1- Relative isotopic mass - mass of an isotope (compared to Carbon-12) 2- Atomic number (protons) is constant between isotopes - defines element. 3- Mass number - number of nucleons (defines isotope) 4- Relative atomic mass - average mass: $A_r = {{RIM \times abundance} \over 100}$ 5- Relative atomic mass ($M_r$) - sum of relative atomic masses 6- Mole $6.02\times 10^{23}$ (= no of atoms in Carbon-12) $=N_A$ (Avogadro's constant) 7- Molar mass - physical mass of 1 mol of a substance ($6.02\times 10^{23}$ atoms) 8- Unit: grams / mole 9 10$$ 11n = {m \over M}