What is radioactivity primarily characterized by?

Radioactivity is primarily characterized by a spontaneous process by atoms with unstable nuclei. This phenomenon occurs when certain isotopes, which possess excess energy or mass, seek stability by emitting radiation in the form of particles or electromagnetic waves. This emission occurs without any external influence, signifying that it is a natural and intrinsic property of the unstable nuclei. The process results in the transformation of the original atom into a different element or a different isotope of the same element, contributing to the concept of half-lives and decay chains.

In contrast to the other options, radioactivity is not dependent on high temperatures or on atoms that are stable. While stable nuclei may undergo reactions under specific conditions, they do not engage in the spontaneous decay that characterizes radioactivity. The emphasis on spontaneity and instability in the correct choice highlights the fundamental nature of radioactive decay, distinguishing it from other processes that require external energy or specific conditions to occur.