Explain why some atoms are radioactive and describe what happens to the nucleus of a radioactive atom when it emits an alpha particle.

Eduqas GCSE Physics — 2.4 The atom, the nucleus and radioactivity · Explain · 4 marks · View as Markdown

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

All atoms contain a nucleus made up of protons and neutrons. Some nuclei are unstable and emit radiation spontaneously.

Model answer (4 marks)

A radioactive nucleus is unstable because its ratio of neutrons to protons is not optimal for stability.

It emits radiation spontaneously, without external influence, to become more stable.

When it emits an alpha particle, the particle consists of 2 protons and 2 neutrons.

The nucleus therefore loses 2 protons and 2 neutrons, so its proton number (Z) decreases by 2 and its mass number (A) decreases by 4.

Examiner tips

  • Use the phrase "unstable nucleus" and "spontaneously emits radiation". Describe the alpha particle as 2p+2n. State the changes in Z and A explicitly.
  • common_mistakes
  • :
  • Saying the nucleus becomes heavier instead of lighter. Forgetting to mention the loss of 2 protons and 2 neutrons. Using vague terms like "changes" without specifying Z and A.

Mark scheme (4 marks)

  1. A radioactive nucleus is unstable
  2. The nucleus emits radiation spontaneously / randomly to become more stable
  3. An alpha particle consists of 2 protons and 2 neutrons
  4. The nucleus loses 2 protons and 2 neutrons so its proton number decreases by 2 and its nucleon/mass number decreases by 4

Key terms in this question

radioactive · nucleus · alpha particle

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