Relative Atomic Mass

The relative atomic mass is the weighted mean mass of an atom of an element compared to 1/12^th the mass of a carbon-12 atom which has a mass of 12.The symbol for relative atomic mass is A_r. Like relative isotopic mass it is a ratio, so it has no units.Most elements exist as a mixture of isotopes, and each isotope makes up a fixed percentage of the element, called its abundance. The relative atomic mass takes both the mass and the abundance of each isotope into account, which is why it is a weighted mean.A_r = Σ(relative isotopic mass × % abundance) ÷ 100Worked example: natural chlorine is 75% chlorine-35 and 25% chlorine-37.A_r(Cl) = (35 × 75 + 37 × 25) ÷ 100 = 35.5The answer is closer to 35 than to 37 because chlorine-35 is the more abundant isotope. A simple mean of 35 and 37 would give 36, which ignores the abundances and is wrong.This is why relative atomic masses are often not whole numbers, even though relative isotopic masses are close to whole numbers. The values given in the Periodic Table are these weighted means, for example A_r(Cl) = 35.5 and A_r(Cu) = 63.5.