P4 - Atoms & Radiation Flashcards
What was Dalton’s model of the atom?
Atoms were solid, indivisible spheres. Each element was made of a different type of sphere
What was Thomson’s model of the atom?
Discovered electrons which could be removed from atoms- disproving Dalton’s theory of indivisibility. Thomson suggested atoms were spheres of positive charge, with negative electrons scattered throughout (plum pudding model)
What was Rutherford’s model of the atom?
Positively charged nucleus, surrounded by a cloud of negative electrons. This was the first nuclear model of the atom
What discovery did Bohr make? How was he proven right?
Electrons orbit the nucleus in fixed positions, called energy levels. His theoretical calculations agreed with experimental data
What discovery was made after electron orbits?
Experiments showed that the nucleus’ positive charge was subdivided between a group of particles, called protons
What discovery did Chadwick make? Why did this make sense?
He proved the existence of neutrons in the nucleus. This explained the imbalance between the atomic and mass numbers.
Who theorised that atoms were solid, indivisible spheres?
Dalton
Who theorised the plum pudding model?
Thomson
Whose discoveries led to the nuclear model of the atom?
Rutherford
Who calculated that electrons had fixed orbits?
Bohr
Who discovered neutrons?
Chadwick
How was Rutherford’s alpha scattering experiment conducted?
Scientists in Rutherford’s lab fired a beam of alpha particles at a thin sheet of gold foil
In Rutherford’s alpha scattering experiment, what did the scientists expect to happen and why?
From the plum pudding model, they expected most particles to be slightly deflected, and a minority to pass straight through the foil
What were the results of Rutherford’s alpha scattering experiment? Why wasn’t this as expected?
The majority of the alpha particles went straight through the gold sheet, and some were deflected more than expected and a few were deflected right back the way they came. The plum pudding model couldn’t explain these results
In Rutherford’s alpha scattering experiment, a few alpha particles were deflected right back and some more than expected. What did the scientists deduce from this?
They realised that most of the atom’s mass was concentrated in a central nucleus, which must also have a positive charge, since it repelled the positive alpha particles
In Rutherford’s alpha scattering experiment, most of the alpha particles passed straight through. What did the scientists deduce from this?
They realised that most of the atom is just empty space, rather than a solid sphere, and that the nucleus is very small relative to this space
What was deduced from the results of Rutherford’s alpha scattering experiment?
1) Because a few alpha particles were deflected back, they realised that most of the atom’s mass was concentrated in a central nucleus, which must also have a positive charge, since it repelled the positive alpha particles.
2) Because most of the alpha particles passed straight through, they realised that most of the atom is just empty space, and that the nucleus is very small relative to this space
The nucleus’ radius is about ___x smaller than the radius of an atom
10,000x
The radius of an atom is about ___m
1x10(^-10)m
Electron shells =
energy levels
What happens when electrons gain energy by absorbing EM radiation?
They move to a higher energy level (further from the nucleus)
What happens when electrons lose energy by releasing EM radiation?
They move to a lower energy level (closer to the nucleus)
What are isotopes?
Isotopes of an element are atoms with the same number of protons, but different numbers of neutrons (so have different mass numbers)
All elements have isotopes, but…
…but there are usually only one or two stable ones
What do unstable isotopes tend to do?
They decay into other elements, giving out radiation to become more stable. This process is called radioactive decay
What is radioactive decay?
The process by which unstable nuclei emit radiation until they reach a stable state
What types of radiation do unstable nuclei release?
Alpha, beta, gamma, neutron
What makes a nucleus unstable?
A bigger nucleus would be less stable because it’s harder to hold all the protons and neutrons together. The less energy holding them together, the less stable
What is alpha radiation?
A particle: helium nucleus (2 protons, 2 neutrons)
What is the symbol for alpha?
α
What is the mass and charge of alpha particles?
Relative mass: large. Charge: +2e
What is the ionising power of alpha radiation?
High
What is the range of alpha particles and why?
Because alpha radiation is so ionising, it quickly loses kinetic energy and is absorbed. It therefore has a small range of a few cm in air, and doesn’t penetrate very far into materials
Alpha particles can be blocked by…
…paper or skin
What is beta radiation?
A particle: high speed electrons released by the nucleus
What is the symbol for beta?
β
What is the mass and charge of beta particles?
Small relative mass. Charge: -1e
What is the ionising power of beta radiation?
Moderate to low
What is the range and penetration of beta radiation and why?
It travels several metres in the air, and penetrates moderately far into materials, because it is only moderately ionising
Beta radiation can be blocked by…
…thin sheet of aluminium and lead
What is gamma radiation?
It is a high energy, high frequency electromagnetic wave (of photons). Released by the nucleus to get rid of excess energy
What is the symbol for gamma?
γ
What is the mass and charge of gamma rays?
No mass and no charge
What is the ionising power of gamma radiation and why?
Very low, because it has no mass or charge
What is the range and penetration of gamma radiation and why?
Both very high, because it has very low ionising power. Eventually, gamma rays will hit something and do damage
Gamma radiation can be “absorbed” by…
several cm of lead / several m of concrete
How is alpha radiation used in smoke detectors?
Smoke alarms release alpha radiation, which ionises air particles, causing a current to flow. If there is smoke in the air, alpha particles bind to the ions. This stops the current, causing the alarm to go off
Why and can beta radiation be useful in testing the thickness of sheets of metal?
Because it’s moderately penetrating
What are beta emitters used for?
Testing the thickness of sheets of metal
Beta radiation is used in testing the thickness of sheets of metal. Why is this used, and not alpha or gamma?
Because beta radiation is moderately penetrating, particles aren’t immediately absorbed, where alpha radiation would be, and don’t penetrate as far as gamma rays. Therefore, slight variations in thickness of the metal affect the amount of radiation passing through the sheet
What is ionisation?
The process by which radiation from radioactive substances ionise (knock electrons out of) atoms
What does ionisation of body cells lead to?
Cell death and cancers
Which type of radiation is the most ionising?
Alpha
Alpha decay decreases the ___ and ___ of the nucleus
Charge and mass
When an atom emits an alpha particle, its atomic number reduces by __ and and its mass number reduces by __
2, 4
What is the rule to remember when forming nuclear equations?
Total mass and atomic numbers must be equal on both sides of the equation
How can alpha particles be shown in nuclear equations?
(4,2)α or (4,2)He
Give the equation for the alpha decay of uranium to thorium:
(238,92)U -> (x,y)Th + alpha
(238,92)U -> (234,90)Th + (4,2)He
What sometimes also happens when a nucleus decays by alpha or beta?
Gamma rays can be released
What happens during beta decay?
A neutron in the nucleus turns into a proton and releases a fast-moving electron
What happens to an atom’s charge during beta decay?
Neutron -> proton + electron. The nucleus’ charge increases by one, and one electron is lost. Overall, the charge increases by 2
Why doesn’t the mass of the nucleus change during beta decay?
Because the nucleus has lost a neutron and gained a proton- which have the same mass
How are beta particles represented in nuclear equations?
(0, -1)e or (0, -1)β
Give the equation for the beta decay of carbon to nitrogen:
(14, 6)C -> (x,y)N + beta
(14, 6)C -> (14, 7)N + (0, -1)e
Why are new elements formed in alpha and beta decay, but not gamma?
The number of protons (atomic number) changes in alpha and beta decay, but doesn’t in gamma decay
How are gamma rays represented in nuclear equations?
(0,0)γ
Give the equation for the gamma decay of uranium:
(238, 92)U ->
(238, 92)U -> (238, 92)U + (0,0)γ
Why is there no change to the mass or atomic number of an atom after gamma decay?
Gamma radiation is not a particle- it is a wave, of energy, and has no mass or charge
How can radiation be measured?
With a Geiger-Muller tube and counter, which records the count-rate
Radioactive decay of a nucleus cannot be ___ ___; it is ___. Half-life can be used to make___ ___ about radioactive sources
1) accurately predicted
2) random
3) estimated predictions
What is activity and what is it measured in?
The number of decays per second- measured in Becquerels, Bq, where 1Bq is one decay per second
What is the half-life of a radioactive isotope?
The time it takes for half the nuclei in the sample to decay, or for the activity to halve
How can substances with a short half-life be dangerous? What is a more positive side to this?
The nuclei are very unstable and rapidly decay, emitting large amounts of radiation. However, they quickly become safer because the activity falls quickly
How can substances with a long half-life be dangerous?
Activity falls slowly, so the source releases small amounts of radiation over a long time. Dangerous because nearby areas can be exposed to radiation for millions of years
What is the formula to calculate the mass/no. nuclei of a sample left where the half-life and time passed is given?
= i x2^-n
where i = initial mass/no. nuclei and n = no. half-lives passed
A 1kg sample of an isotope has a half-life of 3 hours. How much of the sample would you expect to be remaining after 9 hours?
1kg x 2^-3 = 0.125kg
What is the formula to calculate the amount of a sample left where the half-life and time passed is given?
= i x2^-n
where i = initial activity and n = no. half-lives passed
The initial activity of a sample is 640Bq. Calculate the final activity, after two half-lives, as a percentage of the initial activity
1) 640 x 2^-2 = 160Bq
2) (160/640) x 100 = 25%
When measuring radiation, you should always…in order to avoid…
1) measure and subtract the background radiation from your results.
2) systematic errors
What three things contribute to background radiation?
1) Radioactivity of naturally occuring unstable isotopes.
2) Radiation from space: cosmic rays.
3) Radiation due to human activity
Naturally occuring unstable isotopes are found all around us, e.g. in…
Rocks, the air, food
Cosmic rays usually come from __ ___. The ___ protects us from much of this radiation
1) The sun
2) Atmosphere
Radiation due to human activity represents a ___ proportion of the total background radiation on Earth
Tiny
What is radiation dose measured in?
Sieverts (Sv) or millisieverts (mSv)
What is irradiation?
Exposure to radiation
When working with radioactive sources, what are two ways to prevent irradiation?
1) Keeping sources in lead-lined boxes.
2) Staying behind barriers/in a different room and controlling the source remotely
What is contamination?
The presence of radioactive materials in/on an object
Why is contamination more dangerous than irradiation?
A contaminated object will be exposed to radiation for as long as it is contaminated, whereas, if irradiated, it stops being exposed to radiation as soon as it is no longer near the source
What three things are used to prevent contamination?
1) Gloves
2) Tongs
3) Protective suits
What sources of radiation are most dangerous outside the body?
Beta and gamma, because they can penetrate the body and get to sensitive organs. Alpha is less dangerous because it is blocked by a few cm of air, and can’t penetrate through the skin, so is unlikely to get inside your body
Which type of radiation poses the greatest threat to cells when inside the body? Why?
Alpha, because it is the most ionising (due to charge of 2+ and a large mass) and does damage in a very localised area
When working with alpha sources, is the main concern irradiation or contamination?
Contamination
Which type of radiation poses the smallest threat to cells when inside the body? Why?
Gamma, as they usually pass straight through the body since they have a very small ionising power
Why are beta sources less damaging than alpha when inside the body?
Radiation is absorbed over a wider area, and some passes out of the body
How are gamma sources used in medical tracers?
Certain radioactive isotopes are injected or swallowed, and their progress around the body is followed by an external detector. A computer displays where the strongest reading of radioactivity is coming from. This indicates whether substances are being taken in as they should be
Why are gamma emitters used in medical tracers?
Gamma is used so that radiation passes through the body without causing too much ionisation
Why should gamma emitters used in medical tracers have a short half-life?
So that radioactivity inside a patient quickly minimises, preventing too much ionisation
How does radiotherapy work? Give 2 methods.
1) Gamma rays are directed carefully, and at the right dosage, to kill cancer cells without damaging too many normal cells
2) Radiation (usually beta) emitting implants are put next to/inside tumours
Why does radiotherapy make patients feel ill?
Damage is done to non-cancerous cells in the process of killing cancerous ones.
Explain how contamination leads to cell death and cancer
When radiation enters the body, it causes ionisation of atoms and molecules in tissue.
Lower doses of radiation lead to mutated cells which divide uncontrollably, i.e. cancer.
Higher doses cause cell death, which can lead to radiation sickness
Why are many scientists trying to push the production of nuclear power over fossil fuels?
1) Nuclear power produces more energy per unit mass than fossil fuels.
2) Fossil fuels produce carbon dioxide when burnt, contributing to global warming.
Explain what happens during induced nuclear fission
1) High-energy neutron fired at unstable nucleus.
2) Nucleus absorbs neutron and becomes more unstable.
3) Nucleus splits into 2, releasing 2+ neutrons and… 4) Energy not transferred to the KE stores of the products is carried away by gamma rays.
5) If any of the released neutrons are moving enough, they can be absorbed by another nucleus and cause it to split, i.e. a chain reaction
During nuclear fission, energy is carried away by gamma rays and transferred to the kinetic energy stores of the products (neutrons and two nuclei). How is this energy used?
Used to heat water, producing steam to turn turbines and generators
How is nuclear fission controlled?
Lowering down control rods into a nuclear reactor. These absorb neutrons, slowing the chain reaction and controlling the amount of energy released
What happens during uncontrolled fission reactions?
Lots of energy is released as an explosion (this is how nuclear weapons work)
What happens during nuclear fusion?
1) Two small, high-speed nuclei collide and fuse into a larger nucleus.
2) This nucleus is less massive than the total mass of the two initial nuclei. Some of their mass is converted to energy and released as radiation
For a given mass of fuel, which of nuclear fission and fusion produces more energy?
Fusion
Although nuclear fusion produces more energy than fission per unit mass of fuel, it can’t be used to generate energy. Why is this?
The temperatures and pressures needed for fusion are so high that fusion reactors are difficult and expensive to build
Why are sources of alpha radiation more dangerous inside the body than outside the body?
Inside:
1) Alpha radiation is strongly ionising
2) When it enters living cells, it can kill them, damage
them and cause cancer.
3) It is almost certain to be absorbed by living cells and
cause damage.
Outside:
1) It is easily absorbed by thin barriers (e.g. skin) so is
unlikely to reach the body’s delicate organs if outside
the body
