Atom economy weighs the product against everything the reaction makes
Add up the molar mass of every product in the balanced equation, then see what fraction is the product you actually want. That fraction is the atom economy. It measures how much of your starting material is designed to end up as waste, before you even run the reaction.
A worked example: chlorinating methane
CH₄ + Cl₂ → CH₃Cl + HCl. The wanted product is chloromethane (50.5 g/mol); the HCl (36.5 g/mol) is waste.
Only 58% of the product mass is useful; 42% leaves as hydrogen chloride. That is typical for a substitution reaction.
Reaction type predicts the ceiling
| Reaction type | Atom economy | Why |
|---|---|---|
| Addition | 100% | Every atom lands in one product |
| Rearrangement | ~100% | Atoms reshuffle, none lost |
| Condensation | high | Small molecule (water) split off |
| Substitution | <100% | Leaving group becomes waste |
| Elimination | lowest | A whole fragment is removed |
Why it is not the same as yield
- Yield can hide waste. A route can convert nearly all its reactants (high yield) yet still be built to discard half their mass as by-product (low atom economy).
- It is theoretical. Atom economy comes straight from the balanced equation, so it compares synthetic routes on paper before any lab work.
- It drives greener design. This is the second principle of green chemistry: pick the route that builds the most of the reactants into the target, so there is less to dispose of.
Common questions
How is atom economy calculated?
Divide the molar mass of the product you want by the summed molar mass of every product the balanced equation produces, then multiply by 100. It is a property of the equation, so you can work it out before running the reaction.
What is the difference between atom economy and yield?
Yield is how much product you actually recover; atom economy is how much of the reactant mass ends up in the product you want, even in a perfect reaction. A reaction can have 95 percent yield and 30 percent atom economy if it throws off a heavy by-product.
Which reactions have 100 percent atom economy?
Addition reactions, where every atom of the reactants ends up in a single product and nothing is released. Substitution and elimination always fall below 100 percent because a leaving group (often HCl or water) becomes waste.
