Why scientists care
Radioactivity comes from unstable atomic nuclei that transform spontaneously and emit radiation. It is a nuclear phenomenon, distinct from ordinary chemical reactions involving electrons.
The same phenomenon helps us understand nuclei, date materials and build important medical techniques.
A concrete example
After three half-lives, an ideal radioactive population has one eighth of its initial undecayed nuclei remaining.
Try to notice
Science becomes memorable when a definition is connected to something measurable or observable.