The weighted average mass of an element's isotopes relative to one twelfth of a carbon-12 atom.
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Relative atomic mass, Ar, is the weighted average mass of an element's naturally occurring isotopes, compared on a scale where one atom of carbon-12 has a mass of exactly 12. It is dimensionless because it is a ratio, but numerically it equals the average mass in atomic mass units. Each isotope's contribution is weighted by its fractional natural abundance. This is why the atomic masses on the periodic table are rarely whole numbers. Relative atomic mass is the standard input for converting between mass and amount of substance.
Key ideas: Relative atomic mass is the weighted average of isotope masses divided by one twelfth of the mass of 12C. Formula: Ar = sum of (isotope mass x fractional abundance). For chlorine, 0.758 x 34.97 + 0.242 x 36.97 = 35.45. It is dimensionless, unlike atomic mass in u.
Common mistakes and connections: Students average isotope masses without weighting by abundance, or use percent without dividing by 100. The periodic table value is an average, so no single atom has that mass. The method connects to mass spectrometry, molar mass and the mole.
Chlorine's mass spectrum shows peaks at 35 and 37 with a 3 to 1 ratio; the weighted average of these gives relative atomic mass 35.45. The interactive lesson shows this as a live animation or chart.
Ar of chlorine = (0.7577 x 34.97) + (0.2423 x 36.97) = 35.45.
The weighted average mass of an element's isotopes relative to one twelfth of a carbon-12 atom.
Relative Atomic Mass is a beginner topic in Atomic Structure & Periodicity. It needs no earlier chemistry lessons and is a good starting point.
Study these earlier topics in Atomic Structure & Periodicity first: Isotopes, Isobars, Isotones.
The next topics in Atomic Structure & Periodicity are Atomic Mass Unit and Mole.
Yes. The interactive lesson includes a self-check question on Relative Atomic Mass, and the Atomic Structure & Periodicity quiz draws on every topic in this area.
Relative Atomic Mass is topic 24 of 115 in Atomic Structure & Periodicity.