Becquerel's wrapped photographic plate revealed atoms that change into other elements
In 1896 Henri Becquerel wrapped a photographic plate in black paper, set uranium salts on top, and found it fogged anyway. Something invisible was streaming out of the atoms themselves. Within a few years that discovery forced a stark choice: either energy was not conserved, or one chemical element could turn into another.
Becquerel had been testing phosphorescent materials, suspecting a link with X-rays, an interest sparked partly by Henri Poincaré. Every salt failed until uranium. From 1897 Marie Curie studied these rays, showed they belonged to the atoms, and with Pierre Curie coined the word radioactivity and isolated two new elements, polonium and radium. In 1899 Julius Elster and Hans Geitel ruled out air and outer space as the power source by measuring in a vacuum and 300 metres down a Harz mine. Ernest Rutherford saw that decay follows one exponential formula, and with Frederick Soddy recognised transmutation.
Today the process is understood as an unstable nucleus shedding energy, most commonly by alpha, beta or gamma decay. For any single atom, timing is pure chance; no one can say when it will go. Large numbers behave predictably, though, captured by a half-life that ranges from almost instant to far longer than the universe has existed. Earth holds 28 naturally radioactive elements with 35 primordial radionuclides, including uranium, thorium and potassium-40, older than the Solar System.
The hazards were grasped slowly. After Wilhelm Röntgen found X-rays in 1895, reports of burns and hair loss began within a year, yet some doctors denied any harm. Radium enemas and radioactive tonic waters were sold as cures; Marie Curie objected, and she later died of aplastic anaemia probably linked to her exposure. Hermann Joseph Muller showed genetic effects in 1927 and won a Nobel Prize in 1946, and the International Commission on Radiological Protection was founded in London in 1950.
Activity is now counted in becquerels, one disintegration per second. The older curie equals 3.7 × 1010 of them, and European Union rules required phasing it out of public health use by 31 December 1985.
Source: Radioactive decay
