What is radioactivity?

(N/A) Radioactivity is the phenomenon of spontaneous emission of active radiations (such as $\alpha, \beta$,or $\gamma$ rays) from the nuclei of unstable atoms.
It is a nuclear process that occurs regardless of the physical or chemical state of the substance.
The rate of decay is governed by the law of radioactive decay,which states that the number of nuclei decaying per unit time is proportional to the total number of radioactive nuclei present at that instant,expressed as $dN/dt = -\lambda N$,where $\lambda$ is the decay constant.