The emission of a gamma ray does not cause the mass or the charge of the…

The idea

Gamma emission is a way for an unstable nucleus to lose surplus energy without altering what the nucleus is. A gamma ray is a high-energy electromagnetic wave, so it carries energy away but has no mass and no charge. Because the proton number and the nucleon number are untouched, the emitting nuclide keeps the same atomic number and mass number, and the nuclear equation balances with the same symbol on both sides, often written with a gamma term such as γ. For example, technetium-99m emits a gamma ray to become technetium-99: the mass number stays 99 and the atomic number stays 43. This contrasts with alpha emission, which lowers the mass number by 4 and the atomic number by 2, and beta emission, which leaves the mass number unchanged but raises the atomic number by 1.

What earns the marks

• States that a gamma ray is electromagnetic radiation with no mass and no electric charge.
• Explains that gamma emission changes only the energy of the nucleus, not its proton number or nucleon number.
• Applies the rule to a nuclear equation by keeping the same nuclide symbol on both sides, for example Tc-99m → Tc-99 + γ.
• Contrasts gamma emission with alpha emission, which reduces mass number by 4 and atomic number by 2, and beta emission, which raises atomic number by 1 while keeping mass number the same.
• Uses the terms proton number and nucleon number correctly when justifying why the nucleus is unchanged.

Common mistakes and how to put them right

• Thinking a gamma ray removes a particle from the nucleus.

  Put it right by stressing that gamma is energy only, so no proton or neutron is lost.

• Writing a nuclear equation in which the mass number or atomic number changes after gamma emission.

  Put it right by balancing both numbers and showing the same nuclide before and after.

• Confusing gamma emission with beta emission because both may follow a decay.

  Put it right by noting that beta emission changes a neutron into a proton and so raises the atomic number, whereas gamma emission does not.

Exam tips

• When asked to complete a nuclear equation, check that the total mass number and total atomic number are equal on both sides before writing the gamma symbol.
• If a question asks why the nucleus is unchanged, refer explicitly to gamma having zero mass and zero charge rather than saying it is 'just energy'.
• Use the metastable form, such as Tc-99m, when a nucleus emits gamma to reach a lower energy state, and explain that m indicates a higher-energy state of the same nuclide.