What is radioactivity?
Radioactivity is the spontaneous transformation of unstable atomic nuclei with the emission of particles or electromagnetic radiation. Unlike a change in chemical bonding, it involves the nucleus; radioactive processes do not all emit the same type of radiation.
Decay types and half-life
Alpha decay ejects a particle containing two protons and two neutrons. Beta processes can transform protons and neutrons; gamma emission releases excess nuclear energy as high-energy light. Half-life is the time for, on average, half of a large collection of the same nuclei to decay; it cannot predict exactly when one atom will decay. Ordinary chemical conditions such as temperature generally do not readily change the decay rate. A sample does not suddenly vanish after a few half-lives; its amount declines progressively.
Exposure and applications
Ionizing radiation can damage living tissue; risk depends on type, energy, dose, exposure time and route of intake. Alpha particles are readily stopped externally but may be dangerous if inhaled or swallowed. Medical imaging and treatment, industrial measurement and scientific dating are examples of controlled use. Being irradiated by a source does not always make a person radioactive; contamination with radioactive material is different. Safety considers time, distance, shielding and qualified measurements.