Not all atoms of an element are identical - an element is defined by the number of protons in the atom, different numbers of neutrons are hence different isotopes of that element.

Differences Between Isotopes

  • Different molecular weights
  • Similar chemical properties
  • Different physical properties

Average Atomic Mass

The atomic mass (in atomic mass units) listed on the periodic table is the weighted (by abundance) average of all the different isotopic weights of the element.

e.g. if Carbon-12 has weight x and abundance 99%, and Carbon-13 has weight z and abundance 1%, then the Atomic Mass would be said to be:
x*99% + z*1%