Nuclear physics

What the syllabus demands

• —Define mass defect and binding energy; use E = mc²
• —Sketch binding energy per nucleon against nucleon number; explain fission and fusion
• —Define activity and the decay constant: A = λN
• —Use the exponential decay law: N = N₀e^(−λt)
• —Relate half-life and decay constant: t½ = ln 2 ÷ λ

Definitions that earn marks

Clear definitions to practise — check your course mark scheme

Mass defect

The difference between the total mass of the separate nucleons and the mass of the nucleus they form.

Binding energy

The energy required to separate a nucleus into its individual nucleons — equivalently, the energy released when the nucleus forms from them: E = Δmc².

Decay constant (λ)

The probability per unit time that a given nucleus will decay: A = λN.

Activity

The number of decays per unit time of a sample, measured in becquerels (Bq).

The equations

More equations to practise: the Edexcel IAL formula sheet.

Where the marks die

Common mistakes to check

1. 01

   Reading the binding energy curve backwards. Iron-56 sits at the PEAK of binding energy per nucleon — the most stable nucleus. Fusion moves light nuclei up the curve towards it; fission moves heavy nuclei up towards it from the other side. Both release energy because binding energy per nucleon INCREASES.

2. 02

   Mass units chaos: masses given in atomic mass units (u) must become kilograms (1 u = 1.66 × 10⁻²⁷ kg) before E = mc² — or use 1 u = 931.5 MeV directly, but never mix the routes.

3. 03

   Confusing activity with count rate. A detector catches only a fraction of the decays; count rate is proportional to activity, not equal to it, and background must be subtracted first.

4. 04

   Decay-constant logic inverted: a SHORT half-life means a LARGE decay constant (λ = ln 2 ÷ t½) — the nucleus is more likely to decay each second, not less.

One worked example, done properly