ATOM ECONOMY measures how efficiently atoms from reactants are converted into the desired product — how much of the mass of reactants ends up in the useful product vs waste products.
EQUATION:
Atom economy (%) = (sum of relative formula masses of desired products ÷ sum of relative formula masses of ALL products) × 100
Note: this uses ALL products from the reaction (including waste), not just the desired one.
ALTERNATIVELY:
Atom economy (%) = (Mr of desired product ÷ sum of Mr of all products) × 100
A HIGH atom economy means:
Most reactant atoms end up in the desired product — efficient.
Little waste is produced.
A LOW atom economy means:
Many reactant atoms end up in waste products — inefficient.
More waste to dispose of — environmental and cost concerns.
Calculating Atom Economy
EXAMPLE 1 — high atom economy:
Manufacture of iron from iron(III) oxide:
Fe₂O₃ + 3CO → 2Fe + 3CO₂
Desired product: Fe (Mr = 56). Two atoms of Fe = 112.
All products: 2Fe (112) + 3CO₂ (3 × 44 = 132) → total = 244.
Atom economy = (112 ÷ 244) × 100 = 45.9%
EXAMPLE 2 — maximum atom economy (addition reactions):
Addition polymerisation: nCH₂=CH₂ → (—CH₂—CH₂—)ₙ
ONLY ONE PRODUCT formed — all atoms go into the polymer.
Atom economy = 100%
Reason: addition reactions always have 100% atom economy because ALL reactant atoms form the single product with no by-products.
EXAMPLE 3 — low atom economy:
Production of calcium oxide: CaCO₃ → CaO + CO₂
Mr of CaO = 56. Mr of CO₂ = 44. Total = 100.
Atom economy = (56 ÷ 100) × 100 = 56%
(44% of the mass becomes CO₂ waste)
Why Atom Economy Matters
GREEN CHEMISTRY and SUSTAINABILITY:
High atom economy = less waste = more sustainable process.
Green chemistry aims to design reactions with high atom economy.
ECONOMIC BENEFITS:
Fewer raw materials wasted → lower costs.
Less waste to dispose of → lower disposal costs.
ENVIRONMENTAL BENEFITS:
Less waste produced → fewer by-products to release or treat.
More sustainable use of raw materials.
COMPARING ROUTES:
Chemists choose synthetic routes with higher atom economy when possible.
Two routes to the same product may have very different atom economies.
ATOM ECONOMY vs PERCENTAGE YIELD:
These are different measures:
Atom economy: a property of the EQUATION — how efficient the chemistry is.
Percentage yield: a property of how the EXPERIMENT is carried out — how much is actually obtained.
Both should be maximised for efficient, sustainable chemistry.
ADDITION vs SUBSTITUTION reactions:
Addition reactions (one product only): atom economy = 100%.
Substitution/elimination reactions (multiple products): lower atom economy.
⚠️ Common Mistake
Atom economy uses the relative formula masses of the PRODUCTS (from the equation) — not the reactants. Divide the Mr of the DESIRED product by the sum of Mr of ALL products. Do not confuse with percentage yield — atom economy is about the equation; percentage yield is about the actual experiment.
📐 Key Equations
Atom economy (%) = (Mr of desired products ÷ sum of Mr of ALL products) × 100
📌 Key Note
Atom economy = (Mr desired product ÷ sum Mr all products) × 100. High atom economy = less waste = sustainable. Addition reactions: 100% atom economy (one product). Low atom economy = more waste by-products. Distinct from % yield — atom economy is a property of the reaction equation.
🎯 Matching Activity — Atom Economy
Match each scenario to atom economy or a related concept. — drag the symbols on the right to match the component names on the left.
100% atom economy
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Lower atom economy
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Why high atom economy matters
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Atom economy vs % yield
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Atom economy: efficiency of the equation. % yield: how much product is actually collected.
Decomposition of CaCO₃ → CaO + CO₂ — 44% of mass becomes CO₂ waste
Addition polymerisation — only one product formed, all atoms go into the polymer
Less waste, lower disposal costs, more sustainable use of raw materials
⚽ FIFA Worked Examples
Atom Economy Calculation
Calculate the atom economy for making ethanol (C₂H₅OH, Mr = 46) from the reaction: C₂H₄ + H₂O → C₂H₅OH
F
Atom economy = (Mr desired product ÷ sum Mr all products) × 100
I
Only one product: C₂H₅OH, Mr = 46. Sum of all products = 46
F
Atom economy = (46 ÷ 46) × 100
A
Atom economy = 100% — addition reaction, one product only
🔬 Triple Science Only
Atom economy (4.3.3.2) is chemistry-only — not in Combined Science. Students must calculate atom economy and explain its importance for sustainable and economically efficient chemistry.
🎯 Test Yourself
Question 1 of 2
1. A reaction produces 80 g of desired product and 20 g of waste product. What is the atom economy?
2. Why do addition reactions always have an atom economy of 100%?
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