Average atomic mass = sum(isotope fraction x isotope mass). Example: Cl has two isotopes: Cl-35 (75.77%) and Cl-37 (24.23%). Average = 0.7577(35) + 0.2423(37) = 26.52 + 8.97 = 35.49 amu ≈ 35.5. This is why atomic masses on the periodic table are not whole numbers. For calculations: always use the average atomic mass from the periodic table unless specifically working with a single isotope. Mass number (A) = protons + neutrons (always integer). Atomic mass ≈ A for individual isotopes but can differ slightly due to nuclear binding energy.