AS Level Physics · Particle physics. A short explanation of the idea, the rules to remember, the mistake to avoid, a worked example and practice questions with answers.
When α-particles were fired at thin gold foil, most went straight through, so an atom is mostly empty space. A very few bounced back, so the positive charge and nearly all the mass are in a tiny nucleus. Electrons orbit it.
The nucleus contains protons and neutrons (nucleons). A nuclide is written AZX, with nucleon number A on top and proton number Z below. Isotopes have the same Z but different numbers of neutrons.
In every decay the total nucleon number and the total charge are conserved. An α-particle leaves with one fixed energy. A β-particle shares the energy with a neutrino or antineutrino, so β-particles have a continuous range of energies.
In β− decay students make the proton number go down. A neutron becomes a proton, so Z goes up by 1 and A stays the same.
Answer: The final nucleus is 22890Th: nucleon number 228, proton number 90.
Try each one, then open the answer.
Answer: C. β− is an electron emitted when a neutron becomes a proton; β+ is a positron emitted when a proton becomes a neutron.
Answer: C. 1 u = 1/12 of the mass of a 12C atom = 1.66 × 10−27 kg.
Answer: B. n → p + e− + ν̄e. The electron antineutrino accounts for the spread of electron energies in beta decay.
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