Radioactivity is the observed instability of the nuclei of some atoms. Over time, a radioactive substance will change its chemical composition: the radioactive isotopes will disappear, and decay products - usually different elements - will form. The process, called radioactive decay, is often accompanied by the emission of high energy electromagnetic radiation (gamma radiation), electrons (beta radiation) or helium-4 nuclei (alpha radiation). These particles are easy to detect at extremely low levels using fairly simple equipment. Because the number of atoms in a typical laboratory sample is extremely large, methods involving the use of radioactivity are among the most sensitive in all science.
The time it takes for half of a sample of one kind of nucleus (called a nuclide, the time is called a half-life) is normally constant, which allows the dating of materials by looking at how much of a sample has decayed and working out, sometimes with very good accuracy, how long it would have taken for that to happen. This forms the basis of radiometric dating.
 People troubled with radioactivity
There are two main groups of people who have a problem with radioactivity. The first one is biblical literalists, specifically young Earth creationists and some old Earth creationists, because the extent of decay of radioactive elements found on Earth, such as uranium and potassium, suggests that Earth is billions of years old. This obviously contradicts the story of Genesis.
The second group is the anti-nuclear movement, which considers radioactivity that results from the use of nuclear power to be very dangerous and a menace to humans, even though it's actually a tiny drop in the sea of natural radioactivity. Activities of the anti-nuclear movement are partially responsible for the pervasive fear of radiation present in most Western societies.
 See also
- FAQ on radioactivity and nuclear technology
- Radioactive decay
- Radiometric dating
- Evidence against a recent creation
- Second event theory