Explain that it cannot be predicted when a particular nucleus will decay…

The idea

Radioactive decay is random at the level of a single nucleus. No measurement or calculation can say which nucleus will decay next or when a chosen nucleus will decay; the process is spontaneous and unaffected by external conditions such as temperature. However, when a sample contains a very large number of nuclei, the proportion decaying in each half-life is statistically reliable. Half-life therefore lets you predict the activity of the whole source over time even though individual events remain unpredictable.

For example: With a half-life of 6 hours, a source of activity 1200 Bq is predicted to fall to 600 Bq after 6 hours and 300 Bq after 12 hours, while no prediction is possible for one named nucleus.

What earns the marks

• States that the decay of an individual nucleus is random and cannot be predicted.
• Explains that prediction becomes reliable because a very large number of nuclei are involved.
• Uses half-life to predict how activity falls over successive intervals.
• Contrasts the unpredictable single nucleus with the predictable behaviour of the whole source.
• Applies halving to a stated activity over a stated number of half-lives.

Common mistakes and how to put them right

• Claims that half-life predicts exactly when a particular nucleus decays.

  Put it right by stating that individual decay is random and only large-number behaviour is predictable.

• Says decay can be speeded up by heating or by chemical change.

  Put it right by noting decay is spontaneous and unaffected by such conditions.

• Treats the prediction as exact for a small sample.

  Put it right by explaining that statistical reliability needs a very large number of nuclei.

• Assumes activity stays constant until the half-life ends.

  Put it right by describing continuous, gradual halving of activity.

Exam tips

• Separate the two ideas clearly: random for one nucleus, predictable for a very large number.
• Use the phrase very large number of nuclei when justifying why a prediction works.
• Show each halving step when calculating predicted activity so the examiner can follow your reasoning.