Describe what is meant by alpha decay and explain why the nucleus of an atom undergoes radioactive decay.

WJEC GCSE Physics (Wales) — 2.4 The atom, the nucleus and radioactivity · Describe and Explain · 4 marks · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

Unstable nuclei can emit different types of radiation. One type of decay involves the emission of an alpha particle.

Model answer (4 marks)

An alpha particle is a helium‑4 nucleus, consisting of 2 protons and 2 neutrons. In alpha decay the parent nucleus emits this particle, so its atomic number falls by 2 and its mass number falls by 4. The nucleus undergoes radioactive decay because it is unstable – the forces inside the nucleus cannot balance the repulsive forces between protons. Radioactive decay is a random, spontaneous process, so the exact moment a particular nucleus decays cannot be predicted.

Examiner tips

Use the term ‘alpha particle’ and identify it as a helium‑4 nucleus.
Show the change in atomic and mass numbers clearly.
Explain instability as the cause of decay.
Mention the random nature of the process to justify the last point.

Common mistakes

Calling the alpha particle a ‘neutron’ or ‘proton’ only.
Forgetting to state the decrease in both atomic and mass numbers.
Claiming the decay can be predicted or is deterministic.

Mark scheme (4 marks)

An alpha particle consists of 2 protons and 2 neutrons (a helium nucleus)
The proton number (atomic number) decreases by 2 and the mass number decreases by 4
Radioactive decay occurs because the nucleus is unstable
Radioactive decay is a random process / cannot predict when a particular nucleus will decay

Key terms in this question

alpha decay · nucleus · radioactive decay

More The atom, the nucleus and radioactivity questions

▶ Try answering this question with AI marking (free) →