Radioactivity Discovered
Henri Becquerel discovered radioactivity while studying uranium salts
March 01, 1896
Rays From Uranium
In February 1896, French physicist Henri Becquerel was investigating whether uranium salts could emit X-rays — which had just been discovered by Röntgen months earlier. He planned to expose uranium to sunlight, then place it on a photographic plate to see if it released rays. On cloudy days, he set the covered plates aside — and was astonished to find the plates had been exposed anyway, without any sunlight. The uranium was spontaneously emitting rays on its own, with no external energy source. He had accidentally discovered radioactivity, a phenomenon that had no explanation in classical physics.
Marie Curie Takes It Further
Polish-French scientist Marie Curie took up the investigation and coined the word "radioactivity." She discovered that the radiation was a property of the atom itself, not of molecular compounds — a crucial insight. Working with her husband Pierre in a cramped shed, she processed tons of pitchblende ore and isolated two new radioactive elements: polonium (named after her homeland Poland) and radium. She won two Nobel Prizes — in Physics in 1903 and in Chemistry in 1911 — making her the first person to win in two different sciences and the first woman to win a Nobel Prize at all.
What Radioactivity Revealed
Radioactivity proved that atoms were not indivisible and permanent — they could decay, transforming one element into another. This shattered a fundamental assumption of classical chemistry. It led directly to the discovery of the atomic nucleus, the development of nuclear physics, and eventually nuclear energy and nuclear weapons. Radioactive decay also gave scientists a tool: radiometric dating, which uses the predictable decay rates of isotopes to determine the age of rocks, fossils, and archaeological artifacts. The discovery that began with a cloudy day in Paris reshaped science, medicine, energy, and warfare in the century that followed.