What characterizes a radioactive nucleus?

A radioactive nucleus is characterized primarily by its ability to spontaneously decompose, which is a process known as radioactive decay. This decay occurs because the nucleus is unstable, leading it to emit radiation in the form of particles or electromagnetic waves in order to reach a more stable state. This emissions are what make a nucleus radioactive.

In contrast, a stable nucleus does not undergo such spontaneous decomposition, and thus, it would not be characterized as radioactive. While a nucleus can have an electric charge or a high atomic mass, these characteristics do not inherently define radioactivity. Electric charge can be found in both stable and unstable nuclei, while the atomic mass alone does not determine a nucleus's stability or tendency to decay. Therefore, the defining feature of a radioactive nucleus is its intrinsic instability that leads to spontaneous decay.