Relative Atomic Mass

Relative Atomic Mass

Basic Principle

  • The Relative Atomic Mass is a dimensionless physical quantity, symbolised by ‘Ar’.
  • It’s the weighted mean mass of an atom of an element, compared with 1/12 of the mass of an atom of carbon-12.


  • It’s often given to a suitable degree of precision for use in calculations, and the value can usually be found in the Periodic Table.
  • The Relative Atomic Mass takes into account all the isotopes of an element, and their relative abundance, hence why it’s often not a whole number.

Isotopes and Relative Atomic Mass

  • An isotope is an atom of an element with the same number of protons, but differing numbers of neutrons, leading to different mass numbers.
  • Isotopes of an element do not all have the same relative atomic mass, which explains why the Relative Atomic Mass for an element is usually fractional.

Practical Examples

  • For example, chlorine’s Relative Atomic Mass is 35.5, because in a sample of chlorine, 75% of the atoms have a mass of 35 (Cl−35), and 25% have a mass of 37 (Cl−37).
  • An element’s mass number (its number of protons and neutrons) can be found from the Relative Atomic Mass: it’s the closest whole number to the Relative Atomic Mass.

Remember that understanding the concept of Relative Atomic Mass is essential for grasping future topics, such as Atomic Structure, Chemical bonding, Stoichiometry and more.