Mass number is the total mass of the atom, the number of protons and the number of neutrons are added. Elements can have different numbers of neutrons, if the number of neutrons is different then the atoms are isotopes. Radioactivity is the spontaneous emmission of radiation from unstable nuclei. In general, isotopes with more neutrons than protons tend to be unstable and will emit radioactivity. For cancer information, and other fun web stuff, click here.
Radioactive compounds decay, and lose radioactivity.
A half life is the time it takes for a radioisotope to decay to one half of its original radioactivity. Click here to see a decay curve for the decay curve of 99m Tc. Click here to see the decay curve for Tritium 3 H. Note that after 5 half lives, the radioactivity is essentially gone. Radioisotopes are also used for diagostic purposes. 131 I is used to detect and to treat thyroid tumors.