P4 Atomic structure — Section Overview

Atomic structure explains the composition of matter at its most fundamental level and introduces the concept of radioactivity — the spontaneous emission of radiation from unstable nuclei. This section also covers nuclear fission and fusion, which underpin both nuclear power and nuclear weapons.

What this section covers

Sub-topic	Key ideas
P4.1 Atoms and isotopes	Protons, neutrons, electrons; atomic number Z; mass number A; isotopes
P4.2 Radioactive decay	Alpha, beta-minus, beta-plus, gamma; why nuclei decay; decay equations
P4.3 Nuclear radiation	Properties and penetrating power of each type; uses and hazards
P4.4 Half-life	Decay curves; half-life definition; activity calculations
P4.5 Nuclear fission and fusion	Chain reactions; nuclear reactors; fusion in stars; E = mc²

The atom

The atom has a tiny, dense nucleus (protons + neutrons) surrounded by electrons in shells. Proton number Z = number of protons = number of electrons in a neutral atom. Mass number A = protons + neutrons. Isotopes are atoms with the same Z but different neutron numbers.

Radioactive decay

Unstable nuclei decay spontaneously to become more stable. The type of emission depends on the ratio of neutrons to protons:

• Alpha (α): He-4 nucleus; strongly ionising; stopped by a few cm of air or paper.
• Beta-minus (β⁻): fast electron from neutron → proton + electron + antineutrino; moderately ionising; stopped by ~3 mm aluminium.
• Beta-plus (β⁺): positron from proton → neutron + positron + neutrino (physics-only).
• Gamma (γ): high-frequency EM radiation; weakly ionising; needs several cm of lead to attenuate significantly.

Half-life

The time for half the radioactive nuclei (or the activity) to decay. Each half-life reduces activity by half. After n half-lives: remaining fraction = (1/2)^n.

Nuclear reactions

Fission: large nucleus splits into two medium nuclei + neutrons + energy. Chain reactions sustain themselves. Fusion: two light nuclei combine → larger nucleus + energy. Fusion requires extremely high temperatures (stars).

Exam focus

• Write balanced nuclear equations — check A and Z on both sides.
• Use half-life graphs carefully: read the time axis, not the activity axis.
• Explain how a chain reaction is controlled in a nuclear reactor (control rods absorb neutrons).