How this instrument works
Atom economy compares the molar mass of the one product you want against the combined molar mass of everything that went into the reaction, weighted by each reactant's stoichiometric coefficient. A reaction with high atom economy converts nearly all of its reactant atoms into the desired product; a reaction with low atom economy generates a lot of byproduct mass — atoms that were consumed but never became the thing you were trying to make. Chemist Barry Trost introduced the concept in 1991, and it won him the 1998 Presidential Green Chemistry Challenge Award for reframing how chemists judge reaction efficiency.
Atom economy is a purely stoichiometric measure — it only needs a balanced equation and a periodic table, unlike percent yield, which depends on how a specific reaction actually performs in a specific lab on a specific day. A reaction can have perfect, textbook stoichiometry and still have low atom economy if its mechanism inherently produces a lot of byproduct mass, such as a substitution reaction that ejects a heavy leaving group. Conversely, an addition reaction where every reactant atom ends up in the product — with nothing released as a separate byproduct — can reach 100% atom economy even before anyone has run it once.
The metric matters because low-atom-economy processes generate more waste per unit of product, which has real costs: raw materials to purchase, byproducts to separate, and waste to treat or dispose of. Comparing two synthetic routes to the same target molecule by their atom economy is a first-pass way to judge which route is inherently less wasteful, before considering yield, cost, or safety of the specific reagents involved.
- Enter Desired product molar mass (g/mol) — the molar mass of only the product you're trying to make, not any byproducts.
- Enter Sum of all reactant molar masses (g/mol, weighted by stoich. coeff.) — add up every reactant's molar mass multiplied by its coefficient in the balanced equation.
- Read Atom economy (%) directly beneath your inputs.
- Atom economy is calculated from the balanced equation alone — it does not require running the reaction, so you can compare candidate routes before choosing one to attempt.
- The reactant total must be greater than zero; a reaction needs at least some reactant mass for the ratio to mean anything.
Worked example — a 44.01 g/mol product from 58.08 g/mol of reactants
A reaction's desired product has a molar mass of 44.01 g/mol, and the sum of all reactant molar masses, weighted by their coefficients in the balanced equation, comes to 58.08 g/mol. Enter 44.01 into Desired product molar mass (g/mol) and 58.08 into Sum of all reactant molar masses; Atom economy (%) reads 75.7748%.
That means roughly three-quarters of the mass that went into the reaction ends up in the wanted product, with the remaining quarter — about 14.07 g/mol per mole of reaction as written — leaving as byproduct mass. A route with a higher atom economy for the same target product would waste proportionally less of its starting material.
Questions
What's the difference between atom economy and percent yield?
Atom economy is a property of the reaction's stoichiometry alone — it comes entirely from molar masses in the balanced equation and never changes no matter who runs the reaction. Percent yield measures how a specific attempt performed, comparing the actual mass isolated to the theoretical maximum, and depends on lab technique, side reactions, and purification losses. A reaction can have high atom economy on paper and still give a disappointing percent yield in practice, or vice versa — the two numbers answer different questions.
Can a reaction have 100% atom economy?
Yes — any reaction where every reactant atom ends up in the desired product, with nothing else produced, reaches 100% by definition. Addition reactions are the classic example: two molecules combining into one product with no separate byproduct released. Substitution and elimination reactions, by contrast, inherently eject a leaving group or small molecule as byproduct mass, which caps their atom economy below 100% no matter how efficiently the reaction runs.
Why does atom economy matter for green chemistry?
Because it quantifies waste before a single gram is synthesized. A low-atom-economy route generates more byproduct mass per unit of product, meaning more raw material purchased, more waste to separate and dispose of, and often more energy spent processing it. Atom economy became one of green chemistry's founding metrics because it lets chemists compare the inherent wastefulness of competing synthetic routes to the same molecule using nothing more than a balanced equation.
Does atom economy account for solvents or catalysts used in the reaction?
No — atom economy, in its original 1991 definition, only counts the reactants that appear in the balanced stoichiometric equation, not solvents, catalysts, or reagents used in workup and purification. Other green-chemistry metrics, like E-factor (mass of waste per mass of product), are designed to capture that broader picture; atom economy is deliberately narrower and simpler, a first-pass stoichiometric check.
How do I find the 'sum of all reactant molar masses' this instrument asks for?
Take the balanced chemical equation, multiply each reactant's molar mass by its coefficient in that equation, and add the results together. For a reaction written A + 2B → C + D, the total is (1 × molar mass of A) + (2 × molar mass of B). This instrument accepts that pre-summed total as a single input rather than individual reactant fields, since the number of reactants varies reaction to reaction.