The concept of "half-life" does not appear in some foundation GCSE exams.
Check with your teacher if you're not sure which level of exam you'll be taking.

If you point a Geiger counter at a radioactive substance for a period of time, you'll notice that the reading on the meter decreases as you watch. This is shown on the graph.

The radioactivity from some substances dies away very fast - perhaps in a few microseconds. Others take thousands of years before you'll notice that the radioactivity had decreased at all.

In theory, every radioactive substance should stay slightly radioactive for ever - the graph should never actually fall to zero. This means that we can't usefully talk about the "life" of a radioactive source.

Instead, we use the idea of "half-life".
 This is the time it takes for the radioactivity to fall by half.

Watch the animated graph >>

This graph shows what would happen with an imaginary radioactive substance.

Notice that the radioactivity falls by half every 2 hours.

Thus we say that this imaginary substance has a half-life of 2 hours.

The count rate coming from a radioactive source depends on how many unstable atoms it contains.
That's the number of un-decayed atoms.
If the count rate has fallen by half, then the number of unstable atoms has fallen by half.

A typical exam question may be "A radioactive substance has a half-life of 2 hours. How much of the substance will remain after 6 hours have passed?"

Here's how to do it - simply count on your fingers.

First finger: after one half-life (2 hours), half of the substance is left.

Second finger: after two half-lives (another 2 hours), half of that is left, so we're down to a quarter.

Third finger: after three half lives (that's 6 hours) half of that is left, so it's down to an eighth of what we started with.

Thus, for a substance with a half-life of 2 hours, 1/8 of the original atoms will remain after 6 hours.

Note that this also means that 7/8 of the atoms will have decayed in that time.

Take another look at the animated graph above, you'll see that after 6 hours the activity has fallen from 8,000 to 1,000; i.e. it's fallen to 1/8 of the starting value.

That's the key to solving half-life questions - count on your fingers, saying "half", "quarter", "eighth", "sixteenth", "thirty-second", "sixty-fourth"... etc.

At GCSE level you're unlikely to meet a question that needs more than 5 half-lives, and any questions you're asked will always involve whole numbers of half-lives (which makes life much easier!)