Particle Physics: Atoms, Nuclei and Fundamental Particles

This guide covers Topic 11, Particle physics, in full — subtopics 11.1 Atoms, nuclei and radiation and 11.2 Fundamental particles — from Cambridge International AS & A Level Physics 9702, 2025–2027 series. This is the final AS Level topic.

Before studying this

No specific earlier 9702 topic is required, though a basic IGCSE/O Level understanding of protons, neutrons and electrons is assumed.

Syllabus coverage

CAMBRIDGE INTERNATIONAL AS & A LEVEL PHYSICS 9702 — AS Level, Topic 11

11.1 Atoms, nuclei and radiation — inferring the nucleus’s existence and small size from the α-particle scattering experiment; a simple nuclear model (protons, neutrons, orbital electrons); distinguishing nucleon number from proton number; isotopes; the notation for nuclides; conservation of nucleon number and charge in nuclear processes; the composition, mass and charge of α, β and γ radiation (both β⁻ and β⁺); antiparticles, including the positron as the antiparticle of the electron; (anti)neutrinos in β decay; why α particles have discrete energies but β particles a continuous range; writing a radioactive decay equation; the unified atomic mass unit (u).

11.2 Fundamental particles — quarks as fundamental particles with six flavours (up, down, strange, charm, top, bottom); quark and antiquark charges; protons and neutrons as composed of quarks (not fundamental themselves); a hadron as a baryon (three quarks) or meson (a quark and an antiquark); the quark-composition changes during β⁻ and β⁺ decay; electrons and neutrinos as fundamental particles called leptons.

The nuclear atom

The α-particle scattering experiment showed that most of an atom’s mass and all of its positive charge is concentrated in a tiny, dense nucleus, with orbital electrons occupying the much larger surrounding space. The simple nuclear model: protons and neutrons in the nucleus, electrons orbiting.

Nucleon number A is the total number of protons and neutrons; proton number Z is the number of protons only. Isotopes of an element share the same proton number but different nucleon numbers (different numbers of neutrons). A nuclide is written ᴬZX.

Both nucleon number and charge are conserved in nuclear processes — useful for balancing decay equations.

Radioactive decay: α, β and γ

Radiation	Composition	Mass (u)	Charge
α	2 protons + 2 neutrons (helium nucleus)	4	+2
β⁻	electron	~0	−1
β⁺	positron	~0	+1
γ	electromagnetic radiation	0	0

A positron is the antiparticle of an electron — same mass, opposite charge. β⁻ decay emits an (electron) antineutrino; β⁺ decay emits an (electron) neutrino, conserving both energy and other quantities across the decay. Because a neutrino or antineutrino shares the released energy with the β particle in a variable proportion, β particles have a continuous range of energies, unlike α particles, which have discrete energies (the nucleus and α particle share a fixed energy release between just two bodies).

Example α-decay equation:

²³⁸₉₂U → ²³⁴₉₀Th + ⁴₂α

Nucleon number (238 = 234 + 4) and charge/proton number (92 = 90 + 2) both balance.

Fundamental particles

Quarks are fundamental particles with six flavours, each with a fixed fractional charge (in units of e): up (u), charm (c) and top (t) each carry +2/3; down (d), strange (s) and bottom (b) each carry −1/3. Every quark has a corresponding antiquark with the opposite charge (e.g. an anti-up, ū, carries −2/3). Protons and neutrons are not fundamental — each is built from three quarks: a proton is uud (charge 2/3 + 2/3 − 1/3 = +1), a neutron is udd (charge 2/3 − 1/3 − 1/3 = 0). You are expected to recall both the quark charges and these explicit compositions, not just the composition changes during β decay.

A hadron is any particle made of quarks: a baryon (three quarks, like a proton or neutron) or a meson (a quark and an antiquark). Leptons (electrons, neutrinos) are fundamental particles, not made of quarks.

During β⁻ decay, a neutron effectively converts to a proton (one down quark changes to an up quark), emitting an electron and antineutrino. During β⁺ decay, a proton converts to a neutron (an up quark changes to a down quark), emitting a positron and neutrino.

Common mistakes

• Describing β decay as simply “a neutron turning into a proton” without reference to the quark-level change when a fuller explanation is asked for — the syllabus explicitly expects the quark-composition change.
• Forgetting that α particles have discrete energies while β particles have a continuous range — this distinction, and its explanation via the (anti)neutrino sharing energy variably, is a frequently tested point.
• Miscounting nucleon number or charge when balancing a decay equation — always check both totals separately.
• Assuming protons and neutrons are fundamental particles — they are hadrons (baryons), built from quarks. Quarks and leptons are the fundamental matter particles at this level; this doesn’t exclude other fundamental particles outside the matter/hadron classification, such as the photon and other force-carrying (elementary boson) particles.

Quick revision checklist

• The nuclear model of the atom, and what α-particle scattering demonstrated
• Nucleon number vs. proton number; isotopes; nuclide notation
• Composition, mass and charge of α, β⁻, β⁺ and γ radiation
• Why β particles have a continuous energy range and α particles don’t
• Writing and balancing a radioactive decay equation
• Quarks, hadrons (baryons/mesons), leptons, and the quark-level description of β decay

Related resources

• D.C. Circuits: Kirchhoff’s Laws and Potential Dividers — the previous AS topic
• Motion in a Circle — the first A Level topic
• Cambridge AS & A Level Physics hub

Written against Cambridge International AS & A Level Physics 9702, 2025–2027 series. Always check the current syllabus for your examination year.