2.2 Relative Mass

2.2 Relative Mass

1. Relative Isotopic Mass

   • You NEED to know this definition
   • The mass of an isotope relative to 1/12th of the mass of an atom of Carbon-12

2. Relative Atomic Mass

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

3. Relative Molecular Mass

   • you NEED TO KNOW this definition
   • The weighted mean mass of a molecule compared with one -­‐ twelfth of the mass of an atom of carbon -­‐ 12

4. Relative formula mass

   • You NEED TO KNOW this definition - it could be worth one mark in your exam!
   • The weighted mean mass of a formula unit compared with one -­‐ twelfth of the mass of an atom of carbon -­‐ 12

5. The percentage abundances of the isotopes in a sample of an element are found experimentally using a mass spectrometre

   • 1) Sample is placed in the mass spectrometre
   • 2) Sample is vapourised and ionised = positive ions
   • 3) Ions accelerated. Heavier ions move slower and are more difficult to deflect than lighter ions = ions of each isotope are seperated
   • 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