Radioactivity questions

Radioactivity syllabus

Questions on radioactivity can test recall, calculations, explanations, diagrams, data handling and practical skills. You should be able to:

• 7.2 describe the structure of an atom in terms of protons, neutrons and electrons and use symbols such as ¹⁴₆C to describe particular nuclei
• 7.3 know the terms atomic (proton) number, mass (nucleon) number and isotope
• 7.4 know that alpha (α) particles, beta (β−) particles, and gamma (γ) rays are ionising radiations emitted from unstable nuclei in a random process
• 7.5 describe the nature of alpha (α) particles, beta (β−) particles and gamma (γ) rays, and recall that they may be distinguished in terms of penetrating power and ability to ionise
• 7.6 practical: investigate the penetration powers of different types of radiation using either radioactive sources or simulations
• 7.7 describe the effects on the atomic and mass numbers of a nucleus of the emission of each of the four main types of radiation (alpha, beta, gamma and neutron radiation)
• 7.8 understand how to balance nuclear equations in terms of mass and charge
• 7.9 know that photographic film or a Geiger−Müller detector can detect ionising radiations
• 7.10 explain the sources of background (ionising) radiation from Earth and space
• 7.11 know that the activity of a radioactive source decreases over a period of time and is measured in becquerels
• 7.12 know the definition of the term 'half-life' and understand that it is different for different radioactive isotopes
• 7.13 use the concept of the half-life to carry out simple calculations on activity, including graphical methods
• 7.14 describe uses of radioactivity in industry and medicine
• 7.15 describe the difference between contamination and irradiation
• 7.16 describe the dangers of ionising radiations, including: that radiation can cause mutations in living organisms that radiation can damage cells and tissue the problems arising from the disposal of radioactive waste and how the associated risks can be reduced

How to answer radioactivity questions

1. Track mass number and atomic number separately when completing nuclear equations.
2. Compare radiation using both ionising ability and penetrating power.
3. For half-life, repeat the same halving interval and keep the starting quantity clear.
4. Distinguish irradiation from contamination and connect each risk reduction to the exposure route.

Radioactivity example questions and worked answers

These examples show how information in a diagram, graph or experimental context becomes part of a complete exam answer.

Example 1: Plotting three half-lives of iodine-131

June 2024 (R) · Paper 1P · Question 8(a)(iii)

Question 1

This question is about radioactive isotopes used for medical imaging.

Iodine-131 is represented by this symbol.

The cross (×) on the graph shows the initial number of atoms in a sample of iodine-131.

Draw three more crosses (×) on the graph to show how the number of atoms of iodine-131 in the sample changes during three half-lives.

[iodine-131 half-life = 8 days]

Final answer

Draw crosses at (8 days, 8000 atoms), (16 days, 4000 atoms) and (24 days, 2000 atoms).

Mark scheme points

1. M1 A cross at (8 days, 8000 atoms).
2. M2 A cross at (16 days, 4000 atoms).
3. M3 A cross at (24 days, 2000 atoms).

Explanation

The half-life is 8 days, so the number of atoms halves every 8 days. Starting from 16 000 atoms, halve the number at each 8-day interval:

0 days: 16 000 atoms
8 days:   8 000 atoms
16 days:  4 000 atoms
24 days:  2 000 atoms

Plot the three later values as crosses on the graph.

Common mistakes

• The examiner report notes that many candidates correctly recognised that the number of nuclei halves every half-life. Apply that halving repeatedly at 8-day intervals.
• Do not subtract the same number of atoms at each interval: halve the previous number each time.

Example 2: Balancing the beta decay of strontium-90

January 2015 · Paper 2P · Question 7(b)(i)

Question 2

An unstable isotope of strontium has a half-life of 28.8 years.

It is a beta emitter and can be represented by this symbol.

When a strontium-90 nucleus emits a beta particle, it decays to form yttrium-90.

Complete the equation for this decay.

Final answer

The completed decay equation is:

Mark scheme points

1. M1 Write 90 as the upper number for yttrium and 0 as the upper number for the beta particle. Both are required for this mark.
2. M2 Write −1 as the lower number for the beta particle. The minus sign is essential.

Explanation

In beta-minus decay, a neutron changes into a proton and an electron is emitted. The nucleus keeps the same total number of nucleons, but its proton number increases by one.

• Check the upper numbers: 90 = 90 + 0. The emitted electron contains no nucleons.
• Check the lower numbers: 38 = 39 + (−1). The electron has a negative charge, balancing the increase in the nucleus’s proton number.

Common mistakes

• Do not decrease yttrium’s mass number: beta-minus decay leaves it at 90. Complete both upper-number blanks to earn the first mark.
• Do not write 1 instead of −1 below the beta symbol: the negative sign is required.

Practise Radioactivity questions

Build a focused practice set from this part of the Edexcel IGCSE Physics syllabus.