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PhysicsGCSEAQA & EdexcelYears 10–11

GCSE Physics Atomic Structure & Radioactivity Notes

Atoms have a tiny dense nucleus surrounded by electrons. Unstable nuclei decay by emitting radiation — learn the three types and how to use half-life.

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Key concepts

Nuclear model
A small positive nucleus of protons and neutrons contains almost all the mass, with negative electrons orbiting in energy levels (shells).
Isotopes
Atoms of the same element with the same number of protons but a different number of neutrons.
Types of radiation
Alpha (α) — a helium nucleus, highly ionising, stopped by paper. Beta (β) — a fast electron, stopped by aluminium. Gamma (γ) — an EM wave, stopped by thick lead.
  • Radioactive decay is a random process — you cannot predict when a given nucleus will decay.
  • Alpha is the most ionising but least penetrating; gamma is the least ionising but most penetrating.
  • Background radiation comes from rocks, cosmic rays, food and medical sources.

Key formulas

Half-life idea

activity halves every half-life

after n half-lives, activity = initial ÷ 2ⁿ.

Worked example

A sample has an activity of 800 Bq and a half-life of 3 hours. What is the activity after 9 hours?

  1. 1Find the number of half-lives: 9 ÷ 3 = 3 half-lives.
  2. 2Halve once: 800 → 400 Bq.
  3. 3Halve again: 400 → 200 Bq.
  4. 4Halve a third time: 200 → 100 Bq.

Answer: 100 Bq

Common mistakes

✗ Thinking half-life is the time for the sample to fully decay.

✓ It is the time for the activity (or number of undecayed nuclei) to halve.

✗ Confusing the penetration of alpha and gamma.

✓ Alpha is stopped by paper; gamma needs thick lead. Alpha is most ionising, gamma least.

✗ Treating decay as predictable.

✓ Radioactive decay is random; half-life is an average behaviour of many nuclei.

Quick check

Try these, then reveal the answers.

What is an alpha particle made of?Reveal answer

Two protons and two neutrons (a helium nucleus).

Which radiation is stopped by aluminium?Reveal answer

Beta radiation.

A source of half-life 2 days starts at 160 Bq. Activity after 6 days?Reveal answer

6 ÷ 2 = 3 half-lives → 160 → 80 → 40 → 20 Bq

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Exam tips

  • For half-life, count how many times you halve rather than trying a formula.
  • Link ionising power and penetration: more ionising usually means less penetrating.
  • Remember decay is random — use the word 'random' when explaining half-life.

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Related topics

Radioactive Decay & Half-life

Key terms explained

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Frequently asked questions

What is half-life?+

Half-life is the average time taken for half the unstable nuclei in a sample to decay, or for the activity to fall to half its value.

Which type of radiation is the most dangerous inside the body?+

Alpha radiation, because it is the most strongly ionising and deposits all its energy in a small region of tissue if the source is ingested or inhaled.

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