Atomic masses

Atomic masses

1. Relative isotopic mass

   • All atoms in a single isotope are identical
   • For an isotope, the relative isotopic mass is the same as the mass number
   • Relative isotopic mass is the mass of an atom of an element compared with one twelfth of an atom of carbon-12

2. Relative atomic mass (Ar)

   • Most elements contain a mixture of isotopes, each in a different amount and with a different mass.
   • The contribution made by an isotope to the overall mass depends on:
     • The % abundance of the isotope
     • The relative mass of an isotope
   • Relative atomic mass, Ar, is the weighted mean mass of an atom of an element compared with the mass of one twelfth of an atom of carbon-12

3. Relative molecular mass (Mr)

   • Relative molecular mass is the weighted mean mass of a molecule compared to one twelfth of the mass of an atom of carbon-12

4. Relative formula mass

   • Compounds with giant structures do not exist as simple molecules.
   • relative formula mass is the weighted mean mass of a formula unit compared with one-twelfth of the mass of an atom of carbon-12