A substance is radioactive is the atoms of which it is made are unstable. The atoms may decay and turn into atoms of another element or different isotopes of the same element. The probability that a particular atom will decay in each time period is a fixed constant. This means that in each time period of a fixed length, the number of undecayed atoms remaining is reduced by a fixed factor. An exponential decay curve is displayed below. In each period of 25 years the number of undecayed atoms falls to one half of what it was at the start of the time period. This factor of a half is significant. The time period for the number of undecayed atoms, or the mass of undecayed material, or the activity – the number of decays per second of the material – is called the half life and labelled To find the half life from the graph we divided the initial mass by 2 then find the time corresponding to the new mass.

Notice that the amount of material decays by a factor of every 25 years. We may draw up the table

 Tine (years) Mass of Undecayed Substance (grams) 0 100 25 50 50 25 75 12.5 100 6.25 