CIE IGCSE Chemistry

Revision Notes

4.1.3 Ar & Mr

Relative Atomic Mass & Relative Molecular Mass

Relative atomic mass

  • The symbol for the relative atomic mass is Ar
  • This is calculated from the mass number and relative abundances of all the isotopes of a particular element


Mass number and atomic number, IGCSE & GCSE Chemistry revision notesSymbol, mass number and atomic number of chlorine



4.1-Relative Atomic mass equation

The top line of the equation can be extended to include the number of different isotopes of a particular element present.

Example for Isotopes:

The table shows information about the Isotopes in a sample of rubidium

Example for Isotopes table, IGCSE & GCSE Chemistry revision notes


 ( 72   x   85 )   +   ( 28   x   87 ) ÷ 100  =  85.6

Relative Atomic Mass = 85.6


Relative formula (molecular) mass

  • The symbol for the relative molecular mass is Mr and it refers to the total mass of the molecule.
  • To calculate the Mr of a substance, you have to add up the Relative Atomic Masses of all the atoms present in the formula


Substance, atoms present table, IGCSE & GCSE Chemistry revision notes

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