4 Relative mass

• Most elements are made up of atoms of different isotopes (e.g chlorine contains both ³⁵Cl and ³⁷Cl atoms)
• The relative atomic mass (Ar) of an element is an average of the mass of the isotope taking into account the relative abundance of each isotope
• Relative Atomic Mass, Ar is the average mass of an atom of an element relative to 1/12th mass of C¹²
• Relative formula mass, Mr if referring specifically to a molecule (relative molecular mass) the average mass of a molecule of a substance relative to 1/12th the mass of C¹²
• Relative formula mass, Mr more generally for any substance the sum of the relative atomic masses of all the atoms in the formula of a substance