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Everything is made of atoms. Atoms are tiny — they have a radius of about 1 × 10–10 m.
The centre of an atom is the nucleus, which contains protons and neutrons.
Electrons move around the nucleus in shells (energy levels) at different distances.
The nucleus is about 1/10 000 of the radius of the whole atom — most of the atom is empty space.
Each particle has a relative charge and a relative mass:
| Particle | Relative charge | Relative mass |
|---|---|---|
| Proton | +1 | 1 |
| Neutron | 0 | 1 |
| Electron | –1 | very small (≈1/1800) |
Atoms have no overall charge because they contain equal numbers of protons and electrons, whose charges cancel.
If an atom gains or loses electrons it becomes charged — it is then called an ion.
Losing electrons leaves an atom positively charged; gaining electrons makes it negatively charged.
Electrons are arranged at different distances from the nucleus, in different energy levels. An electron can only move between energy levels if it absorbs or emits energy.
Excitation: if an atom absorbs electromagnetic radiation, an electron can move further from the nucleus, to a higher energy level.
De-excitation: if an electron moves closer to the nucleus, to a lower energy level, electromagnetic radiation is emitted.
Atoms contain three types of particle.
(a) Draw a ring around the correct answer to complete the sentence. (1 mark)
The particles in the nucleus of the atom are electrons and neutrons / electrons and protons / neutrons and protons.
(b) Complete the table to show the relative charges of the atomic particles. (2 marks)
| Particle | Relative charge |
|---|---|
| Electron | –1 |
| Neutron | |
| Proton |
(c) Complete the table to show the relative masses of the atomic particles. (2 marks)
| Particle | Relative mass |
|---|---|
| Electron | |
| Neutron | |
| Proton | 1 |
(d) (i) A neutral atom has no overall charge. Explain this in terms of its particles. (2 marks)
(ii) Complete the sentence. (2 marks)
An atom that loses an electron is called an and has an overall charge.
The atomic number is the number of protons in an atom. It defines which element the atom is.
The mass number is the total number of protons + neutrons.
We write a nuclide as AZX, where A = mass number (top) and Z = atomic number (bottom).
Isotopes are atoms of the same element with the same number of protons but different numbers of neutrons.
Because the proton number is the same, isotopes have the same atomic number but different mass numbers.
Isotopes of an element have the same chemical properties (same electron arrangement).
Given a nuclide symbol, you can read off the protons and electrons (atomic number) and work out the neutrons.
Remember: in a neutral atom, electrons = protons = atomic number.
| Element | Protons | Electrons | Neutrons |
|---|---|---|---|
| Neon (2010Ne) | |||
| Beryllium (94Be) | |||
| Nitrogen (147N) | |||
| Sodium (2311Na) | |||
| Aluminium (2713Al) |
The diagram represents an atom of beryllium.
(a) Complete the following statements by writing one of the letters, J, K or L, in each box. Each letter should be used only once. (2 marks)
The particle with a positive charge is
The particle with the smallest mass is
The particle with no charge is
(b) Give the reason why all atoms have a total charge of zero. (1 mark)
(c) Complete the following sentence. (1 mark)
There are several isotopes of beryllium. Atoms of different beryllium isotopes will have different numbers of
(d) What happens to the structure of an atom to change it into an ion? (1 mark)
Atoms are different sizes. One of the heaviest naturally occurring stable elements is lead. Two of its isotopes are lead-206 (20682Pb) and lead-208 (20882Pb).
(a) (i) What is meant by ‘isotopes’? (2 marks)
(ii) How many protons are in the nucleus of a 20682Pb atom? (1 mark)
(iii) How many neutrons are in the nucleus of a 20682Pb atom? (1 mark)
Scientists once thought the atom had no inner structure. After the electron was discovered by J.J. Thomson, he suggested the plum pudding model.
In this model the atom is a ball of positive charge with negative electrons embedded in it (like fruit in a pudding).
Rutherford (with Geiger and Marsden) fired alpha particles at a thin sheet of gold foil.
Most alpha particles passed straight through; some were deflected; a very few bounced straight back.
This disproved the plum pudding model and led to the nuclear model: a tiny, dense, positively charged nucleus with electrons around it.
Niels Bohr showed that electrons orbit the nucleus at fixed distances (shells), which matched experimental observations.
Later, James Chadwick provided evidence for the neutron in the nucleus.
The ‘plum pudding’ model of the atom was used by scientists in the early part of the 20th century to explain atomic structure.
(a) Those scientists knew that atoms contained electrons and that the electrons had a negative charge. They also knew that an atom was electrically neutral overall.
What did this allow the scientists to deduce about the ‘pudding’ part of the atom? (1 mark)
(b) An experiment, designed to investigate the ‘plum pudding’ model, involved firing alpha particles at a thin gold foil.
If the ‘plum pudding’ model was correct, then most of the alpha particles would go straight through the gold foil. A few would be deflected, but by less than 4°.
The results of the experiment were unexpected. Although most of the alpha particles did go straight through the gold foil, about 1 in every 8000 was deflected by more than 90°.
Why did this experiment lead to a new model of the atom, called the nuclear model, replacing the ‘plum pudding’ model? (1 mark)
(c) The diagram shows the paths, A, B and C, of three alpha particles. The total number of alpha particles deflected through each angle is also given.
(i) Using the nuclear model of the atom, explain the three paths, A, B and C. (3 marks)
A
B
C
(ii) Using the nuclear model, the scientist E. Rutherford devised an equation to predict the proportion of alpha particles that would be deflected through various angles. The results of the experiment were the same as the predictions made by Rutherford.
What was the importance of the experimental results and the predictions being the same? (1 mark)
A nucleus is radioactive (unstable) if it has too many or too few neutrons compared with protons.
An unstable nucleus decays by emitting radiation. This is a random process — you cannot predict when a given nucleus will decay.
Activity is the rate at which nuclei decay, measured in becquerel (Bq). 1 Bq = one decay per second.
Activity is measured using a Geiger-Müller tube and counter (the count rate).
There are three main types of nuclear radiation, and these are all ionising. Ionising means they can cause atoms to lose electrons and become ions.
Alpha (α): a helium nucleus (2 protons + 2 neutrons), charge +2. Very ionising; stopped by paper or a few cm of air.
Beta (β): a fast-moving electron, charge –1. Moderately ionising; stopped by a few mm of aluminium.
Gamma (γ): an electromagnetic wave, no charge. Weakly ionising; reduced by thick lead or concrete.
| Alpha 42α | Beta 0−1β | Gamma 00γ | |
|---|---|---|---|
| What it’s made of | |||
| Charge | |||
| Atomic mass | |||
| Penetrating power | |||
| Ionising effect | |||
| Effect of electromagnetic field |
Radiation is dangerous because it can ionise atoms — knocking electrons off them. This can damage cells and cause cancer.
Outside the body, the most penetrating radiation (gamma) is most dangerous. Inside the body, the most ionising (alpha) is most dangerous.
Workers who use ionising radiation reduce their exposure by: 1. Keeping as far away as possible. 2. Spending as little time as possible near the radioactive source. 3. Shielding themselves with thick concrete/lead.
(a) What is alpha, beta and gamma radiation made of? (3 marks)
(b) Sam and Kris are arguing about alpha and gamma radiation.
Sam says that alpha radiation is more dangerous.
Kris disagrees. He thinks that gamma radiation is more dangerous. What do you think? Explain your answer as fully as you can. (4 marks)
(c) Tritium is one of the elements found in the waste material of the nuclear power industry. The diagram below shows a worker behind a protective screen. The container holds a mixture of different waste materials which emit alpha (α), beta (β) and gamma (γ) radiation.
Suggest a suitable material for the protective screen. The material should prevent radiation from the container reaching the worker. Explain your answer. (2 marks)
When a nucleus decays, mass and charge are conserved. That means the totals balance on each side for both the mass number and the atomic number.
In alpha decay, the nucleus emits an alpha particle 42α. The mass number falls by 4 and the atomic number falls by 2.
The element changes because the number of protons changes.
To complete a decay equation, make the top numbers (mass numbers) balance, and the bottom numbers (atomic numbers) balance.
Then use the atomic number with a periodic table to identify the new element.
In beta decay, a neutron in the nucleus turns into a proton and emits a fast electron (the beta particle 0−1β).
The mass number stays the same and the atomic number increases by 1.
Emitting a gamma ray does not change the mass number or atomic number — it only removes energy.
Gamma radiation is an electromagnetic wave — it has no charge and no mass. When gamma is emitted, the nucleus only loses energy; it does not change into a new element.
Sometimes in exams we may need to identify the type of radiation emitted from a decay equation. To do this, check:
Double check by adding the numbers to the equation and making sure the top and bottom balance.
When the nucleus of a radium-225 atom decays, it changes into a nucleus of actinium-225.
What type of radiation is emitted by radium-225? Draw a ring around your answer.
alpha beta gamma
Explain the reason for your answer. (3 marks)
Q2. (a) Uranium atoms do not always have the same number of neutrons. What are atoms of the same element that have different numbers of neutrons called? (1 mark)
(b) By emitting an alpha particle, an atom of uranium-235 decays into an atom of thorium. An alpha particle, which is the same as a helium nucleus, is represented by the symbol 42He.
The decay can be represented by the equation below. Complete the equation by writing the correct number in each of the two boxes. (2 marks)
Over time, the activity of a radioactive source decreases as unstable nuclei decay.
The half-life is the time taken for the count rate (activity) to fall to half its value.
OR
The half-life is the time for half the unstable nuclei to decay.
Because decay is random, half-life is an average that is reliable for large numbers of nuclei.
Half-life can be very small, e.g. polonium-214 has a half-life of about 164 microseconds (0.000164 s), or really long, e.g. uranium-238 has a half-life of about 4.5 billion years.
You can find the half-life of an isotope by reading its activity–time graph: pick a starting activity, find the time it takes to fall to half that value.
Notice how a short half-life (Technetium-99m) produces a curve that drops away quickly, while a long half-life (Iodine-131) looks almost flat over the same time period.
| Isotope | Half-life |
|---|---|
| Radon-220 (thoron) | 55.6 s |
| Fluorine-18 | 109.8 minutes |
| Iodine-123 | 13.2 hours |
| Iodine-131 | 8.02 days |
| Cobalt-60 | 5.27 years |
| Tritium (hydrogen-3) | 12.3 years |
To solve half-life problems, halve the count rate once for each half-life that passes.
You can also find the number of half-lives that have passed from the fraction remaining.
A question may ask for the net decline in the radioactive emission rather than for an activity. The net decline is the ratio of the activity that is left to the activity you started with.
Each half-life halves the activity, so after n half-lives the fraction remaining is 1 ÷ 2n, and the net decline is written as the ratio 1 : 2n.
Half-life is used in carbon dating: living things take in carbon-14, which decays after death, so the remaining amount indicates age.
(a) The graph shows how a sample of barium-143, a radioactive isotope with a short half-life, decays with time.
(i) What is meant by the term isotope? (1 mark)
(ii) What is meant by the term half-life? (1 mark)
(iii) Use the graph to find the half-life of barium-143. (1 mark)
(b) Humans take in the radioactive isotope carbon-14 from their food. After their death, the proportion of carbon-14 in their bones can be used to tell how long it is since they died. Carbon-14 has a half-life of 5700 years.
(i) A bone in a living human contains 80 units of carbon-14. An identical bone taken from a skeleton found in an ancient burial ground contains 5 units of carbon-14. Calculate the age of the skeleton. Show clearly how you work out your answer. (2 marks)
(ii) Why is carbon-14 unsuitable for dating a skeleton believed to be about 150 years old? (1 mark)
(a) A radiation detector and counter were used to detect and measure the radiation emitted from a weak source. The graph shows how the number of counts recorded in one minute changed with time.
Even though the readings from the counter were accurately recorded, not all the points fit the smooth curve. What does this tell us about the process of radioactive decay? (1 mark)
(b) The radioactive isotope sodium-24 injected into the bloodstream can be used to trace blood flow to the heart. Sodium-24 emits both beta particles and gamma rays.
(i) What is a beta particle? (1 mark)
(ii) What is a gamma ray? (1 mark)
(iii) The count rate from a solution containing sodium-24 decreases from 584 counts per minute to 73 counts per minute in 45 hours. Calculate the half-life of sodium-24. Show clearly how you work out your answer. (3 marks)
(iv) Give one advantage of using sodium-24 to trace blood flow compared to using an isotope with a half-life of: [A] ten years; [B] ten seconds. (2 marks)
Contamination is when radioactive atoms get onto or into a material or person. The contaminating atoms keep decaying and emitting radiation.
Irradiation is when an object is exposed to radiation from an outside source. It does not make the object radioactive.
Contamination is especially dangerous because the source stays with the person and continues to emit radiation.
Medical tracers: a source that emits gamma (or beta) radiation is put into the body and tracked by a detector to study organs.
Tracers have a short half-life so the patient’s exposure is limited.
Radiotherapy: high-energy gamma rays are aimed at a tumour from several directions to destroy cancer cells while limiting damage to healthy tissue.
Thickness control: a beta source and detector measure the thickness of paper or foil — if the reading changes, the rollers adjust.
Sterilising: gamma radiation kills bacteria on medical equipment and can be used on food without making it radioactive.
Food irradiation is a process that exposes food to radiation. Irradiation can be used to kill the bacteria that cause food poisoning or to slow down the ripening of fresh fruit and vegetables. Frozen foods and food inside packaging can also be irradiated.
(a) The table gives information about five radioactive isotopes.
| Isotope | Half-life | Radiation emitted |
|---|---|---|
| Caesium-134 | 2.1 years | beta |
| Cobalt-60 | 5.3 years | gamma |
| Curium-242 | 160 days | alpha |
| Strontium-90 | 28 years | beta |
| Technetium-99 | 6 hours | gamma |
Which of these radioactive isotopes would be most suitable for irradiating food?
Explain the reasons for your choice. (3 marks)
(b) Many people think that food should not be irradiated. Consumer groups have said that they are worried about the nutritional value and safety of eating irradiated foods.
(i) Suggest one reason why some people may be concerned about the safety of eating irradiated food. (1 mark)
(ii) Independent scientific committees in several countries, including Sweden, Canada and the UK, have concluded that it is safe to eat irradiated food. These scientific committees need to be independent from government influence. Suggest why. (1 mark)
(iii) One group of scientists compared the vitamin content of non-irradiated and irradiated foods. The table below gives the data obtained for 1 kg of cooked chicken.
| Vitamin | Non-irradiated (mg) | Irradiated (mg) |
|---|---|---|
| B6 | 1.22 | 1.35 |
| B12 | 21.00 | 28.00 |
| E | 3.30 | 2.15 |
| Niacin | 58.00 | 55.50 |
| Riboflavin | 2.10 | 2.25 |
Considering only the data in the table, is it valid to conclude that irradiated food is less nutritional than non-irradiated food? Explain your answer. (2 marks)
(iv) In a restaurant, meals with ingredients that have been irradiated must be clearly identified on the menu. It is important that people eating in a restaurant are given this information. Suggest why. (1 mark)
(c) The isotope caesium-137 decays by emitting beta radiation. Caesium-137 has a half-life of 30 years.
(i) What is a beta particle, and from which part of an atom is a beta particle emitted? (1 mark)
(ii) A sample containing caesium-137 has a count rate of 600 counts per minute. Calculate how long it would take for the count rate from the sample to fall to 75 counts per minute. Show clearly how you work out your answer. (2 marks)
Background radiation is the low level of ionising radiation that is around us all the time, from a variety of sources.
Most background radiation comes from natural sources: radon gas (from rocks and soil), rocks and soil themselves, cosmic rays from space, and the food and drink we consume.
A smaller part comes from man-made (artificial) sources, mainly medical uses (such as X-rays and radiotherapy) and, to a much lesser extent, the nuclear industry and fallout from nuclear weapons testing.
Radiation dose (a measure of the risk of harm) is measured in sieverts (Sv). 1 millisievert (mSv) = 0.001 Sv.
Background radiation varies with location and occupation: for example, people who live at high altitude or fly frequently (e.g. pilots) receive more cosmic ray dose, and people who work with radioactive sources receive a higher occupational dose.
When measuring the activity of a source, the background count rate (measured with the source absent or removed) must always be subtracted from the total reading to find the count rate due to the source alone.
The pie chart shows the average proportions of background radiation from different sources in the UK.
(a) What percentage of background radiation comes from radon gas? (1 mark)
(b) Only two of the following statements are true. Tick the boxes next to the two true statements. (2 marks)
Radon gas is the single largest source of background radiation.
Cosmic rays contribute more than food and drink.
All background radiation comes from natural sources.
Medical sources are the largest artificial source of background radiation.
(c) The table shows the average radiation dose received in one year by a person in the UK, in dose units, based on the proportions in the pie chart.
| Source | Dose (units) |
|---|---|
| Radon gas | 1250 |
| Medical | 350 |
| Ground & buildings | 350 |
| Food & drink | 275 |
| Cosmic rays | 250 |
| Other | 25 |
| TOTAL | 2500 |
Each dental X-ray delivers 25 dose units. How many dental X-rays would give the same dose as the yearly average from medical sources? (2 marks)
(a) Classify each of the following as a natural or artificial (man-made) source of background radiation. (4 marks)
Cosmic rays:
Medical X-rays:
Radon gas:
Nuclear industry waste:
(b) Background radiation from cosmic rays increases with altitude. For every 500 m gained in altitude, the cosmic ray dose increases by 1 unit.
Two towns are built at different heights on a hillside: Town A is at a height of 200 m, and Town B is at a height of 1700 m.
(i) Calculate the difference in cosmic ray dose between the two towns. Show clearly how you work out your answer. (2 marks)
(ii) Suggest one occupation where a person might receive an increased dose of cosmic radiation, for a similar reason to Town B. (1 mark)
Nuclear fission is the splitting of a large, unstable nucleus (such as uranium-235 or plutonium-239) into two smaller nuclei, releasing energy.
Fission is usually induced: the nucleus must first absorb a neutron to become unstable enough to split.
Each fission also releases two or three neutrons and a large amount of energy. These released neutrons can be absorbed by other nuclei, causing them to split too — this is a chain reaction.
The neutrons released by one fission can be absorbed by other nuclei, causing them to split too — this is a chain reaction.
In a controlled chain reaction (a reactor), some neutrons are absorbed so the rate stays steady.
In an uncontrolled chain reaction, the rate grows rapidly — this is what happens in a nuclear bomb.
The energy released by fission is used to generate electricity in a nuclear power station.
The reactor produces heat, which heats water (via a coolant) to produce steam; the steam powers a turbine and generator, and is then cooled and condensed by a cooling tower.
Advantages of nuclear power: low carbon emissions, and a reliable supply of electricity.
Disadvantages of nuclear power: produces radioactive waste, and has high construction and decommissioning costs.
The first commercial nuclear power station in the world was built at Calder Hall in Cumbria. The fuel used at the Calder Hall power station is uranium. Natural uranium consists mainly of two isotopes: uranium-235 and uranium-238. The nucleus of a uranium-235 atom is different to that of a uranium-238 atom.
(a) (i) Where is the nucleus in an atom? (1 mark)
(ii) Name the two types of particle found in the nucleus. (2 marks)
(iii) How is the nucleus of a uranium-238 atom different to the nucleus of a uranium-235 atom? (2 marks)
(b) In the nuclear reactor, fission of uranium atoms takes place in reactions such as the one shown below. The nuclear reactions are carefully controlled in the power station so that a chain reaction takes place.
Explain, as fully as you can: (4 marks)
(i) how fission of uranium atoms takes place in a nuclear reactor;
(ii) how this leads to a chain reaction;
(iii) why it can be used to generate electricity.
Nuclear power stations generate electricity through nuclear fission. Electricity can also be generated by burning shale gas.
(a) Shale gas is natural gas trapped in rocks. Shale gas can be extracted by a process called fracking. There is some evidence that fracking causes minor earthquakes. Burning shale gas adds carbon dioxide to the atmosphere.
Describe the advantages of nuclear power compared with the use of shale gas to generate electricity. (3 marks)
(b) What is the name of one fuel used in nuclear power stations? (1 mark)
(c) Describe the process of nuclear fission. (4 marks)
Nuclear fusion is the joining of two small nuclei to form a larger nucleus, releasing energy.
Fusion is the source of the energy released by stars, including the Sun.
Fusion needs extremely high temperatures and pressures so the nuclei move fast enough to overcome their repulsion and join.
Fission: a large nucleus splits into smaller ones. Fusion: small nuclei join to make a larger one. Both release energy.
Fusion releases more energy per kilogram of fuel than fission and produces little radioactive waste, but is very hard to achieve on Earth.
Fusion uses isotopes of hydrogen — deuterium (from seawater) and tritium (made from lithium).
These fuels are abundant, making fusion a promising 'clean' energy source if the engineering can be solved.
Many countries use nuclear power stations to generate electricity. Nuclear power stations use the process of nuclear fission to release energy.
(a) (i) What is nuclear fission? (1 mark)
(ii) Plutonium-239 is one substance used as a fuel in a nuclear reactor. For nuclear fission to happen, the nucleus must absorb a particle. What type of particle must be absorbed? (1 mark)
(b) Nuclear fusion also releases energy. Nuclear fusion happens at very high temperatures. A high temperature is needed to overcome the repulsion force between the nuclei.
(i) Why is there a repulsion force between the nuclei of atoms? (1 mark)
(ii) Where does nuclear fusion happen naturally? (1 mark)
(c) In 1991, scientists produced the first controlled release of energy from an experimental nuclear fusion reactor. This was achieved by fusing the hydrogen isotopes, deuterium and tritium. Deuterium is naturally occurring and can easily be extracted from seawater. Tritium can be produced from lithium. Lithium is also found in seawater.
The table gives the energy released from 1 kg of fusion fuel and from 1 kg of fission fuel.
| Type of fuel | Energy released from 1 kg of fuel in joules |
|---|---|
| Fusion fuel | 3.4 × 1014 |
| Fission fuel | 8.8 × 1013 |
(i) Suggest two advantages of the fuel used in a fusion reactor compared with plutonium and the other substances used as fuel in a fission reactor. (2 marks)
1.
2.
(ii) Some scientists think that by the year 2050 a nuclear fusion power station capable of generating electricity on a large scale will have been developed. Suggest one important consequence of developing nuclear fusion power stations to generate electricity. (1 mark)
(d) Tritium is radioactive. After 36 years, only 10 g of tritium remains from an original sample of 80 g. Calculate the half-life of tritium. Show clearly how you work out your answer. (2 marks)