In November 1897, Marie Curie decided to conduct systematic research on the ‘uranic rays’ found in a number of elements, compounds and minerals. She measured their ability to electrify the air surrounding them with a piezoelectric quartz electrometer invented by Pierre and Jacques Curie, capable of measuring very low values of electric intensity.
Marie Curie carried out these measurements and discovered that thorium also emits these ‘Becquerel rays’, and that uranium minerals such as pitchblende and chalcolite have infinitely more intense emissions that pure uranium. Uranium is a rare mineral (present in 3 parts per million on the Earth’s crust). Pitchblende is a mineral rich in uranium, that was used in the 19th Century to add colour to Bohemian crystals. Like pitchblende, chalcolite is a mineral often found in lead mines.
Marie Curie then proposed the hypothesis that the property of emitting these rays was a more general property of matter, which she named ‘radioactivity’. She added that the origin of the phenomenon must be inside the atom, as radioactivity occurs on a submolecular level.
Pierre Curie immediately understood the importance of these observations and together with his wife discovered two previously unknown elements in 1898: polonium and radium, present in trace amounts in pitchblende.
From several tonnes of pitchblende residue, Marie Curie was able to isolate pure radium, an element which is a million times more radioactive than uranium. Radium salts also possess a remarkable quality: they glow in the dark, are warm to the touch and seem to give off an inexhaustible supply of heat.
In 1902, Mendeleyev’s periodic table still had gaps. Marie Curie was able to show that radium filled in one of these pieces in the jigsaw, and demonstrated that it had an atomic mass of 226. The atomic mass is today defined as the total number of protons and neutrons forming up a nucleus. Marie Curie was able to narrow down the accuracy on radium atomic mass to 1 nucleon.
In 1903, Pierre et Marie Curie were rewarded by a Nobel Prize shared with Henri Becquerel (in 1911 Marie Curie was again rewarded by a second Nobel Prize).
The new element, the radium, became an extraordinary tool for the early exploration of the structure of matter and also for its therapeutical applications.
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