Step 1: Describe why it happens.
Some nuclei have an unfavourable balance of protons and neutrons, which makes them unstable. To move towards a stable configuration, such a nucleus breaks up on its own and throws out energy in the form of nuclear radiation. This self-driven break-up is what we call radioactivity.
Step 2: Name the process and its discoverer.
The effect was first observed by Becquerel with uranium salts and studied further by Marie and Pierre Curie. Because the emission comes from the nucleus itself, it is also called nuclear radioactivity.
Step 3: Summarise the emissions and their nature.
• \(\alpha\)-decay: the nucleus emits a helium nucleus, so its mass number falls by 4 and charge by 2.
• \(\beta\)-decay: the nucleus emits an electron (with an antineutrino), and its atomic number changes by one.
• \(\gamma\)-emission: an excited nucleus releases surplus energy as a high-energy photon, without any change in mass or charge number.
Step 4: State the governing behaviour.
The decay is random for any single nucleus but, for a large sample, follows the exponential law \(N=N_{0}e^{-\lambda t}\), and it cannot be sped up or slowed down by ordinary physical or chemical means.
Result: Radioactivity is the spontaneous, temperature-independent disintegration of unstable heavy nuclei with emission of \(\alpha\), \(\beta\) or \(\gamma\) radiation.