25.3 Half-life and activity

• Radioactive decay is random; we can not predict when a particular nucleus in a sample will decay or which one will decay. Each nucleus within a sample has the same chance of decaying per unit time.
• Radioactive decay is spontaneous because the decay of nuclei is not affected by the presence of other nuclei in the sample and external factors such as pressure.
• The half-life of an isotope is the average time it takes for half the number of active nuclei in the sample to decay.
• The activity A of a source is the rate at which nuclei decay or disintegrate.
• Activity = Decay constant x number of nuclei in the sample.
• The decay constant has SI unit of s^-1; it is defined as the probability of decay of an individual nucleus per unit time.