Some atoms are born restless. Their nuclei contain an uneasy balance of protons and neutrons — too many of one, too few of the other, or simply too much energy packed inside. To reach a more stable state, these nuclei spontaneously break apart or eject particles, releasing energy in the process. This is radioactivity, and it was discovered almost by accident in 1896 when Henri Becquerel noticed that uranium salts fogged a photographic plate stored in a dark drawer. Marie and Pierre Curie took the discovery further, isolating polonium and radium — work that earned the trio a Nobel Prize and opened an entirely new chapter in physics.
There are three main types of radioactive decay. Alpha decay ejects a cluster of 2 protons and 2 neutrons (essentially a helium nucleus), reducing the element's atomic number by 2. Beta decay converts a neutron into a proton (or vice versa), shooting out an electron or positron and shifting the element one place on the periodic table. Gamma decay releases pure electromagnetic energy — high-frequency photons that carry away excess energy without changing the element at all. Each type has different penetrating power: alpha particles are stopped by a sheet of paper, beta particles by a thin sheet of aluminum, but gamma rays require thick lead or concrete to block.
Radioactivity is both feared and essential. Nuclear power plants harness the energy of uranium fission to generate about 10% of the world's electricity. In medicine, radioactive isotopes diagnose cancer (PET scans use fluorine-18), treat tumors (cobalt-60 radiation therapy), and sterilize surgical equipment. Archaeologists use carbon-14 decay to date ancient artifacts. Smoke detectors in your home contain a tiny amount of americium-241. Radioactivity is woven into modern life far more than most people realize — the key is understanding it well enough to use it safely.