Explain why alpha particles are described as highly ionising but weakly penetrating.

Cambridge International IGCSE Physics (0625) — 5.2 Radioactivity · Explain · 4 marks · View as Markdown

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

Model answer (4 marks)

Alpha particles carry a +2 charge, so they have a large positive charge.
They interact strongly with the electrons of atoms in the material, removing electrons and forming ions.
Because they ionise so frequently, they lose energy rapidly.
Consequently they are stopped after a very short distance – a few centimetres of air or a sheet of paper.

Examiner tips

Use the charge (+2) to explain the strong interaction with electrons. Show the link between frequent ionisation and rapid energy loss. Mention the short stopping distance to emphasise weak penetration.
Use the exact terminology: "highly ionising" and "weakly penetrating".
Avoid vague phrases such as "they are strong" – specify the charge and ionisation mechanism.

Common mistakes

Not stating the +2 charge of alpha particles. Failing to explain that ionisation removes electrons. Claiming they are weakly penetrating without linking to rapid energy loss.

Mark scheme (4 marks)

Alpha particles have a large (positive) charge / carry charge of +2
The large charge means alpha particles interact strongly with / remove electrons from surrounding atoms, forming ions
Because alpha particles ionise so frequently / lose energy rapidly to surrounding matter
So they are stopped after a short distance / stopped by a few centimetres of air or a sheet of paper

Key terms in this question

alpha particle · ionising · penetrating · ion