These neutrons can become unstable, and when they do, they release energy and undergo decay. Radioactivity occurs when the nucleus contains an excess amount of neutrons.
Scientists use a technique called radiometric dating to estimate the ages of rocks, fossils, and the earth.
The results showed that Ötzi died over 5000 years ago, sometime between 33 BC. It is found in the air in carbon dioxide molecules.
The amount of carbon-14 in the air has stayed the same for thousands of years.
There is a small amount of radioactive carbon-14 in all living organisms because it enters the food chain.
Once an organism dies, it stops taking in carbon-14.