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No atoms are gained or lost in a chemical reaction. However, some atoms in the reactants may not end up in the desired product. They instead form other products, and so are regarded as by-products.

For example, hydrogen can be manufactured by reacting methane with steam:

methane + steam → hydrogen + carbon monoxide

CH₄(g) + H₂O(g) → 3H₂(g) + CO(g)

In this reaction, carbon and oxygen atoms in the reactants do not form the useful product. Carbon monoxide is a waste gas.

The atom economy of a reaction is a measure of the amount of starting materials that end up as useful products. It is important for sustainable development and for economic reasons to use reactions with high atom economy.

Calculating percentage atom economy

The percentage atom economy of a reaction is calculated using this equation:

The highest possible value of atom economy is 100%, when all the reactant atoms end up in the desired product. If the atom economy is 50%, for example, then half the reactant atoms end up in the desired product or products.

Example

Hydrogen can be manufactured by reacting methane with steam:

CH₄(g) + H₂O(g) → 3H₂(g) + CO(g)

Calculate the atom economy for the reaction. (A_r of H = 1, A_r of C = 12, A_r of O = 16)

M_r of H₂ = 2 × 1 = 2

M_r of CO = 12 + 16 = 28

total M_r of all products = 3 × 2 + 28 = 34

A_r of H₂ = (2 × 1) = 2

total M_r of desired product = 3 × 2 = 6 (there are three H₂ in the balanced equation)

atom economy = 17.6%