radioactivity
/ RAY-dee-oh-ak-TIV-ih-tee /
Some atoms are restless. Deep in their core sits the nucleus, and in certain atoms that nucleus holds too much energy to stay still — so it throws a bit of itself away to settle down. That spontaneous flinging-off of energy and particles is radioactivity. Picture an over-wound clockwork spring that, every so often and entirely on its own, lets out a sharp little kick.
What comes flying out takes three classic forms, named after the first three Greek letters. Alpha is a heavy, slow chunk that even a sheet of paper can stop. Beta is a fast, light particle (an electron) that a few millimetres of aluminium can block. Gamma is pure high-energy light, so piercing it takes thick lead or concrete to tame. The atom usually changes its identity in the bargain — one element quietly turning into another.
Here's the part that trips people up: you can never say when a single atom will decay, only how fast a whole crowd of them will. That pace is the half-life — the time for half the atoms in a sample to decay. It can be a blink or billions of years, and nothing you do, no heat, no pressure, no chemistry, can hurry it or slow it down.
Carbon-14 has a half-life of about 5,730 years — which is exactly how archaeologists date ancient bones and wood.
Half-life turns decay into a clock.
Marie Curie introduced the term radioactivity in 1898, building on the Latin radius ("ray"). With her husband Pierre and Henri Becquerel she shared the 1903 Nobel Prize in Physics for research on radiation. She received a second Nobel Prize, in Chemistry, in 1911 for discovering polonium and radium and isolating radium — making her the first person to win two Nobel Prizes, and still the only person to win in two different sciences.