The number of protons in the nucleus of an atom. This determines an element's structure, properties and location on the periodic table of elements. One-half the distance between two adjacent atoms in crystals of elements. This varies according to interatomic forces. The atomic mass of an element divided by its density. An Italian chemist who first stated the principle behind stoichiometry in 1811. Also see Avogadro's Number and Molarity, Molality And Normality. This is the number of atoms in a..