Revision notes · Atomic structure

Atoms and nuclear radiation

Radioactive decay and nuclear radiation4.4.2.1

Definition: Radioactive decay is a random process: an unstable nucleus decays and gives out radiation to become more stable. You cannot predict when any particular unstable nucleus will decay.

Activity is the rate at which a radioactive source decays, measured in becquerels (Bq) — 1 Bq is one decay per second. Count-rate is the number of decays detected per second by a detector (such as a Geiger-Müller tube), which is usually lower than the true activity because a detector doesn't pick up every decay.

Radiation	What it is	Ionising power	Penetration (range in air)
Alpha (α)	2 protons + 2 neutrons (a helium nucleus)	Strongly ionising	Weakly penetrating — absorbed by a few cm of air or a sheet of paper
Beta (β)	A fast-moving electron ejected from the nucleus	Moderately ionising	Moderately penetrating — travels ~1 m in air, absorbed by a few mm of aluminium
Gamma (γ)	Electromagnetic radiation from the nucleus	Weakly ionising	Strongly penetrating — absorbed only by many cm of lead or metres of concrete

Penetration of alpha, beta and gamma radiation

🧠 Remember: Ionising power and penetrating power trade off against each other: alpha is the most strongly ionising but the least penetrating (it loses its energy quickly by ionising nearby atoms), while gamma is the most penetrating but the least ionising (it interacts with matter only weakly).

Some unstable nuclei also decay by directly emitting a neutron.

Nuclear equations4.4.2.2

A nuclear equation shows the mass number and atomic number of the nucleus before and after a decay, and must balance on both sides: total mass number and total atomic number are conserved.

Decay	Effect on the nucleus	Example
Alpha (α)	Mass number decreases by 4, atomic number decreases by 2 — an alpha particle (⁴₂He) is emitted	²²⁶₈₈Ra → ²²²₈₆Rn + ⁴₂He
Beta (β)	Mass number unchanged, atomic number increases by 1 — a neutron turns into a proton and an electron (⁰₋₁e) is emitted	¹⁴₆C → ¹⁴₇N + ⁰₋₁e
Gamma (γ)	No change to mass number or atomic number — the nucleus loses energy only	no change to the nucleus's composition

Writing nuclear decay equations

⚠️ Common mistake: Beta decay increases the atomic number (a neutron becomes a proton, releasing an electron) — it's easy to mix this up with alpha decay, which decreases it. Gamma decay changes neither the mass number nor the atomic number, only the energy of the nucleus.

Half-lives and the random nature of radioactive decay4.4.2.3

Definition: Half-life is the time it takes for the number of unstable nuclei in a sample to halve, or equivalently, the time it takes for the activity (or count-rate) of a sample to fall to half its initial value.

Radioactive decay is random at the level of an individual nucleus, but with a very large number of nuclei the overall activity follows a predictable pattern — this is why half-life is a useful, reliable measure even though decay itself is unpredictable.

1Example: a sample starts with an activity of 80 Bq.
2After one half-life, the activity has halved to 40 Bq.
3After a second half-life, it has halved again to 20 Bq.
4If this drop from 80 Bq to 20 Bq took 10 minutes (two half-lives), the half-life is 10 ÷ 2 = 5 minutes.

🧠 Remember: A short half-life means activity is initially very high (a high immediate hazard) but drops away quickly. A long half-life means activity stays low but persists for a very long time (a long-term, low-level hazard).

This trade-off is why americium-241 (half-life 432 years, an alpha emitter) is used in household smoke alarms: its weakly-penetrating alpha radiation ionises the air inside a small chamber, allowing a tiny electric current to flow. Smoke entering the chamber absorbs the alpha radiation, reducing the ionisation and the current — this drop triggers the alarm. Its long half-life means the source doesn't need replacing for decades, and its low activity keeps continuous exposure safe.

Radioactive contamination4.4.2.4

	Contamination	Irradiation
What happens	Unwanted radioactive atoms get onto or into an object or person	Exposure to radiation from a source outside the body, without the source touching it
Does the object/person become radioactive?	Yes — the contaminating material itself keeps emitting radiation	No — once removed from the source, exposure stops
Duration of hazard	Long-lasting, until the contaminating material is removed or decays away	Only while exposed to the source

🧠 Remember: Precautions differ because of this: protecting against contamination means preventing radioactive material getting onto skin, clothing, or being inhaled/ingested (e.g. protective suits, sealed containers); protecting against irradiation means increasing distance from the source, reducing exposure time, or shielding (e.g. lead, concrete).

Because radiation hazards affect public health and safety, findings from research into radiation risks must go through peer review before publication — this lets other scientists check the methods and conclusions are sound before the results influence safety guidance.

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