Introduction Atomic mass:/The first scientists to measure atomic mass were John Dalton (between 1803 and 1805) Jons Jacoband Berzelius (between 1808 and 1826). Early atomic mass theory was purposed by the English chemist William Prout in a series of published papers in 1815 and 1816. Known was Prout's Law, Prout propsed that the known elements all had atomic weights that were whole number multiples of the atomic mass of hydrogen. Berzelius discovered that this was not always true by showing that Chlorine(Cl) had a mass of 35.45 which was not a whole number multiple of hydrogen's mass. The SI unit for atomic mass is the atomic mass unit (u), or Dalton (Da). The unit Dalton was derived from the carbon-12 isotope, as 12 u is the exact atomic mass of that isotope. So 1 u is 1/12 of the mass of a carbon-12 isotope (see Isotopic Atomic Mass section below). Later, in the 1860s, Cannizzario applied Avogadro's number to atomic masses, allowing stoichiometric conversions to be carried out. Over time, our knowledge of atomic mass has expanded greatly.