Atomic structure and radioactivity
The nuclear atom
An atom is mostly empty space. The nucleus (protons + neutrons) sits at the centre and contains nearly all the mass. Electrons orbit in shells. Atomic number Z = protons; mass number A = protons + neutrons.
Isotopes are atoms of the same element with the same Z but different A. Some isotopes are unstable — they decay, releasing radiation.
Three types of radiation
| Type | What it is | Range in air | Stopped by | Ionising power |
|---|---|---|---|---|
| Alpha (α) | Helium nucleus, 2p + 2n | a few cm | a sheet of paper / skin | Very high |
| Beta (β) | Fast electron from the nucleus | ~1 m | a few mm of aluminium | Medium |
| Gamma (γ) | High-energy electromagnetic wave | many m | thick lead / metres of concrete | Low |
Alpha decay reduces A by 4 and Z by 2. Beta decay leaves A unchanged but increases Z by 1 (a neutron becomes a proton + electron). Gamma decay alone changes neither Z nor A — the nucleus simply releases excess energy.
Half-life
Half-life = the time taken for the activity (or the number of undecayed nuclei) to fall by half. After n half-lives, activity = (1/2)ⁿ × original.
Example: a sample has activity 800 Bq and a half-life of 5 minutes. After 15 minutes (3 half-lives), activity = 800 × (1/2)³ = 100 Bq.
Uses and dangers
- Smoke alarms — americium-241 (alpha source).
- Medical tracers — technetium-99m (gamma, short half-life).
- Cancer treatment — cobalt-60 (gamma).
Risks: ionising radiation damages DNA → cancer or cell death. Workers wear film badges, use lead shielding, and minimise exposure time.
CCEA tip
For "compare alpha and beta", give a property AND a consequence: "alpha is more strongly ionising B1 so causes more damage if ingested B1".
AI-generated · claude-opus-4-7 · v3-ccea-combined-science-leaves