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Atomic mass mass of an atom units: amu or u (atomic mass unit) Da (Dalton) amu is expressed as the ratio of mass of an atom to that of a chosen standard atom today amu scale is based on an isotope of carbon, 12 C amu is defined by assigning a mass of exactly 12 amu to the 12C isotope of carbon 1 amu = 1 the mass of 12 12 C Microscopic amounts - usually at the level of individual atoms Chemistry is done on the macroscopic level Mole -- SI unit for the amount of substance -- Used to bridge the gap between the microscopic and macroscopic levels of substances -- invented to provide a simple way of reporting a huge number of atoms and molecules in a visible sample 1 mole = 6.0221367 x 1023 particles = usually rounded to 6.022 x 1023 particles = Avogadros’s number or Avogadros’ constant particle = could be atom, molecule, formula unit or ion spreading 6.022 x 1023 marbles over the entire surface of the Earth would produce a layer of 3 miles thick 1 mole has a mass in grams numerically equal to the atomic mass in amu 12.01 amu = mass of one atom of C 1.01 amu = mass of one atom of H 12.01 g = mass of one mole of C 1.01 g = mass of one mole of H 12.01 g = mass of one mole of C = 6.022 x 1023 atoms of C = molar mass of C = mass of one mole of C 12.01 amu = mass of one atom of C = atomic mass of C 1.01 g = mass of one mole of H = 6.022 x 1023 atoms of H = molar mass of H = mass of one mole of H 1 mole of atoms of any element = 6.022 x 1023 atoms