On this day in 1776, Amedeo Avogadro, an Italian scientist known as one of the founders of physical chemistry, was born. Avogadro studied the properties of electricity and liquids, but his best known work was with gases. It was known by 1809 that all gases, when heated equally, expand by the same amount. Avogadro went further to hypothesize that two equal volumes of gas, if they were at the same temperature and pressure, contained the same number of particles--which could be either atoms or molecules. This observation, now known as Avogadro's law, was published in 1811, but was not widely accepted until the 1850s. He was the first to make a distinction between molecules of a substance and its atoms. From Avogadro's law, it follows that one molar volume of any gas contains the same number of molecules, 6.02252 × 1023, now called Avogadro's number--or a mole.
To find out about the National Mole Day Foundation, a celebration of Avogadro's chemical legacy, click here.