Explain what happens to the nucleus of an atom during alpha decay, including how the proton number and nucleon number of the nucleus change.

Eduqas A-Level Physics — 1.7 Particles and nuclear structure · Explain · 5 marks · View as Markdown

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

Radioactive decay is a process in which an unstable nucleus loses energy by emitting radiation. Alpha decay is one type of radioactive decay.

Model answer (5 marks)

During alpha decay an α‑particle (a helium nucleus of 2 p and 2 n) is emitted from the parent nucleus.

The emission removes 2 protons and 2 neutrons, so the proton number (Z) of the nucleus decreases by 2.

The total nucleon number (A) decreases by 4.

The remaining nucleus has different Z and A, therefore it is a different element – a daughter nucleus is formed.

Examiner tips

Use the word "alpha particle" and state it is 2 p + 2 n; this shows you know its composition.
Show the two changes – Z ↓ 2 and A ↓ 4 – and link them to the new element.
Mention the daughter nucleus to cover the element change.

Common mistakes

Forgetting to state that the alpha particle contains 2 protons and 2 neutrons.
Only saying the proton number changes, omitting the nucleon number change.
Not recognising that the remaining nucleus is a different element.

Mark scheme (5 marks)

An alpha particle is emitted from the nucleus
An alpha particle consists of 2 protons and 2 neutrons (or is a helium nucleus)
The proton number of the nucleus decreases by 2
The nucleon number of the nucleus decreases by 4
The nucleus becomes a different element (a new nucleus / daughter nucleus is formed)

Key terms in this question

alpha decay · proton number · nucleon number · nucleus