Changes in the nucleus
Some nuclei are unstable and decay spontaneously, releasing particles or electromagnetic radiation. A daughter nucleus may be stable or may decay again. Nuclear transformations differ from ordinary chemical reactions, which primarily rearrange electrons rather than changing the nucleus itself.
Different forms of radiation
Alpha radiation involves helium nuclei, beta decay can release electrons or positrons, and gamma radiation consists of energetic photons. The forms differ in charge, mass and interaction with matter, so radiation is not one uniform particle or a single degree of penetrating power.
Half-life as a statistical relation
Half-life is the time for the undecayed population of a large collection of identical radioactive nuclei to fall by half. It is not a fixed appointment for each individual atom, and one elapsed half-life does not make the remaining sample nonradioactive.
References: [2]
Activity and other measured quantities
Activity measures decays per unit time. Radiation energy, location and interaction with matter determine other measured quantities. Research, medicine and dating use different nuclide properties; interpreting a particular application requires identifying the nuclide and measurement conditions.