Radioactivity

An element is radioactive if the nuclei of its atoms are unstable. Radioactivity is the phenomenon in which such an unstable nucleus spontaneously emits a nuclear particle and/or gamma radiation and, in the process, transforms into a different nuclide -- elements such as uranium and radium are classic naturally radioactive elements.

A defining and important feature of radioactivity is that the probability of a given nucleus decaying does NOT depend on external, chemical-level conditions at all: it is unaffected by the state of chemical combination the atom happens to be in, by temperature, by pressure, by the presence of a catalyst, or even by how long that particular nucleus has already existed (its 'age'). This is a genuinely NUCLEAR phenomenon, governed entirely by what is happening inside the nucleus, and it is precisely this chemical-independence that distinguishes a nuclear reaction from an ordinary chemical one (see the comparison in section 13.7.1). …