Amedeo Avogadro is best-known for having discovered the law and accompanying number that are named in his honor. Avogadro's constant and Avogadro's number were critical breakthroughs in the development of modern chemistry.
Avogadro's number, which equals 6.02214129 × 10²³, defines the number of units in one mole of a substance. This number was derived from his discovery that, at equal temperature and pressure, equal volumes of gases contain the same number of molecules. Avogadro also calculated that, at standard temperature and pressure (32 degrees Fahrenheit and 1 atmosphere), 1 gram-mole of gas occupies about 0.791 cubic foot; this number is the same for all gases, which is why Avogadro's law is also considered to be a constant.