We measure the rate of radioactive decay of a substance in half-lives. We can't predict when one individual atom is going to undergo radioactive decay so a half-life tells us the time it will take for half the atoms in a sample decay. This video gives another explanation of the idea of a half-life.

A half-life is not a linear concept, so we can't say that after two half-lives everything will have decayed. After one half-life 50% of the sample will have decayed and 50% will remain. After

**another**half-life we will have half of the 50% remaining, so that means we now 25% remaining and so on and so on. The mathematical term for this is "exponential" decay. This graph shows the rate of sample decaying.Notice that 50% of the sample is remaining after 1.3 billion years, so for Element X the half-life is 1.3 billion years. Here is a video showing how to work out a half life from a decay curve.

Once we have a graph showing the radioactive decay we can also use it to make predictions about how long it will take for a sample to decay a certain amount or about how much of a sample will be remaining after a certain time. Do this by drawing lines from the x- and y-axes to the decay curve.

We can also make predictions about decay rates by generating a table of values like in the examples below. Note that the amount of radioactive substance present is often described as the "count rate". The starting count rate represents 100% on a graph such as the one above. Take the time to work through these two examples.

We can also make predictions about decay rates by generating a table of values like in the examples below. Note that the amount of radioactive substance present is often described as the "count rate". The starting count rate represents 100% on a graph such as the one above. Take the time to work through these two examples.