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January 31st is Theodore William Richards' birthday. Richards was an American chemist who gave the first chemical evidence of the existence of isotopes. An element is determined by the number of protons that make up its nucleus. An isotope of an element is determined by the number of neutrons in the nucleus. Each different isotope has a slightly different atomic mass. Richards spent an entire career systematically obtaining pure samples of elements and carefully determining their atomic weights. From 1889 to 1932, he had obtained atomic weights for 55 different elements. He also showed that an element's atomic weight could vary by small amounts from sample to sample. The explanation for this would not appear until 1913, when J.J. Thomson first identified two different isotopes of neon.

Richards would receive the 1914 Nobel Prize in Chemistry for his work and contributions to atomic structures. His work would be quickly surpassed by British chemist Francis Aston, who invented the mass spectrometer. This device would quickly perform many of the same tasks Richards would painstakingly spend his life's work completing. Find out what else occurred on this day in science history.

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