Key Concepts: Relative Masses

Relative atomic mass (Ar) is a measure of the mass of atoms. It compares the mass of a particular atom to one twelfth the mass of a carbon-12 atom.
Carbon-12 is used as a reference because the mass of its atoms is constant and well-defined.
All atomic masses are therefore fractions compared to the carbon-12 atom.
Relative isotopic mass refers to the mass of a specific isotope of an element, also compared to one twelfth of the mass of a carbon-12 atom.
Isotopes are atoms of the same element that have the same number of protons, but a different number of neutrons.
Relative molecular mass (Mr) is calculated by adding together the relative atomic masses of all the atoms in a molecule.
Similarly, the relative formula mass for an ionic compound is found by adding the relative atomic masses of all the atoms or ions in its formula unit.
The relative atomic, isotopic or molecular masses do not have units because they are comparisons or ratios.
The principles of relative mass play an integral role in many areas of chemistry, including stoichiometry, structure determination, and identification of unknown substances.
In the context of chemical reactions, the law of conservation of mass applies. This signifies that the total mass of reactants entering a reaction must equal the total mass of products exiting the reaction.
Calculation of relative masses has practical implications as well, aiding in balancing chemical equations and understanding how much of a particular reactant is needed or product is formed in a chemical reaction.