2. (1) Relative Isotopic Mass - the mass of one atom of an isotope of an element relative to 1/12 the mass of an atom of carbon-12 isotope. (2) Relative Atomic Mass, Ar - the average mass of one atom of an element relative to 1/12 the mass of an atom of carbon-12 isotope. - the weighted average of the relative isotopic masses of the element e.g. Ar of Cl = 75/100 x 35 + 25/100 x 37 = 35.5 Definitions © my-chem-tutor.blogspot.com

3. (3) Relative Molecular Mass, Mr - the average mass of one molecule of a compound relative to 1/12 the mass of an atom of carbon-12 isotope. - sum of the individual Ar of the atoms in the molecular formula e.g. Mr of CH 4 = 12 + (1 x 4) = 16 (4) Relative Formula Mass - the average mass of one formula unit of ionic compound relative to 1/12 the mass of an atom of carbon-12 isotope. © my-chem-tutor.blogspot.com CH 4 16 “units” Relative Masses may be visualized as the number of “balls” or “units” required to balance the isotope, atom, molecule or ionic compound. TIP

4. Mole & Avogadro’s Constant Mole - is the amount of substance which contains the same number of particles ( atoms, molecules, ions, electrons etc ) as there are atoms in exactly 12g of 12 C. Avagadro’s constant, L - is the exact number of carbon atoms in 12g of 12 C. (i.e. L= 6.02 x 10 23 ). Molar Mass - the mass of one mole of a substance (g/mol). © my-chem-tutor.blogspot.com Mole may be visualized as just another quantitative term! TIP 6.02 x 10 23 units A Mole 12 units A Dozen 2 units A Pair