Isotopic Abundance & Relative Atomic Mass (Cambridge (CIE) A Level Chemistry): Revision Note

Calculating Relative Atomic Mass

• Isotopes are different atoms of the same element that contain the same number of protons and electrons but a different number of neutrons.

  • These are atoms of the same elements but with different mass numbers

• Because of this, the mass of an element is given as relative atomic mass (A_r) by using the average mass of the isotopes

• The relative atomic mass of an element can be calculated by using the relative abundance values

  • The relative abundance of an isotope is either given or can be read off the mass spectrum

• $A_r=\frac{\left(\mathrm{relative}{\mathrm{abundance}}_{\mathrm{isotope}1}{\mathrm{mass}}_{\mathrm{isotope}1}\right)+\left(\mathrm{relative}{\mathrm{abundance}}_{\mathrm{isotope}2}{\mathrm{mass}}_{\mathrm{isotope}2}\right)\mathrm{etc}.}{100}$