+ Possible decay modes of unstable nuclei including α, β , − β and…

The idea

Unstable nuclei adjust their proton number Z and neutron number N to move towards stability. An alpha particle is a helium nucleus, ⁴₂He, so alpha emission lowers Z by 2 and N by 2. Beta-minus decay converts a neutron into a proton, emitting an electron and an antineutrino, so Z increases by 1 while N falls by 1. Beta-plus decay converts a proton into a neutron, emitting a positron and a neutrino, so Z decreases by 1 while N rises by 1. Electron capture pulls an inner electron into the nucleus, combining with a proton to form a neutron and a neutrino, again lowering Z by 1 and raising N by 1. Each mode changes the nucleus differently, so the N-Z plot reveals which decay suits a given nuclide.

What earns the marks

• Alpha emission removes two protons and two neutrons, so the daughter nuclide has Z decreased by 2 and N decreased by 2, with mass number A decreased by 4.
• Beta-minus decay changes a neutron into a proton, so Z increases by 1 and N decreases by 1 while A stays constant; an electron and an antineutrino are emitted.
• Beta-plus decay changes a proton into a neutron, so Z decreases by 1 and N increases by 1 while A stays constant; a positron and a neutrino are emitted.
• Electron capture combines an inner-shell electron with a proton to form a neutron, so Z decreases by 1 and N increases by 1 while A stays constant; a neutrino is emitted.
• The N-Z plot shows that neutron-rich nuclides tend to undergo beta-minus decay, proton-rich nuclides tend to undergo beta-plus decay or electron capture, and heavy nuclides tend to undergo alpha emission.

Common mistakes and how to put them right

• Treating beta-minus decay as loss of an electron from the nucleus.

  Put it right: A neutron transforms into a proton and the emitted electron is created in the decay.

• Confusing beta-plus decay with electron capture because both lower Z by 1.

  Put it right: Beta-plus emits a positron while electron capture absorbs an inner electron.

• Assuming alpha emission changes only Z.

  Put it right: Both Z and N fall by 2 and A falls by 4.

Exam tips

• Write the parent and daughter nuclides with correct A and Z before balancing any decay equation.
• Check conservation of A and Z after every decay to confirm the mode is consistent.
• Use the N-Z plot to justify why a particular nuclide decays by one mode rather than another.