Relative Masses

Relative Mass: No units as it is ratio of two masses- it’s relative.
• Carbon-12 Isotope: One atom of carbon 12 is 12 atomic mass units. 1 atomic mass unit is 1/12th
of an atom of carbon- 12 = mass of a proton or neutron.
• Relative Isotopic Mass: It is the mass of an isotope compared to 1/12th of the mass of an atom of
• Relative Atomic Mass: Ar. Is the weighted mean mass of an atom of an element compared to
1/12th of the mass of one atom of carbon-12. Shown in the periodic table. Most elements contain
mixture of isotopes.
• Calculating Ar: Sum of- percentage abundance of each isotope multiplied by their relative isotopic
masses. All divided by 100. Gives relative atomic mass.