(c) nuclear decay equations for alpha, beta- minus and beta-plus decays;…

The idea

Nuclear decay equations track changes in proton and nucleon numbers. Alpha emission removes a helium nucleus, so the parent loses two protons and two neutrons: for example, ²³⁸U → ²³⁴Th + ⁴He. Beta-minus decay converts a neutron into a proton and emits an electron and an antineutrino: ¹⁴C → ¹⁴N + ⁰e + antineutrino. Beta-plus decay converts a proton into a neutron and emits a positron and a neutrino: ²²Na → ²²Ne + ⁰e + neutrino. Balancing means the total nucleon number and total proton number are equal on both sides. Use the nuclide notation with mass number as a superscript and atomic number as a subscript, and include the emitted particle with its own numbers.

What earns the marks

• Alpha decay: parent nucleus loses 2 protons and 2 neutrons, emitting a helium nucleus (alpha particle).
• Beta-minus decay: a neutron changes into a proton, emitting an electron and an antineutrino; proton number increases by 1, nucleon number unchanged.
• Beta-plus decay: a proton changes into a neutron, emitting a positron and a neutrino; proton number decreases by 1, nucleon number unchanged.
• Balancing nuclear equations: sum of mass numbers on each side must be equal, and sum of atomic numbers on each side must be equal.
• Correct notation for emitted particles: alpha as ⁴₂He, beta-minus as ⁰₋₁e, beta-plus as ⁰₊₁e, with neutrinos/antineutrinos included where required.

Common mistakes and how to put them right

• Forgetting to balance nucleon numbers.

  Put it right: Check both mass and atomic number sums.

• Confusing beta-minus and beta-plus changes.

  Put it right: Link beta-minus to neutron-rich nuclei and beta-plus to proton-rich nuclei.

• Omitting the neutrino or antineutrino in beta decay equations.

  Put it right: Include them to conserve lepton number and energy, as required by the specification.

• Using incorrect symbols for particles, such as writing e⁻ without the mass and atomic numbers.

  Put it right: Use ⁰₋₁e for beta-minus and ⁰₊₁e for beta-plus.

Exam tips

• In multiple-choice questions, check that both mass number and atomic number are conserved before selecting an answer.
• Practise writing decay equations for common isotopes, such as ²³⁸U, ¹⁴C and ²²Na, to build speed and accuracy.
• Remember that alpha decay reduces both mass number by 4 and atomic number by 2; beta decay leaves mass number unchanged.