Some materials give off radiation. We call those materials “radioactive”. Radioactive materials are often dangerous to people and other living things.
There are many different kinds of radioactive materials. Some give off particle radiation, like alpha or beta particles or neutron radiation. Some give off electromagnetic radiation, such as gamma rays or X-rays.
Most elements come in various “versions”, called isotopes. Some isotopes are radioactive. Other isotopes are not. Isotopes that are not radioactive are called “stable” isotopes. During radioactive decay, a radioactive isotope gives off some type of radiation. The isotope is transformed into a different isotope or sometimes into a different element. For example, carbon-14 is a rare isotope of carbon. Carbon-14 is radioactive. When a carbon-14 atom decays, it gives off a beta particle. The carbon-14 atom is transformed into a nitrogen atom.
Radioactive isotopes can be dangerous to living things. They can also cause damage to equipment such as electronics. Radioactive isotopes are not always dangerous, though. Some only give off tiny amounts of radiation. There are radioactive isotopes in nature all around us. Most of them cause us little or no harm.
Humans make use of radioactive materials in many ways. We use them in medicine to treat cancer. We use them to find out how old artifacts are by using carbon-14 dating. We also use them for nuclear power and in nuclear weapons.
source : windows to the universe