Atomic Structure
An atom has a radius of about 1 x 10^-10 m, with a nucleus, containing protons and neutrons, that is less than 1/10 000 of that radius yet holds almost all of the atom's mass. Isotopes are atoms of the same element with different numbers of neutrons, and unstable isotopes decay at random, emitting alpha, beta or gamma radiation, with the activity measured in becquerels and the half-life defined as the time taken for the activity to halve.
Before you start
No specific prerequisites - this is a good place to start.
Method
- Learn the three radiations as a set of contrasts: alpha is stopped by paper or a few centimetres of air, is the most strongly ionising and travels the shortest distance; beta passes through paper but is stopped by a few millimetres of aluminium and is moderately ionising; gamma is only reduced by thick lead or metres of concrete and is the least ionising per unit distance.
- Write nuclear equations by conserving mass number and atomic number: in alpha decay the mass number falls by 4 and the atomic number by 2; in beta decay the mass number is unchanged and the atomic number increases by 1; gamma emission changes neither.
- Define half-life carefully: the time it takes for the number of unstable nuclei in a sample, or the activity, to halve. Decay is random, so half-life is a statistical average and you cannot predict when any individual nucleus will decay.
- For half-life calculations, count the halvings: divide the starting activity by 2 repeatedly until you reach the final value, count how many times you did it, and multiply that number by the half-life to get the total time.
- Distinguish irradiation from contamination, since it is examined nearly every year: irradiation is being exposed to radiation from an outside source and does not make the object radioactive, while contamination is the unwanted presence of radioactive atoms on or in a material, and it continues to expose the object until the atoms are removed or decay.
- Match a source to its use through half-life and penetration: a source used as a medical tracer must emit gamma, so the radiation escapes the body, and must have a short half-life so it does not continue to irradiate the patient, while a thickness gauge for aluminium foil needs a beta source, since alpha would be stopped by the foil entirely and gamma would pass through unchanged.
Worked example
A radioactive sample has an activity of 800 Bq. After 18 hours the activity has fallen to 100 Bq. Calculate the half-life of the sample.
- Halve the starting activity repeatedly and count the steps: 800 to 400 is one half-life.
- 400 to 200 is a second half-life.
- 200 to 100 is a third half-life.
- So three half-lives have passed in 18 hours.
- Divide the total time by the number of half-lives: 18 / 3 = 6.
- Final answer: the half-life is 6 hours.
Practice questions
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Q1State the approximate radius of an atom.Show answer
Answer: About 1 x 10^-10 m.
Q2What is an alpha particle made of?Show answer
Answer: Two protons and two neutrons, the same as a helium nucleus.
Q3What happens inside the nucleus when a beta particle is emitted?Show answer
Answer: A neutron changes into a proton and a high-speed electron, which is emitted as the beta particle.
Q4Which type of radiation is stopped by a sheet of paper?Show answer
Answer: Alpha.
Q5Define half-life.Show answer
Answer: The time taken for the number of unstable nuclei in a sample, or its activity, to halve.
Q6A source has a half-life of 5 days and an initial activity of 1600 Bq. What is its activity after 15 days?Show answer
Answer: 15 days is three half-lives: 1600 to 800 to 400 to 200, so the activity is 200 Bq.
Q7State the difference between irradiation and contamination.Show answer
Answer: Irradiation is exposure to radiation from an external source and does not make the object radioactive; contamination is radioactive atoms being present on or in the object, which continues to emit radiation.
Exam-style questions
Written in the style of a GCSE Science exam paper, with a full mark scheme.
Uranium-238 (atomic number 92) decays by alpha emission to form thorium. State the mass number and atomic number of the thorium produced, and explain how you worked them out.
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A hospital needs a radioactive tracer to follow the flow of blood through a patient's kidneys. Explain why a gamma emitter with a half-life of six hours is more suitable than an alpha emitter with a half-life of ten years.
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Free printable worksheet
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This topic is chapter 5 of GCSE Physics Workbook, the whole course as one free printable PDF.
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