The activity of all radioactive substances decreases over time. The time it takes for the activity of a material to reduce by half is called the

**of the material. Radioactivity is a random process. We don’t know which atom will be the next one to decay – a bit like we don’t know which piece of popcorn will be the next one to pop.***half life*However, we do know the time it takes for half of the atoms in a radioisotope to decay – this is our definition of half life.

These notes will show you how to do different types of half life calculations. The maths is straightforward – just make sure you can multiply and divide by 2!

There are 6 different problems to try as you read through the notes, answers are provided on the last page.