Stable and unstable nuclei: Unstable nuclei; alpha and beta decay.

The idea

Unstable nuclei have an imbalance of protons and neutrons, or too much energy, and decay randomly to become more stable. Alpha decay emits an alpha particle (2 protons and 2 neutrons, a helium nucleus), reducing proton number by 2 and nucleon number by 4; it usually occurs in heavy nuclei where electrostatic repulsion is large. Beta-minus decay converts a neutron into a proton, emitting an electron and an antineutrino, increasing proton number by 1 while nucleon number stays the same; beta-plus decay converts a proton into a neutron, emitting a positron and a neutrino, decreasing proton number by 1. For example, carbon-14 beta-minus decays to nitrogen-14. Each decay is spontaneous and random, and the nucleus often emits gamma radiation afterwards as it loses excess energy.

What earns the marks

• Defines an unstable nucleus as one with an imbalance of protons and neutrons or excess energy that leads to spontaneous decay.
• Describes alpha decay as emission of an alpha particle (2 protons and 2 neutrons), decreasing proton number by 2 and nucleon number by 4.
• Describes beta-minus decay as a neutron changing into a proton with emission of an electron and an antineutrino, increasing proton number by 1.
• Describes beta-plus decay as a proton changing into a neutron with emission of a positron and a neutrino, decreasing proton number by 1.
• Applies conservation of nucleon number and charge to write or check nuclear decay equations.

Common mistakes and how to put them right

• Writing alpha decay as changing proton number by 4 and nucleon number by 2.

  Put it right: Proton number decreases by 2 and nucleon number decreases by 4.

• Forgetting the antineutrino in beta-minus decay or the neutrino in beta-plus decay.

  Put it right: Include the appropriate neutrino or antineutrino to conserve energy and momentum.

• Treating beta particles as orbiting electrons.

  Put it right: Beta particles are emitted from the nucleus when a neutron or proton changes type.

Exam tips

• Balance nuclear equations by checking that the total nucleon number and total charge are the same on both sides.
• Use the notation for alpha, beta-minus and beta-plus particles correctly, including the antineutrino or neutrino, and state that decay is spontaneous and random.
• When explaining stability, link the type of decay to whether the nucleus has too many neutrons, too many protons or is simply too heavy.