How Radioactivity Was Discovered

The discovery of radioactivity opened a window into the atomic nucleus and transformed physics, medicine and the modern world. It began by accident, with a cloudy sky and a forgotten photographic plate, and was driven forward by one of the most remarkable scientific partnerships in history.

An accident in a drawer

In 1896, the French physicist Henri Becquerel was investigating whether certain minerals, after being exposed to sunlight, would emit penetrating rays. His method was to place a uranium compound on a photographic plate wrapped in black paper and leave it in the sun; if rays were emitted, they would expose the plate. The experiment seemed to be working. But then the weather turned cloudy, and Becquerel put his materials away in a drawer, plate and uranium together, to wait for sunshine.

Days later, for reasons that remain a little mysterious, he developed the plate anyway — expecting only a faint image, since the uranium had received no sunlight. Instead, the plate showed a strong, clear image. The uranium was emitting penetrating rays entirely on its own, with no sunlight needed. The energy was coming from within the uranium itself.

The Curies take up the trail

Becquerel's strange rays caught the attention of a young researcher, Marie Curie, who chose them as the subject of her doctoral work. Together with her husband Pierre, she began a painstaking investigation. Marie coined the term "radioactivity" for the phenomenon, and made a profound realisation: the radiation was an atomic property, coming from inside individual atoms, not from any chemical reaction between them. This hinted that atoms, long thought indivisible, had a hidden inner structure.

New elements from tonnes of rock

The Curies noticed that some uranium ore was far more radioactive than uranium alone could explain. They concluded it must contain unknown, intensely radioactive elements. To isolate them, they processed tonnes of a heavy mineral called pitchblende by hand, in a leaky shed, through gruelling chemistry. Their labour yielded two new elements: polonium, named for Marie's native Poland, and radium, which glowed faintly in the dark with its own eerie light.

Triumph and cost

In 1903, Becquerel and the Curies shared the Nobel Prize in Physics for the discovery and study of radioactivity. Marie Curie went on to win a second Nobel Prize, in Chemistry, in 1911 — becoming the first person ever to win two. But the work exacted a terrible price. The dangers of radiation were not yet understood; the Curies handled radioactive materials with bare hands and carried glowing samples in their pockets. The exposure ruined their health, and Marie eventually died of an illness almost certainly caused by her lifelong radiation exposure. Her notebooks remain so radioactive that they are still stored in lead-lined boxes.

A new physics

Radioactivity proved that atoms are not the changeless, indivisible units they had been assumed to be. They can transform, breaking apart and releasing energy from within. This insight opened the door to nuclear physics, to the understanding of the atom's structure, and eventually to both nuclear medicine and nuclear power. A forgotten plate in a drawer had revealed that matter itself holds hidden, restless energy.

Key takeaways

• Becquerel discovered radioactivity in 1896 when uranium exposed a plate with no sunlight.
• The energy came from within the uranium — radiation is spontaneous.
• Marie and Pierre Curie named it, showed it was an atomic property, and isolated polonium and radium.
• The discovery revealed the atom has inner structure and opened the era of nuclear physics — at great personal cost to Marie Curie.