Radioactive decay is random.

The idea

Radioactive decay is random at the level of a single nucleus. You cannot predict which nucleus in a sample will decay next, and you cannot predict when any particular nucleus will decay; an old nucleus is no more likely to go than a fresh one. What is fixed is the chance: every undecayed nucleus of a given isotope has the same constant probability of decaying in the next second. The dice model captures this, because each die has a fixed one in six chance of showing a six on every throw and you cannot say which die will do it. Because a real sample holds an enormous number of nuclei, those individual gambles average out into a smooth, predictable decay curve, and the measured count rate only wobbles a little from reading to reading.

What earns the marks

• Saying you cannot predict which nucleus in the sample will decay next.
• Saying you cannot predict when a particular nucleus will decay.
• Linking the model to the fixed chance of rolling a six on any throw.
• Stating that the average behaviour of a very large number of nuclei is still predictable.

Common mistakes and how to put them right

• Saying that because decay is random the half-life cannot be predicted either.

• Claiming nuclei decay in turn, so that the oldest or outermost nuclei go first.

• Writing that decay speeds up or slows down as a nucleus 'gets ready' to decay.

• Treating the small wobble in successive count-rate readings as a mistake in the experiment.

• Describing the dice model as random because the thrower cannot aim, rather than because the outcome per die is unpredictable.

Exam tips

• When a question asks for two reasons why the dice model is good, give one about which die and one about when, because they are separate marks.
• Examiners accept answers written in terms of 'rolling a six', so use the model's own language rather than translating it.
• Say randomness applies to individual nuclei before you talk about the sample, so the examiner sees you have separated the two ideas.