Historical Background

Discoverer: Pierre and Marie Curie

Discovered at: France

Discovery Date: 1898

Henri Becquerel found that uranium salts without exposure to light, emitted some rays of unknown nature. Becquerel's discovery fascinated the Curies. Marie felt and could soon prove that the unknown radiation was an atomic property. Even though the experiment had only been observed with uranium, nothing proved that uranium was the only chemical element capable of producing such radiation.

Marie decided to examine all known chemical bodies, either in pure state or in compounds. And the result was that: compounds of another element, thorium, also emitted rays like those of uranium and of similar intensity. The physicist had been right; the surprising discovery was not the property of uranium alone. It became necessary to give it a distinct name. Marie Curie suggested the name of radioactivity.

She knew ahead of time, what she would learn from an examination of the minerals. The specimens which contained neither uranium nor thorium would be revealed as totally "inactive". The others that contained uranium or thorium, would be radioactive. Experiment confirmed this hypothesis. Marie overlooked the inactive minerals, and measured the radioactivity of the active minerals. Then came a surprising outcome: the radioactivity was a lot stronger than should normally be in the quantity of uranium or thorium that was examined.

After a lot of trials she was forced to believe the evidence: the quantities or uranium and of thorium found in these minerals did not justify the intensity of the radiation she observed. In her later experiments, Marie examined all known chemical elements. The minerals contained a radioactive substance, which was a chemical element unknown until this day: a new element-Radium (from the Latin word "radius" meaning "ray".)

The element was isolated in 1911 by Marie Curie and Debierne by the electrolysis of a solution of pure radium chloride.