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Atoms have very little mass so their relative atomic masses are used. The relative atomic mass of an element, symbol A_r, is the relative mass of its atoms compared to the mass of a carbon-12 atom. The A_r values for elements are given in the periodic table. Since A_r is a measure of relative mass, it has no units.

Calculating relative formula mass

The relative formula mass of a substance made up of molecules is the sum of the relative atomic masses of the atoms in the numbers shown in the formula.

Relative formula mass has the symbol, M_r. To calculate the M_r for a substance:

1. work out how many atoms of each element there are in the chemical formula
2. add together the A_r values for all the atoms of each element present

For example, the formula for carbon dioxide is CO₂. It consists of one carbon atom (A_r = 12) and two oxygen atoms (A_r = 16):

M_r of CO₂ = 12 + 16 + 16 = 44

It could also be calculated this way:

M_r of CO₂ = (1 × 12) + (2 × 16) = 12 + 32 = 44

Like A_r values, M_r values are just numbers. They have no units because they are relative masses.

Relative formula masses of ionic compounds

Ionic compounds such as sodium chloride do not exist as molecules. However, their relative formula masses are calculated in the same way, from the numbers shown in the formula.

Worked example

Relative formula masses in balanced symbol equations

Because of the law of conservation of mass, as long as a chemical equation is balanced, the sum of the M_r values of the reactants equals the sum of the M_r values of the products.