2.2 Relative Mass

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• You NEED to know this definition

1. The mass of an isotope relative to 1/12th of the mass of an atom of Carbon-12

1. The weighted mean mass of an atom of an element relative to 1/12th of the mass of an atom of Carbon-12
2. Can be worked out from isotopic abundances
   1. 1) Different isotopes of an element occur in different quantities or isotopic abundances
   2. 2) To work out the relative atomic mass of an element, you need to work ou thte average mass of all is atoms
   3. If you're given the isotopic abundances in percentages:
      1. 1) Multiply each Ar by its % relative isotopic abundance, and add up results
         1. 2) Divide by 100

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• you NEED TO KNOW this definition

1. The weighted mean mass of a molecule compared with one -­‐ twelfth of the mass of an atom of carbon -­‐ 12

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• You NEED TO KNOW this definition - it could be worth one mark in your exam!

1. The weighted mean mass of a formula unit compared with one -­‐ twelfth of the mass of an atom of carbon -­‐ 12

1. 1) Sample is placed in the mass spectrometre
2. 2) Sample is vapourised and ionised = positive ions
3. 3) Ions accelerated. Heavier ions move slower and are more difficult to deflect than lighter ions = ions of each isotope are seperated
4. 4) The ions are detected on a mass spectrum as a mass-to-charge ratio (m/z). Each ion reaching the detector adds to the signal, so the greater the abundance, the larger the signal