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Imagine running an industrial synthesis with a perfect 100%100\% yield, yet watching four-fifths of your expensive starting materials leave the plant as toxic waste.
How can a reaction be experimentally complete, yet fundamentally wasteful?

Percentage Yield vs. Atom Economy

Percentage yield measures how efficiently your lab technique avoids mechanical losses, while atom economy measures the theoretical waste built directly into the balanced chemical equation.
In 1991, chemist Barry Trost introduced atom economy to shift the focus from merely maximizing reaction yield to minimizing waste at the molecular design stage.
Because mass is conserved in any chemical reaction, we calculate atom economy as the ratio of the desired product's molar mass to the total molar mass of all reactants.
Atom Economy=Molar Mass of Desired ProductTotal Molar Mass of All Reactants×100%\text{Atom Economy} = \frac{\text{Molar Mass of Desired Product}}{\text{Total Molar Mass of All Reactants}} \times 100\%
Can a reaction's basic mechanism immediately tell you its maximum possible atom economy?

Reaction Types and Inherent Waste

In an addition reaction, all reactant atoms combine into a single compound, giving an atom economy of exactly 100%100\%.
Conversely, substitution reactions and elimination reactions swap out leaving groups or shed fragments, inherently generating byproducts that lower atom economy.