History of Chemistry


When was Avogadro’s constant discovered?

Amedeo Carlo Avogadro published a paper in 1811 describing his theory that a volume of gas (at a given temperature and pressure) contains a certain number of atoms or molecules regardless of what gas it is. Avogadro didn’t actually determine what that number was, however. It took just over fifty years for someone to make progress on that: Johann Josef Loschmidt, in 1865, estimated the average size of molecules in air. It’s nothing short of amazing that he ended up being off by only a factor of two. Jean Perrin, a French physicist, accurately determined the constant using a few different techniques. He was awarded the Nobel Prize in Physics in 1926 for the work, but Perrin proposed that the constant be named for Avogadro—and the name stuck. (For more on the use of the constant, see “Atoms and Molecules.”)


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