The atom economy (atom utilisation) is a measure of the amount of starting…

The idea

Atom economy, also called atom utilisation, measures how much of the mass of the reactants becomes the desired product rather than waste or by-products. It is calculated from the balanced equation, not from an experiment, so it is a theoretical measure of a reaction's efficiency. A reaction with high atom economy converts most reactant atoms into the useful product; addition reactions typically have 100% atom economy because all reactant atoms appear in the single product, while substitution and elimination reactions often produce by-products and therefore lower atom economy. For example, in an addition reaction such as ethene reacting with hydrogen to form ethane, all atoms become ethane, so the atom economy is 100%. Atom economy differs from percentage yield: yield depends on how much product is actually collected, whereas atom economy depends only on the reaction pathway chosen.

What earns the marks

• Define atom economy as the proportion of reactant mass that becomes the desired product, distinguishing it from percentage yield.
• Identify the desired product and all reactants from a balanced equation before any calculation.
• Explain that atom economy is a theoretical value determined by the equation and is independent of experimental losses.
• Compare two reaction pathways and state which has higher atom economy from the balanced equations, linking this to fewer unwanted by-products.
• Recognise that addition reactions generally have higher atom economy than substitution or elimination reactions because no atoms are wasted.

Common mistakes and how to put them right

• Confusing atom economy with percentage yield.

  Put it right by stating that atom economy comes from the equation while yield comes from measured amounts.

• Including only some reactants in the denominator.

  Put it right by summing the relative formula masses of all reactants shown in the balanced equation.

• Assuming a high atom economy guarantees a high yield.

  Put it right by explaining that a reaction can be efficient in theory but slow, incomplete or difficult to separate in practice.

Exam tips

• Write the balanced equation first and label the desired product clearly before calculating anything.
• Use the term atom utilisation if it appears in the question, showing you recognise it as a synonym for atom economy.
• When comparing pathways, quote the atom economy values or the by-products formed to justify your choice rather than asserting one is better.