Explain what gamma radiation is and describe why emitting gamma radiation does not change the mass number or atomic number of a nucleus. Include in your answer a description of what happens inside the nucleus during gamma emission.

WJEC A-Level Physics (Wales) — 3.5 Nuclear decay · Explain · 5 marks · View as Markdown

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

Some radioactive nuclei release energy after emitting an alpha or beta particle by emitting gamma radiation.

Model answer (5 marks)

Gamma radiation is a high‑frequency, short‑wavelength electromagnetic wave. It has no mass (mass number zero) and no charge (atomic number zero). Because it carries no mass or charge, the emitting nucleus keeps the same mass number and atomic number. During gamma emission the nucleus, which has been left in an excited state after the alpha or beta decay, releases the excess energy by emitting a gamma photon and falls to a lower, more stable energy level.

Examiner tips

  • Define gamma as EM wave, state zero mass and charge, explain unchanged A and Z, mention energy release to lower state

Common mistakes

  • Confusing gamma with alpha or beta particles, claiming gamma changes mass or charge, omitting the energy‑state change

Mark scheme (5 marks)

  1. Gamma radiation is a high-frequency / short-wavelength electromagnetic wave / ray
  2. Gamma radiation has no mass (or mass number of zero)
  3. Gamma radiation has no charge (or atomic number / proton number of zero)
  4. Because gamma radiation carries no mass and no charge, the mass number and atomic number of the nucleus remain unchanged
  5. The nucleus releases excess energy (moves from a higher energy state to a lower / more stable energy state) during gamma emission

Key terms in this question

gamma radiation · mass number · atomic number

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