Radioactivity http://www.ducksters.com/science/chemistry/radiation_and_radioactivity. php

Stable and unstable isotopes Elements can be made up of different isotopes. Isotopes are atoms with the same number of protons and electrons, but a different number of neutrons. Sometimes isotopes are stable and happy. These are the elements that we see around us and find in nature. However, some isotopes are unstable. These isotopes are called radioactive isotopes.

Radioactive decay When isotopes are unstable they emit energy in the form of radiation. There are three main types of radiation or radioactive decay depending on the isotope. 

Types of radiation Alpha particles : are helium nuclei - made up of two protons and two neutrons; they have a positive charge and move at speeds up to one tenth the speed of light Beta particles β: are high speed negatively charged particles; they can travel a few metres through air but are stopped by objects such as a sheet of aluminium or a centimetre thickness of wood Gamma rays γ: are not particles but a type of electromagnetic radiation, like light, UV and X-rays; they have very high energy and can travel km through the air and can be stopped by concrete or lead

Half life of an isotope The half-life of an isotope is the time on average that it takes for half of the atoms in a sample to decay. 

Using radioisotopes Nuclear medicine: in radiotherapy, a radioisotope is placed in a shielded box with an opening through which a beam of gamma rays emerges, the beam is aimed where the cancer is Industry: small leaks in pipe systems can be traced by putting radioisotope into the pipe and follow the movement of the liquid Smoke detectors: uses americium-241, and the alpha particles emitted by this create a small current that stops the alarm from sounding, as soon as smoke enters the detector, it absorbs the alpha particles and cuts off the current setting off the alarm Forensic analysis: samples collected at a crime scene can be bombarded by neutrons, made radioactive so they can be identified Radioactive dating: by measuring how much carbon 14 we can date how old an object is