AQA A Level Chemistry

Revision Notes

1.2.1 Relative Atomic Mass & Relative Molecular Mass

Relative Masses

Atomic Mass Unit

  • The mass of a single atom is so small that it is impossible to weigh it directly
  • Atomic masses are therefore defined in terms of a standard atom which is called the unified atomic mass unit
  • This unified atomic mass is defined as one-twelfth of the mass of a carbon-12 isotope
  • The symbol for the unified atomic mass is u (often Da, Dalton, is used as well)
  • 1 u = 1.66 x 10-27 kg

Relative atomic mass, Ar

  • The relative atomic mass (Ar) of an element is the ratio of the average mass of the atoms of an element to the unified atomic mass unit
  • The relative atomic mass is determined by using the average mass of the isotopes of a particular element
  • The Ar has no units as it is a ratio and the units cancel each other out

Atomic Mass Unit equation

Relative isotopic mass

  • The relative isotopic mass is the mass of a particular atom of an isotope compared to the value of the unified atomic mass unit
  • Atoms of the same element with a different number of neutrons are called isotopes
  • Isotopes are represented by writing the mass number as 20Ne, or neon-20 or Ne-20
    • To calculate the average atomic mass of an element the percentage abundance is taken into account
    • Multiply the atomic mass by the percentage abundance for each isotope and add them all together
    • Divide by 100 to get average relative atomic mass
    • This is known as the weighted average of the masses of the isotopes

Atomic Structure 1, downloadable AS & A Level Chemistry revision notes

Relative molecular mass, Mr

  • The relative molecular mass (Mr) is the ratio of weighted average mass of a molecule of a molecular compound to the unified atomic mass unit
  • The Mr has no units

Relative Masses equation 3

  • The Mr can be found by adding up the relative atomic masses of all atoms present in one molecule
  • When calculating the Mr the simplest formula for the compound is used, also known as the formula unit
    • Eg. silicon dioxide has a giant covalent structure, however the simplest formula (the formula unit) is SiO2

Atoms, Molecules & Stoichiometry Relative Masses Table, downloadable AS & A Level Chemistry revision notes

Relative formula mass, Mr 

  • The relative formula mass (Mr) is used for compounds containing ions
  • It has the same units and is calculated in the same way as the relative molecular mass
  • In the table above, the Mr for potassium carbonate, calcium hydroxide and ammonium sulfates are relative formula masses
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