Leptons & Fundamental Particles

• A fundamental particle cannot be split into anything smaller. As far as we know, quarks and electrons are fundamental.
• A proton is not fundamental, because it is made of three quarks. We say it is composite. The mark scheme wants the exact idea: a fundamental particle cannot be subdivided. (9702/22/O/N/25 Q6(a))

• Particles split into two families, by whether they are built from quarks.
• Hadrons are made of quarks. Protons and neutrons are hadrons.
• Leptons are fundamental and are not made of quarks. The electron is a lepton, and so is the neutrino (the small, chargeless particle in beta decay). (9702/22/F/M/24 Q8)
• So the electron and the proton are in different families. The electron is a fundamental lepton. The proton is a composite hadron. A positron is also a lepton (it is the antiparticle of the electron). (9702/11/O/N/25 Q40)

Made of quarks → hadron (proton, neutron). Fundamental, no quarks → lepton (electron, positron, neutrino). Fundamental means it cannot be split.