Radioactive decay happens on its own, from deep inside the nucleus, without any outside cause. Heating, cooling, squeezing or chemically combining a substance does nothing to change it, which is very different from ordinary chemical reactions.
It is also random: for any one nucleus, all we can state is the chance that it will decay in a given time, never the exact moment. Two identical nuclei side by side may decay years apart. You can see this randomness in the irregular clicks of a Geiger counter, which never come at even intervals.
Yet order emerges from large numbers. With billions of nuclei present, the fraction decaying each second is steady, which is why a well-defined half-life exists even though each individual event is unpredictable.
In SPM you should describe decay as spontaneous and random, and connect this to the erratic count rate and the smooth decay curve of a large sample.
Common misconceptions
- Heating a radioactive source makes it decay faster -> Decay is unaffected by temperature, pressure or chemical state.
- We can predict exactly when a given nucleus will decay -> We can only give the probability; the exact moment is unpredictable.
- Randomness means the count rate is completely unpredictable -> Individual events are random, but a large sample decays at a steady, predictable average rate.
The physics behind it
Radioactive decay is a nuclear process: an unstable nucleus emits an alpha particle, a beta particle or a gamma ray entirely on its own. Because the trigger comes from inside the nucleus, no external agent controls it.
Two facts define it: it is spontaneous (nothing outside starts it) and random (you cannot say which nucleus goes next).
For a single nucleus you can state only a probability of decaying in a given time. For a large sample, a steady fraction decays each second, so the number of undecayed nuclei falls by half over a fixed interval called the half-life, symbol T½.
Activity is measured in becquerel (Bq), where 1 Bq = 1 decay per second.
Worked line: a sample has 8.0 × 10²⁰ undecayed nuclei with T½ = 6 hours. After 12 hours, that is 2 half-lives, so N = 8.0 × 10²⁰ × (½)² = 2.0 × 10²⁰ nuclei.
The individual events are unpredictable, yet the total follows a smooth curve.
See it in daily life
Hold a Geiger-Muller counter near a weak radioactive source and you hear irregular clicks. The clicks never arrive at even, ticking intervals; sometimes several come in a rush, then a pause.
That erratic rhythm is randomness you can hear directly, because each click marks one nucleus decaying at a moment nobody can schedule.
The same randomness underlies smoke detectors, which use a tiny americium source whose steady average emission keeps the detector armed, and carbon dating, which relies on the fixed half-life of carbon-14 in old wood and bone. In both cases no one predicts a single decay, yet the average behaviour of trillions of nuclei is reliable enough to trust.
Notice too that leaving a source in the sun, the fridge, or a chemical bath changes nothing. Temperature, pressure and chemical reactions all leave the decay rate untouched, which is very unlike burning or rusting, and shows the process is buried deep in the nucleus.
How this comes up in SPM
In Paper 2 of SPM Physics (4531) this idea is examined with the command words describe and explain: describe radioactive decay as a spontaneous and random process, and explain why the count rate from a Geiger-Muller tube is erratic while the decay curve of a whole sample is smooth.
You are also asked to state the meaning of half-life and to determine it from a graph of activity or number of nuclei against time, reading off successive halvings. A common structured task gives you a starting count and a half-life and asks you to calculate the amount remaining after a whole number of half-lives.
Neighbouring content standards cover the types of radiation (alpha, beta and gamma) and their properties, the uses of radioisotopes, and radioactive safety. Keep the language precise: say a decay is spontaneous and random, and never claim any outside factor can speed it up or slow it down.
Source: DSKP KSSM Physics Form 4 and 5 (Versi English) (Bahagian Pembangunan Kurikulum (BPK), KPM)