 10(-10) m
What is the relative mass and charge of the proton?
- 1
- +1
What is the relative mass and charge of the neutron?
- 1
- 0
What is the relative mass and charge of the electron?
- 1/2000
- -1
Do atoms have an overall charge? Why? (2)
- No overall charge.
- The charge of an electron is equal and opposite to the charge of a proton meaning the number of protons always equals the number of electrons in a neutral atom.
Can electrons move within or leave energy levels of an atom? How?
- Electrons can move within or sometimes leave the energy levels of an atom.
- If they gain energy by absorbing EM radiation they move to a higher energy level, further from the nucleus.
- If they release EM radiation, they move to a lower energy level that is closer to the nucleus.
What is the atomic number?
The number of protons in the nucleus of an atom.
What is the mass number?
The number of protons plus the number of neutrons in the nucleus of an atom.
What is an ion? (2)
- Atoms are neutral, but if some electrons are added or removed, the atom becomes a charged particle called an ion.
- The ions still have the same number of protons and neutrons as usual, but a different number of electrons.
What is ionisation?
If an atom has had electrons added or removed. and has become an ion, it is said to have been ionised. This process is called ionisation.
What are isotopes? (3)
- Isotopes are different forms of the same element.
- Isotopes have atoms with the same number of protons but a different number of neutrons.
- This means they have the same atomic number (same charge on nucleus), but different mass numbers.
Do all elements have different isotopes? What quality do unstable isotopes have? (2)
- All elements have different isotopes, but there are usually only one or two stable ones.
- The unstable isotopes are radioactive, meaning they decay into other elements and give out radiation.
What is radioactive decay?
Unstable isotopes tend to decay into other elements and give out radiation as they try to become more stable. This process is called radioactive decay.
Is the process of radioactive decay random? What does this mean? (3)
- The process is completely random.
- This means that if you have a load of unstable nuclei, you can’t say when any one of them is going to decay, and neither can you do anything at all to make a decay happen.
*It’s completely unaffected by physical conditions like temperature, or by any sort of chemical bonding, etc.
What do radioactive substance emit? What is this? What does the ionising power of a radiation source tell you? (3)
- Radioactive substances emit ionising radiation.
- Ionising radiation is radiation that knocks electrons off atoms, creating positive ions.
- The ionising power of a radiation source tells you how easily it can do this.
What do radioactive substances release as they decay? (2)
- One or more types of ionising radiation.
- Neutrons, as they try to rebalance their atomic and mass numbers.
What types of ionising radiation am I required to know about? (3)
- Alpha
- Beta
- Gamma
What is an alpha particle?
It is two neutrons and two protons - the same as a helium nucleus.
What is alpha radiation/ decay? (definition)
When an alpha particle is emitted from the nucleus. This means an alpha particle (two neutrons and two protons) is lost from the unstable nucleus.
How does alpha radiation work? How does it affect the atomic and mass numbers? (process) (3)
- When an atom decays by emitting an alpha particle, two protons and two neutrons are lost from the nucleus.
- As protons have a relative charge of +1, alpha emission decreases the charge on the nucleus and the atomic number by 2.
- The mass number decreases by 4, as protons and neutrons each have a relative mass of 1.
What are 3 main characteristics of alpha particles? What does this mean? (3)
- Alpha particles are relatively big, heavy and slow-moving.
- This means they don’t penetrate very far into materials and are stopped quickly.
- They only travel a few centimetres in air and are absorbed by a piece of paper.
What does the size of the alpha particles mean?
Because of their size, they are strongly ionising - they bash into a lot of atoms and knock electrons off them before they slow down, which creates a lot of ions.
How is alpha radiation used in smoke detectors? (3)
- It ionises air particles, causing a current to flow.
- If there is smoke in the air, the smoke binds to the ions, reducing the number available to carry a current.
- The current falls and the alarm sounds.
In summary, what part(s) of the nucleus does alpha decay change?
- The mass
- The charge
What is a beta particle?
A beta particle is a fast-moving electron released by a nucleus. It has virtually no mass and a relative charge of -1.
What is beta decay? (process) (2)
- When a nucleus decays by beta decay, a neutron turns into a proton in the nucleus, releasing a beta particle.
- This increases the charge on the nucleus (and the atomic number) by 1 but leaves the mass number unchanged.
What are 6 key facts/ characteristics of beta particles?
- Move quite fast
- Quite small
- Moderately ionising
- Penetrate moderately far into materials before colliding
- Have a range in air of a few metres
- Can be absorbed by a sheet of aluminium (around 5 mm thick)
How are beta emitters used to test the thickness of thin sheets of metal?
Beta emitters are used to test the thickness of thin sheets of metal, as the particles are not immediately absorbed by the material like alpha radiation would be, and do not penetrate as far as gamma rays.
In summary, what part(s) of the nucleus does beta decay change?
Only the charge of the nucleus.
What are gamma rays? (2)
- Gamma rays are very short wavelength electromagnetic waves released by the nucleus.
- They have no mass, and no charge.
What are 4 key facts/ characteristics about gamma rays?
- They penetrate fair into materials without being stopped
- They pass straight through air
- They are weakly ionising
- They can be absorbed by thick sheets of lead, or metres of concret
Why are gamma rays weakly ionising?
Because they tend to pass through rather than collide with atoms. Eventually they hit something and do damage.
In summary, what part(s) of the nucleus does gamma decay change?
Changes nothing, neither the charge or mass.
What are nuclear equations and what can they be written for? (2)
- You can write alpha and beta decays as nuclear equations but not gamma decays.
- They are equations that show what atoms you start with, what radiation is emitted and what atoms you’re left with.
What must be balanced on each side of a nuclear equation? (2)
- The mass numbers
- The atomic numbers
Why can’t you write nuclear equations for gamma decays?
Because they do. not change the atomic or mass number of the atom.
What does gamma emission do?
Gamma emission is just a way of getting rid of excess energy, and can happen after an alpha or beta decay.
Look at physical flashcard on wall of alpha and beta symbols.
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What do radioactive substances give out no matter what?
Radiation from the nuclei of their atoms.
How can radiation be measured? (3)
- With a Geiger-Muller tube and counter.
- This records the count-rate.
- The count-rate is the number of radiation counts reaching the Geiger-Muller tube and counter per second.
What can you find out about radioactive decay, despite it being completely randomised?
The time it takes for the amount of radiation emitted by a source to halve. This is known as the half-life.
What can the half-life be used for?
- To make predictions about radioactive sources, even though their decays are random.
- To find the rate at which a source decays - its activity.
What is activity measured in?
Becquerels, Bq (where 1 Bq is 1 decay per second).
What always happens to the radioactivity of a sample over time? Why? (2)
- It decreases.
- Each time a radioactive nucleus decays to become a stable nucleus, the activity as a whole will decrease - so older sources emit less radiation.
Does how quickly the activity drops off vary?
Yes, a lot.
What is the issue with trying to measure the activity?
The activity never reaches zero, which is why we have to use the idea of half-life to measure how quickly the activity drops off.
What is the definition of half-life?
Half-life is the time it takes for the number of nuclei of a radioactive isotope in a sample to halve.
What is another more unofficial way to explain half-life?
It is the time it takes for the count rate (the number of radioactive emissions detected per unit of time) or activity from a sample containing the isotope to fall to half its initial level.
What does a short half-life mean? (2)
- A short half-life means the activity falls quickly, because the nuclei are very unstable and rapidly decay.
- Sources with a short half-life can be dangerous because of the high amount of radiation they emit at the start, but they quickly become safe.
What does a long half-life mean?
- A long half-life means the activity falls more slowly because most of the nuclei don’t decay for a long time - they just sit there, releasing small amounts of radiation over a longer period.
- This can be dangerous because nearby areas are exposed to radiation for years - perhaps even millions of years.
What measurement is half-life given in?
Time - i.e. minutes, hours etc.
If have time, look at or do some half-life calculation questions.
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How can you use a graph to work out the half-life of a radioactive isotope?
- Plot or use a graph of radioactive activity against time to work out the half-life of a radioactive isotope.
- 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.
How is the graph of radioactive activity against time always shaped?
Starts at the top of the y-axis and curves downwards to the x-axis.
Look at the example of the graph of radioactive activity against time in photos.
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What happens to objects near a radioactive source? What does this mean? (3)
- Objects near a radioactive source are irradiated by it.
- This simply means they’re exposed to the radiation.
- Irradiating something does not make it radioactive, put exposure to radiation can be harmful to living things.
What are some ways you can reduce the risks or irradiation? (4)
- Keeping sources in lead-lined boxes.
- Standing behind barriers.
- Being in a different room.
- Using remote-controlled arms to handle sources.
What is contamination with an example?
If unwanted radioactive atoms get onto or into a material, then it is said to be contaminated. E.g, if you touch a radioactive source without wearing gloves, your hands would be contaminated.
How is contamination dangerous?
- If you touch a radioactive source without wearing gloves, your hands would be contaminated and these contaminating atoms might then decay, releasing radiation which could cause you harm.
- Contamination is especially dangerous because radioactive particles could get inside your body.
How can contamination be prevented? (2)
- Using gloves and tongs when handling sources, to avoid particles getting stuck to your skin or under your nails.
- Some industrial workers wear protective suits to stop them breathing in particles.
What are the most and least dangerous radioactive sources outside the body? Why? Is contamination or irradiation a bigger threat in this case? (4)
- Outside the body, beta and gamma sources are the most dangerous.
- This is because they can penetrate the body and get to the delicate organs.
- Alpha is less dangerous because it can’t penetrate the skin and is easily stopped by a small air gap.
- High levels of irradiation from all sources are dangerous, but especially from ones that emit beta and gamma.
What are the most and least dangerous radioactive sources inside the body? Why? Is contamination or irradiation a bigger threat in this case? (4)
- Inside the body, alpha sources are the most dangerous.
- They do all their damage in a very localised area.
- Beta and gamma sources are less dangerous inside the body because they mostly pass straight out without doing much damage (they have a lower ionising power).
- So contamination, rather than irradiation, is the major concern when working with alpha sources.
What does how likely you are to suffer damage if you’re exposed to nuclear radiation depend on?
The radiation dose.
What is radiation dose?
Radiation dose is a measure of the risk of harm to your body due to exposure to radiation.
What does radiation dose depend on?
The type and amount of radiation you’ve been exposed to.
What are you more at risk of with a higher radiation dose?
Developing cancer.
What is radiation dose measured in? (2 bullet points)
- Sieverts (Sv).
- Radiation dose due to background radiation is small, so it is given in millisieverts (1 Sv = 1000 mSv)
What is background radiation?
Background radiation is low-level radiation that is present at all times, all around us, wherever you go. The background radiation we receive comes from many sources.
What are 3 examples of background radiation sources?
- 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.
- Radiation from space, known as cosmic rays. These come mostly from the sun. The Earth’s atmosphere absorbs a lot of the radiation from cosmic rays, but at very high altitudes a lot more of them can get through.
- Radiation due to man-made sources, e.g. fallout from nuclear weapons tests, nuclear accidents (such as Chernobyl) or dumped nuclear waste.
How is the radiation you’re exposed to - your radiation dose - effected by your location? (2)
- Certain underground rocks (e.g. granite) can cause higher levels of radiation at the surface, especially if they release radioactive radon gas, which tends to get trapped inside people’s houses.
- People who live at high altitudes are exposed to more background radiation, in the form of cosmic rays, than people who live at sea level.
How can the amount of radon in your house be monitored and controlled?
A radon detector can tell you if your house has a dangerous level of radon, and a radon pipe can be used to keep the level down.
How is the radiation you’re exposed to - your radiation dose - effected by your occupation? (3)
- Nuclear industry workers and uranium miners are typically exposed to 10 times the normal amount of radiation.
- Radiographers work in hospitals using ionising radiation and so have a higher risk of radiation exposure.
- Underground (e.g. in mines, etc) the radiation dose increases because of the rocks all around, posing a risk to miners.
How do nuclear industry workers and uranium miners protect themselves from the radiation they’re exposed to?
They wear protective clothing and face masks to stop them from touching or inhaling the radioactive material, and monitor their radiation doses with special radiation badges and regular check-ups.
How do radiographers protect themselves from the radiation they’re exposed to?
They wear lead aprons and stand behind lead screens to protect them from prolonged exposure to radiation.
How is ionising radiation harmful to living cells?
Alpha, beta and gamma radiation will enter living cells and collide with molecules. These collisions cause ionisation, which damages or destroys the molecules.
What are the impacts of lower doses of ionising radiation? (3)
- Lower doses tend to cause minor damage without killing the cell.
- This can give rise to mutant cells which divide uncontrollably.
- The cells keep dividing, making more cells and forming a tumour - this uncontrolled cell division is cancer.
What are the impacts of lower doses of ionising radiation?
Higher doses tend to kill cells completely, which causes radiation sickness if a lot of body cells are killed at once.
What two things does the extent of the harmful effects of radiation mainly depend on?
- How much exposure you have to the radiation.
- The energy and penetration of the radiation, since some types are more hazardous than others.
What are medical tracers?
Certain radioactive isotopes can be injected into people or swallowed, and their progress around the body can be followed using an external detector. These isotopes are known as medical tracers.
How are medical tracers and external detectors used by doctors? (2)
- A computer converts the readings from the external detector to a display showing where the strongest readings of the radioactive isotopes are.
- This can help doctors to investigate whether the patient’s internal organs are functioning as they should.
What is an example of a medical tracer? What does it do? (3)
- An example is the use of iodine-123 or iodine-131.
- These are absorbed by the thyroid gland in the neck just like normal iodine-127, but give out gamma radiation.
- The radiation can be detected to indicate whether the thyroid gland is taking in iodine as it should.
Which types of radioactive isotopes can be taken into the body and which ones can’t? Why? (3)
- All isotopes which are taken into the body must be gamma or beta emitters, so that the radiation passes out of the body.
- Alpha sources should never be used as they are highly ionising and do their damage in a localised area.
- The source should only last a few hours too, so that the radioactivity inside the patient disappears quickly (i.e. they should have a short half-life).
What is radiotherapy and how is it used? (3)
- Radiotherapy is the treatment of cancer using ionising radiation, e.g. gamma rays.
- It can be used to control or destroy cancer cells.
- Higher doses of radiation will kill all living cells, including cancer cells.
How is radiotherapy used in a way that isn’t too harmful and how is it monitored?
- The radiation has to be directed carefully and at just the right dosage so as to kill the cancer cells without damaging too many normal cells.
- Radioactive implants (usually beta-emitters) can also be put next to or inside tumours.
Does radiotherapy still damage normal cells?
- A fair bit of damage is done to normal cells, which makes the patient feel very ill.
- But if the cancer is successfully killed off in the end, then it’s worth it.
What is it important to consider when using radioactive materials?
For every situation, it’s worth considering both the benefits and risks of using radioactive materials.
How might someone with cancer weigh up the risks of using radioactive materials?
Whilst prolonged exposure to radiation poses risks and causes many side effects, many people with cancer choose to have radiotherapy as it may get rid of the cancer entirely. For them, the potential benefits outweigh the risks.
What is perceived risk?
Perceived risk is how risky a person thinks something is. It’s not the same as the actual risk of a procedure and the perceived risk can vary from person to person.
What is nuclear fission?
Nuclear fission is 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.
What has to happen for nuclear fission to begin? (2)
- Spontaneous (unforced) fission rarely happens.
- Usually, the nucleus has to absorb a neutron before it will split.
When the atom splits by nuclear fission, what is formed/released? (2)
- It forms two new lighter elements that are roughly the same size and have the same energy in their kinetic energy stores.
- Two or three neutrons are also released.
How is the process of nuclear fission a chain reaction?
If any of the neutrons released after the atoms have split are moving slowly enough to be absorbed by another nucleus, they can cause more fission to occur. This is a chain reaction.
What is the chain reaction process of nuclear fission used for?
It is used to generate power in a nuclear power plant.
What happens to any extra energy from nuclear fission not transferred to kinetic energy stores of the products? What can it be used for? (2)
- The energy not transferred to the kinetic energy stores of the products is carried away by gamma rays.
- The energy carried away by the gamma rays, and in the kinetic energy stores of the remaining free neutrons and the other decay products, can be used to heat water, making steam to turn turbines and generators.
How is the energy released by fission in a nuclear reactor controlled? (2)
- The amount of energy released by fission in a nuclear reactor is controlled by changing how quickly the chain reaction can occur.
- This is done using control rods, which are lowered and raised inside the nuclear reactors to absorb neutrons, slow down the chain reaction and control the amount of energy released.
What do uncontrolled nuclear fission chain reactions lead to?
Uncontrolled chain reactions quickly lead to lots of energy being released as a nuclear explosion - this is how atomic bombs work.
What is nuclear fusion with an example? (3)
- Nuclear fusion is the opposite of nuclear fission.
- 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 form a helium nucleus.
How does the mass of the two separate light nuclei compare to the final heavy nucleus in a nuclear fusion reaction? (2)
- The heavier nucleus does not have as much mass as the two separate, light nuclei did.
- Some of the mass of the lighter nuclei is converted into energy and released.
Have scientists discovered a way of using fusion to generate energy? Why? (2)
- So far, scientists haven’t found a way of using fusion to generate energy for us to use.
- The temperatures and pressures needed for fusion are so high that fusion reactions are really hard and expensive to build.
