What you'll learn
Atomic structure is the unit of AQA GCSE Combined Science: Trilogy physics that looks inside the atom and at what happens when a nucleus is unstable. It shares its opening with the chemistry course — the historical development of the atomic model — but then goes somewhere chemistry does not, into radioactive decay, half-life and the difference between being irradiated and being contaminated. It is also the clearest example in the whole specification of how scientific models change when new experimental evidence arrives. By the end of this unit you should be able to describe the structure of the atom with the relevant sizes, explain how the alpha scattering experiment overturned the plum pudding model, define isotopes, describe the four types of nuclear radiation and compare their penetrating power and ionising ability, complete nuclear decay equations, define and calculate half-life, and distinguish contamination from irradiation. This unit is assessed on Physics Paper 1.
Key terms and definitions
Atom — the smallest part of an element that can exist, with a radius of about 1 times 10 to the power minus 10 metres
Nucleus — the tiny central part of an atom containing protons and neutrons, with a radius less than one ten-thousandth that of the atom
Isotope — an atom of the same element with the same number of protons but a different number of neutrons
Radioactive decay — the random process by which an unstable nucleus gives out radiation as it becomes more stable
Activity — the rate at which a source of unstable nuclei decays, measured in becquerels
Count-rate — the number of decays recorded each second by a detector such as a Geiger–Müller tube
Alpha particle — a decay product consisting of two neutrons and two protons, the same as a helium nucleus
Beta particle — a high speed electron ejected from the nucleus as a neutron turns into a proton
Gamma ray — electromagnetic radiation emitted from the nucleus, with no mass and no charge
Half-life — the time it takes for the number of nuclei of an isotope in a sample to halve, or for the count-rate to fall to half its initial level
Irradiation — exposing an object to nuclear radiation without the object becoming radioactive
Contamination — the unwanted presence of materials containing radioactive atoms on or in other materials
Core concepts
The structure of the atom
Atoms have a radius of about 1 times 10 to the power minus 10 metres. The nucleus sits at the centre and contains protons and neutrons; its radius is less than one ten-thousandth of the radius of the atom, yet it contains almost all of the atom's mass. The electrons are arranged at different distances from the nucleus, and these distances can change when the atom absorbs or emits electromagnetic radiation.
The number of protons in an atom of an element is always the same, and it is called the atomic number. Atoms of different elements have different numbers of protons. Atoms have no overall electrical charge because the number of electrons equals the number of protons. The total number of protons and neutrons is the mass number.
If an atom loses one or more outer electrons it becomes a positively charged ion.
The development of the model
The model of the atom has changed repeatedly in response to new evidence, and the sequence is explicitly examinable.
Before the electron was discovered, atoms were thought to be tiny spheres that could not be divided. The discovery of the electron led to the plum pudding model, in which the atom was a ball of positive charge with negative electrons embedded in it.
The alpha particle scattering experiment provided evidence that overturned this. Alpha particles were directed at a very thin sheet of gold foil. Most passed straight through, showing that the atom is mostly empty space. A small number were deflected through large angles, and a very few came back towards the source, showing that the positive charge and nearly all the mass must be concentrated in a very small volume at the centre. This led to the nuclear model.
Niels Bohr then adapted the model by proposing that electrons orbit the nucleus at specific distances, and his theoretical calculations agreed with experimental observations. Later experiments identified the positive particles in the nucleus as protons, and about twenty years after that James Chadwick provided evidence for the existence of neutrons.
The point examiners want drawn out is the process itself: new experimental evidence may lead to a scientific model being changed or replaced, and the results must be checked by other scientists before the new model is accepted.
Radioactive decay
Some atomic nuclei are unstable. The nucleus gives out radiation as it changes to become more stable, and this is radioactive decay. The process is entirely random, so it is impossible to predict which nucleus will decay next or when a particular nucleus will decay.
Activity is the rate at which a source decays, measured in becquerels, where one becquerel is one decay per second. Count-rate is what a detector actually records, and a Geiger–Müller tube is the standard instrument.
The four types of radiation
An alpha particle consists of two neutrons and two protons, identical to a helium nucleus. It is the most strongly ionising of the three main types but the least penetrating: it is stopped by a sheet of paper and has a range in air of only a few centimetres.
A beta particle is a high speed electron ejected from the nucleus as a neutron turns into a proton. It is moderately ionising and moderately penetrating: it is stopped by a few millimetres of aluminium and has a range in air of around a metre.
A gamma ray is electromagnetic radiation emitted from the nucleus. It has no mass and no charge, is the least ionising and the most penetrating, requiring thick lead or metres of concrete to reduce it significantly, and has an effectively unlimited range in air.
A neutron may also be emitted. The general pattern is that the more strongly ionising a radiation is, the less far it penetrates, because it transfers its energy to the material more rapidly.
Nuclear equations
Nuclear equations show what happens to the mass number and the atomic number during decay, and both must balance on each side.
Alpha decay causes the mass number to decrease by four and the atomic number to decrease by two, because two protons and two neutrons leave the nucleus. Since the atomic number changes, the element changes.
Beta decay causes no change to the mass number but increases the atomic number by one, because a neutron has turned into a proton. Again the element changes.
Gamma emission causes no change to either the mass number or the atomic number, because a gamma ray has no mass and no charge. Only the energy of the nucleus changes.
Half-life
The half-life of a radioactive isotope is the time it takes for the number of nuclei of that isotope in a sample to halve, or equivalently the time for the count-rate from a sample to fall to half its initial level.
Because decay is random, half-life is a statistical measure that applies to large numbers of nuclei rather than a prediction about any individual nucleus.
Calculations follow a simple pattern. After one half-life half the original nuclei remain; after two half-lives a quarter remain; after three an eighth, and so on. Counting the halvings is more reliable than trying to use a formula.
Questions often ask for the net decline, which is the ratio of the remaining activity to the initial activity. After three half-lives the net decline is one eighth, which can also be expressed as 12.5 per cent remaining.
Contamination and irradiation
These two terms are frequently confused and the distinction is worth a great deal.
Irradiation is the process of exposing an object to nuclear radiation. The irradiated object does not become radioactive itself, and once it is removed from the source, or the source is removed, the irradiation stops. Suitable shielding and keeping the source at a distance reduce irradiation.
Contamination is the unwanted presence of materials containing radioactive atoms on or in other materials. The hazard is that the contaminating atoms continue to decay wherever they are, and the type of radiation emitted affects the level of hazard. Gloves, tongs and protective clothing reduce contamination.
The key difference: irradiation stops when the source is removed, but contamination stays with the object until the contaminating material is physically removed.
Because the effects of radiation on the human body can be serious, it is important that the conclusions of studies into these effects are published and shared with other scientists so that they can be checked by peer review.
Worked examples
Example 1: Completing an alpha decay (3 marks)
An isotope with mass number 238 and atomic number 92 emits an alpha particle. State the mass number and atomic number of the nucleus produced.
An alpha particle consists of two protons and two neutrons, so the mass number decreases by four and the atomic number decreases by two. The new mass number is 238 minus 4, which is 234. The new atomic number is 92 minus 2, which is 90. Because the atomic number has changed, the nucleus is now a different element.
Example 2: A half-life calculation (4 marks)
A sample has an activity of 800 becquerels. Its half-life is 6 days. Calculate the activity after 24 days.
The number of half-lives is the total time divided by the half-life, which is 24 divided by 6, giving 4 half-lives. Halving four times: 800 gives 400, then 200, then 100, then 50. The activity after 24 days is 50 becquerels, which is one sixteenth of the original.
Example 3: Distinguishing contamination from irradiation (4 marks)
A worker handles a radioactive source with bare hands and later finds their hands are giving a count-rate above background. Explain what has happened and how it differs from irradiation.
Some of the radioactive material has transferred onto the worker's hands, so they are contaminated: radioactive atoms are physically present and continue to decay there, emitting radiation until they are removed. This differs from irradiation, in which an object is exposed to radiation from a source but does not become radioactive itself and stops being affected as soon as it is moved away from the source. Contamination is reduced by wearing gloves and using tongs; irradiation is reduced by shielding and by increasing the distance from the source.
Common mistakes and how to avoid them
The most costly confusion in this unit is between contamination and irradiation. Remember that irradiated objects do not become radioactive, while contaminated objects carry the radioactive atoms with them.
Students frequently say alpha radiation is the most dangerous, or the least dangerous, without qualifying it. Alpha is the most ionising, so it is very dangerous inside the body, but it is stopped by skin or paper, so it is far less hazardous outside. The answer depends on whether the source is inside or outside.
In nuclear equations, many answers change the mass number during beta decay. The mass number is unchanged, because a neutron becomes a proton and the total number of protons and neutrons stays the same.
Another routine slip is defining half-life as the time for the substance to halve in mass. It is the time for the number of undecayed nuclei, or the count-rate, to halve.
Finally, students often forget that radioactive decay is random. Any answer suggesting a nucleus is due to decay, or that decay happens at regular intervals, misunderstands the process.
Exam technique for "Physics: Atomic Structure"
For half-life calculations, count the halvings on paper rather than attempting the arithmetic mentally. Writing 800, 400, 200, 100, 50 takes seconds and is checkable.
When a question gives a graph of count-rate against time, read the half-life from the graph by finding the initial value, halving it, and reading across to the curve and down to the time axis. Marks are given for showing those construction lines.
In questions about the atomic model, always link the evidence to the conclusion. Most alpha particles passing through shows mostly empty space; a few deflected back shows a small, dense, positively charged nucleus.
Where a question asks which radiation is suitable for a task, reason from penetrating power. If the radiation must pass through something, gamma or beta is needed; if it must be absorbed, alpha is appropriate.
Quick revision summary
Atoms have a radius of about 1 times 10 to the power minus 10 metres with a nucleus less than one ten-thousandth of that, holding nearly all the mass. The atomic number is the proton count and the mass number is protons plus neutrons; isotopes differ only in neutron number. The model developed from indivisible spheres through the plum pudding model, overturned by alpha scattering, to the nuclear model, then Bohr's shells, protons and neutrons. Decay is random; activity is measured in becquerels and count-rate with a Geiger–Müller tube. Alpha is most ionising and least penetrating, stopped by paper; beta is intermediate, stopped by a few millimetres of aluminium; gamma is least ionising and most penetrating, needing thick lead or concrete. Alpha decay reduces mass number by four and atomic number by two; beta decay leaves mass number unchanged and raises atomic number by one; gamma changes neither. Half-life is the time for the number of undecayed nuclei or the count-rate to halve. Irradiation stops when the source is removed; contamination persists until the radioactive atoms are removed.