Step 1: Explain the phenomenon in plain terms.
Some naturally occurring heavy elements (such as uranium, thorium and radium) have nuclei that carry too much energy to remain stable. Such a nucleus, without any external cause, breaks up and throws out invisible penetrating radiation. This self-driven emission of radiation from an unstable nucleus is called radioactivity.
Step 2: Note who and how it was found.
Becquerel first noticed it with uranium salts, and the Curies later isolated other radioactive elements. The activity depends only on the nucleus, so heating, cooling or forming compounds cannot switch it off.
Step 3: Classify the emitted radiations, which fixes the number of decay types.
When the radiation is passed through an electric or magnetic field it splits into three parts, showing there are three kinds of decay:
(i) \(\alpha\)-decay, giving positively charged helium nuclei;
(ii) \(\beta\)-decay, giving negatively charged electrons;
(iii) \(\gamma\)-decay, giving uncharged electromagnetic radiation of very short wavelength.
Hence radioactive decay is of three types.
\[\boxed{\alpha,\ \beta \text{ and } \gamma \text{ decay (three types)}}\]