Relative atomic mass: The average mass of one atom of an element, taking the natural abundances of the isotopes of that element into account. Relative atomic mass is measured on a scale whereby one carbon-12 atom has a mass of exactly 12.

Relative atomic mass = \(\frac{\text{Total mass of the atoms}}{\text{Total number of atoms}} \)

Most periodic tables show the relative atomic mass instead of the atomic mass.

Relative formula mass: The average mass of one molecule of a compound, where one carbon-12 atom has a mass of one unit.

The relative formula mass of a compound can be found by adding up all the relative atomic masses of all the atoms in that compound.

Example: Find the relative formula mass of calcium carbonate.

Answer: Calcium carbonate has the formula \(CaCO_3\). This means one molecule of calcium carbonate contains one calcium atom, one carbon atom and three oxygen atoms. Its relative formula mass is the sum of the relative atomic masses of all these atoms.

Relative atomic mass of carbon: 12.01

Relative atomic mass of oxygen: 15.99

Relative atomic mass of calcium: 40.08.

Therefore, the relative formula mass of calcium carbonate can be given by: \[(Ca \times 1)+(C \times 1) + (O \times 3) = (40.08 \times 1)+(12.01 \times 1) + (15.99 \times 3) = 100g\]