Nuclear radiation
Atomic nuclei can store far more energy than chemical bonds. When a high energy nucleus becomes a low energy nucleus, it needs to shed that excess energy somehow. It does this by emitting particles or waves that carry away most of that energy. These particles or waves move away from the nuclear material in all directions – they radiate away, and hence this phenomenon is called nuclear radiation.
Radioactivity
Nuclei with stored energy can be unstable. Given time, they can spontaneously decay, releasing their energy as radiation. These unstable nuclei are called radioactive, and the process of their decay is radioactivity. Note that, despite having similar sounding names, radioactivity is separate from radiation – if you protect yourself from one, you are not necessarily protecting yourself from the other. Penetrating radiation emerging from radioactivity can get through barriers that will keep the radioactive material out, and all the shielding in the world will not help you if the radioactive material can get to your side of the shielding.
Radioactive material where you do not want it is called radioactive contamination.
In any given span of time, a given fraction of the radioactive material in any sample (as measured from that present at the start of that span of time) will decay. It is convenient to find the time it takes for exactly half of the radioactive material to decay, this is called the <i.half-life and is commonly denoted with in equations.
