Isotopic Abundance and Average Atomic Mass Units

Isotopic Abundance and Average Atomic Mass

Atomic Mass Units

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  In a periodic table, the atomic mass is usually given in atomic mass units (u).
  
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  The atomic mass unit is defined as the mass of 1/12^th of the mass of a carbon-12 atom.
  
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  These masses are referred to as relative atomic masses, because the mass is measured relative to the carbon-12 atom.
  
  • 
    We now know that one atomic mass unit is 1.66 x 10^-24 g.
    

Average Atomic Mass

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  Many elements have two or more naturally occurring stable isotopes.
  
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  In order to determine the atomic mass of an element with more than one isotope, you need to find the average atomic mass.
  

The Average Atomic Mass of an element reflects the natural abundance of its isotopes, or isotopic abundance.

Relative Atomic Mass is calculated as follows:

(Atomic Mass of Isotope 1) x (Isotopic Abundance) + (Atomic Mass of Isotope 2) x (Isotopic Abundance) ...

Example: Isotopic Abundance of Rubidium

A sample of rubidium is 72.17% Rb-85 (mass = 84.91 u) and 27.83% Rb-87 (mass = 86.91 u). Calculate the average atomic mass of rubidium.