History: On The Human Side

  1. The mole concept is attributed to Amedeo Avogadro, who postulated that equal volumes of gases at the same temperature and pressure contain equal numbers of molecules. He based this hypothesis on Gay-Lussac's Law of Combining Volumes (when two gases react, their volumes are in whole number ratios) and Dalton's Atomic Theory (substances combine in integral ratios of atoms). Avogadro proposed that gases exist as molecules that can be composed of more than one atom of an element and that these molecules can break up during reaction. Avogadro, however, never determined what we know today as Avogadro's number.

  2. Because Dalton and other influential scientists of his day did not accept his hypothesis, Avogadro's idea was not accepted for many years. After his death, Avogadro's hypothesis was "rediscovered" and accepted. When the number of molecules in a mole was estimated by Perrin during the early years of this century, he suggested that the number be called Avogadro's number. Since that time, Avogadro's number has been measured by several different methods, including a measurement by Rutherford using alpha particles and the measurement by Millikan in his famous oil-drop experiment in which he determined the charge on an electron. More recently, Avogadro's number has been measured by X-ray crystallographic measurements.