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What is inside an atom, and how does radioactive decay work?

The structure of the atom and the development of the nuclear model, isotopes, the types of nuclear radiation (alpha, beta and gamma), radioactive decay and nuclear equations, half-life, and the dangers and uses of radiation.

A focused answer to the AQA GCSE Combined Science: Trilogy Atomic structure topic, covering the nuclear model of the atom, isotopes, alpha, beta and gamma radiation, radioactive decay and nuclear equations, half-life, and the dangers and uses of radiation.

Generated by Claude Opus 4.88 min answer

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  1. What this topic is asking
  2. The nuclear model and isotopes
  3. Types of radiation and decay
  4. Half-life and the uses of radiation

What this topic is asking

AQA wants you to describe the nuclear model of the atom and how it developed, define isotopes, describe alpha, beta and gamma radiation, write nuclear decay equations, define and use half-life, and discuss the dangers and uses of radiation.

The nuclear model and isotopes

Isotopes are atoms of the same element with the same number of protons but different numbers of neutrons. Some isotopes have unstable nuclei (too many or too few neutrons) and become more stable by emitting radiation, which is what makes them radioactive.

Types of radiation and decay

In an alpha decay the nucleus loses 2 protons and 2 neutrons, so the mass number falls by 4 and the atomic number by 2 (a new element is formed). In a beta decay a neutron turns into a proton and emits a fast electron, so the mass number is unchanged but the atomic number rises by 1. Gamma emission carries away energy only and changes neither number. Nuclear equations must balance: the total mass number and the total atomic number must be the same on both sides, which is how you work out an unknown particle. Activity (the rate of decay) is measured in becquerels (Bq) using a Geiger-Muller tube.

Half-life and the uses of radiation

Because decay is random but the half-life is constant, the activity falls by half each half-life: after one half-life it is one half, after two one quarter, after three one eighth, and so on. Radiation is dangerous because it ionises atoms (knocks electrons off them), which can damage or kill living cells and cause mutations or cancer; exposure is reduced by shielding, by keeping a distance, and by limiting the time near a source. Despite the risks, radiation is useful for medical tracers (a gamma source followed through the body), for treating cancer (targeted gamma rays), for sterilising surgical equipment and food, and in smoke detectors (which use a weak alpha source).

Exam-style practice questions

Practice questions written in the style of AQA exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.

AQA 20194 marksA radioactive isotope has a half-life of 6 hours. A sample has an activity of 800 counts per second. Calculate the activity after 18 hours, and explain why the decay of any single nucleus cannot be predicted.
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A Physics Paper 1 half-life calculation. Method: 18 hours is 186=3\dfrac{18}{6} = 3 half-lives. The activity halves each half-life: after 6 hours 400, after 12 hours 200, after 18 hours 100 counts per second. (Equivalently, 800×(12)3=800÷8=100800 \times \left(\dfrac{1}{2}\right)^3 = 800 \div 8 = 100.) For the explanation: radioactive decay is a random process, so although we know the probability that a nucleus decays in a given time, we cannot say which nucleus will decay or when; only the behaviour of a large number of nuclei is predictable. Markers credit the number of half-lives, the halving, and the idea that decay is random.

AQA 20214 marksCompare the penetrating power and ionising ability of alpha, beta and gamma radiation, and explain why alpha radiation is the most dangerous if a source is swallowed.
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A Physics Paper 1 comparison. Reward: alpha is the most strongly ionising but least penetrating (stopped by paper or a few centimetres of air); beta is moderately ionising and more penetrating (stopped by a few millimetres of aluminium); gamma is the least ionising but most penetrating (reduced by thick lead or concrete). Inside the body, alpha radiation is the most dangerous because it is highly ionising and is absorbed by nearby tissue over a very short range, so it deposits all its energy in a small region of cells, causing the most damage. (Outside the body alpha is least dangerous because skin stops it.) Markers credit the ranked comparison and the link from high ionisation and short range to tissue damage when swallowed.

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