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Relative Atomic Mass

Chemistry: Edexcel IGCSE, topic 3.1.5

Relative Atomic Mass (Ar)

The relative atomic mass (Ar) is the average mass of all of the isotopes of an element. It takes into account how often each isotope is found (the isotope abundance).

Calculating relative atomic masses (Ar)

Calculating relative atomic masses (Ar)

  • We can calculate relative atomic masses (Ar) using the expression shown above.
Equation explained

Equation explained

  • The Σ means 'sum of'.
  • The numerator is 'the sum of the isotope abundance times the isotope mass'.
  • The denominator is 'the sum of all the isotope abundances'.
E.g. ar of chlorine

E.g. ar of chlorine

  • Cl may exist as 2 isotopes, 35Cl and 37Cl.
  • Isotope abundances:
  • 75% 35Cl
  • 25% 37Cl
    • Ar = (0.75 x 35) + (0.25 x 37) = 35.5.
  • The existence of isotopes means that the relative atomic mass is not always a whole number.

Test yourself on Relative Atomic Mass

  • 75% of Cl atoms have a mass number of 35, whilst the remaining 25% have a mass number of 37. What is the relative atomic mass of chlorine?

  • What is the shorthand for relative atomic mass?

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