To understand natural radioactivity , we need a quick review of chemistry. All matter consists of elements or combinations of elements. The smallest subdivision of an element that still has properties of that element is an atom. Atoms consist of a central nucleus, containing protons and neutrons, and an outer cloud of orbiting electrons. In contrast, the number of neutrons in atoms of some elements can vary, and the various forms of such elements are called isotopes .
Some isotopes are stable and remain unchanged through time.
Others are unstable and tend to break down, or decay over time, by giving off subatomic particles and energy. In the same way that the number of annual rings gives the age of a tree, the key is to know the rate at which a process occurs, measure its resultant product and calculate how long the process was going on. However, the process of radioactive decay is rather different from the yearly growth of trees. Imagine that a parent decides to give his daughter exactly half of his money each day. The first day he gives her half of his total wealth.