A carbon-14 nucleus holds too many neutrons to stay as it is. Sooner or later it breaks down.
When it does, one of its neutrons turns into a proton and shoots out a fast electron. The nucleus now has 7 protons and 7 neutrons, so the atom is nitrogen-14 again, the atom it was made from. This breaking down is radioactive decay.
Nobody can say when one particular carbon-14 atom will decay: it might be tomorrow, or in 20,000 years. In a crowd of billions, though, the numbers are very steady. In any 5,700 years, half of the carbon-14 atoms decay. That time is carbon-14's half-life.
The halving goes on however little is left: a half, a quarter, an eighth, a sixteenth. After ten half-lives, 57,000 years, about a thousandth of the carbon-14 remains.
How well it is known
Scientists have measured the half-life many times, and the value they accept today is 5,700 years, give or take 30. Willard Libby, who invented carbon dating, used 5,568 years. Laboratories still work out a first radiocarbon age with Libby's number, so that ages measured since the 1950s can all be compared, and calibration, at the end of this lesson, turns it into a calendar date.