GCSE Combined Science (AQA)

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Relative Atomic Mass

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Atomic structure and the periodic table A simple model of the atom, symbols, relative atomic mass, electronic charge and isotopes

Relative Atomic Mass

6:57 Isotopes and Relative Atomic Mass of Elements
Spec 4.1.1.6
Previous Isotopes Next Electronic Structure
  • All elements consist of different isotopes.
  • Isotopes of an element have different mass numbers due to varying numbers of neutrons.
  • The periodic table lists each element with its proton number and relative atomic mass.
  • Relative atomic mass (Ar) is a weighted average of the masses of an element's isotopes.
  • Ar considers the abundance of each isotope, which is how much exists in nature.
  • Chlorine has two major isotopes: chlorine 35 (75% abundance) and chlorine 37 (25% abundance).
  • Relative atomic mass is a decimal number because it is an average, not a whole number.
  • The relative atomic mass is calculated using the masses and abundance of isotopes.
  • The formula for Ar is the total mass of all atoms divided by the total number of atoms.
  • Example calculation for chlorine: (75 x 35 + 25 x 37) / 100 = 35.5
  • For exams, students should be able to calculate Ar given the percentage abundance of isotopes.
  • Example calculation for copper: (63 x 69 + 65 x 31) / 100 = 63.62
  • Relative atomic mass is called "relative" because it uses carbon-12 as a reference point.
  • Carbon-12 is the most abundant stable isotope of carbon.
  • The relative atomic mass of an element is the average mass of its atoms compared to 1/12 the mass of a carbon-12 atom.
  • Carbon is said to have a relative atomic mass of 12, as carbon-12 makes up almost 99% of carbon on Earth.

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