Atoms That Fall Apart

Most atoms are stable — their nuclei hold together indefinitely. But some atoms carry too many protons, too many neutrons, or too much trapped energy.

Henri Becquerel discovered radioactivity by accident in 1896 when he left uranium salts on a photographic plate and found the plate had been exposed. Marie Curie took the discovery and ran with it, coining the term "radioactivity" and finding two new elements — polonium and radium.