Carbon dating halflife
This means a human adult has a radioactivity of around 3000-4000 becquerels due to carbon-14. When a living thing dies the cells are no longer replaced so no new carbon enters it.
The radioactivity of the carbon-14 begins to decrease. By measuring the radioactivity we can tell how long ago the living thing died.
It's useful because it has a half-life of the order of a minute and a pure sample can be prepared simply by shaking a bottle of liquid.
It's not that radioactive isotopes 'have' a half-life.
We can use the same idea to find out how long it would take for a sample with radioactivity 120 Bq to drop to 30 Bq. We can use radioactive decay to calculate the age of things.
So all living things contain exactly the same proportion of carbon-14 compared to carbon-12: the proportion in the atmosphere. A living adult human body contains about a billionth of a gram of carbon-14.
We can use our graph to show that it always takes 10 years for the radioactivity to drop by a half regardless of where you are on the graph.
A common school experiment is to find the half-life of an isotope called protactinium-234m.
Carbon dating can be used to date things up to about 60 000 years old. Many people think that plants grow by taking food from the soil through their roots but this is not true. Animals eat plants (or other animals that eat plants) so animals are also mostly rearranged carbon dioxide.
All green plants make their own food in their leaves. A tiny fraction of carbon atoms are the radioactive isotope carbon-14.