How to Revise Topic 5: Nuclear Physics

Topic 5 is one of the shorter topics but is conceptually demanding. Students who understand the particle model and nuclear notation tend to find it straightforward.

What Topic 5 covers

1. The nuclear model of the atom (protons, neutrons, electrons, isotopes, Rutherford scattering)
2. Radioactivity (alpha, beta, gamma; properties, detection, background radiation)
3. Half-life (definition, calculations, graphical determination)
4. Nuclear reactions (fission, fusion, nuclear equations)

Priority areas

Must-know facts

• Proton: charge +1, mass 1, in nucleus
• Neutron: charge 0, mass 1, in nucleus
• Electron: charge -1, mass 1/1836, orbiting nucleus
• Atomic (proton) number Z = number of protons
• Mass (nucleon) number A = protons + neutrons
• Isotopes: same Z, different A

Must-know radiation properties

Property	Alpha	Beta	Gamma
Identity	Helium nucleus (2p + 2n)	High-speed electron	EM wave
Charge	+2	-1	0
Mass	4	~0	0
Penetration	Stopped by paper	Stopped by aluminium (~5 mm)	Reduced by thick lead or concrete
Ionising ability	Strongest	Moderate	Weakest
Deflection in fields	Deflected (toward negative plate)	Deflected (toward positive plate)	Not deflected

Must-know calculations

• Half-life from a decay graph or table
• Remaining activity or mass after n half-lives: N = N0 x (1/2)^n
• Nuclear equations balancing (conserve mass number and atomic number)

Must-know concepts

• Why radioactive decay is random and spontaneous (cannot be predicted or controlled)
• Sources of background radiation (cosmic rays, rocks, medical, food, radon gas)
• Why background count must be subtracted from measured count
• The difference between irradiation and contamination
• Safety precautions for handling radioactive sources
• How fission produces a chain reaction (neutrons released hit more nuclei)
• How fusion requires extremely high temperatures (to overcome electrostatic repulsion)

Revision strategy for Topic 5

2-day intensive plan

• Day 1: Atomic structure (learn nuclear notation, isotopes, Rutherford scattering experiment and its conclusions). Radiation types (learn the table above, practise comparing alpha, beta and gamma). Nuclear equations.
• Day 2: Half-life (definition, graph reading, calculations). Fission and fusion (describe each, compare, explain why fusion is difficult). Safety and applications. Past paper questions.

Common mistakes in Topic 5

• Confusing atomic number with mass number.
• Saying alpha radiation is “absorbed” by paper (it is stopped or blocked, not absorbed in the physics sense).
• Writing unbalanced nuclear equations (always check that mass numbers and atomic numbers balance).
• Confusing fission and fusion (fission splits large nuclei; fusion joins small nuclei).
• Saying half-life means “half the radiation is gone” (it means half the unstable nuclei have decayed, or the activity has halved).
• Not subtracting background radiation when analysing experimental data.

Self-test checklist

After revision, you should be able to:

• State the properties of protons, neutrons and electrons.
• Define isotope and give an example.
• Compare alpha, beta and gamma radiation.
• Read half-life from a graph or table.
• Calculate remaining activity after multiple half-lives.
• Write and balance nuclear equations.
• Describe fission and fusion and their differences.
• State safety precautions for handling radioactive sources.