Atom economy is an important GCSE Chemistry topic that commonly appears in exam questions because it helps chemists measure the efficiency of chemical reactions. Reactions with a high atom economy produce less waste and are usually more sustainable.
In this guide, you will learn what atom economy means, how to calculate it, and how to answer GCSE exam questions step-by-step.
Summary
- Reactions with a high atom economy produce less waste.
- Atom Economy = (Mr of desired product / Total Mr of all reactants) ×100
- Atom economy is different from percentage yield.
- Balanced equations are essential for accurate calculations.
What Is Atom Economy?
Atom economy measures how many of the atoms from the reactants end up in the desired product.
A high atom economy means most of the atoms become desired products. A low atom economy means lots of atoms are wasted as unwanted by-products.
Atom economy is solely based on the chemical reaction used. It is distinct from percentage yield which depends on the experimental method used to produce the substance.
Difference between atom economy and percentage yield
| Atom Economy | Percentage Yield |
|---|---|
| Measures how many reactant atoms end up in the desired product | Measures how much product is actually produced |
| Based on the chemical equation only | Based on practical experimental results |
| Linked to waste and sustainability | Linked to losses during reactions and purification |
| High atom economy means less waste | High percentage yield means more product obtained |
Learn more about calculating percentage yield in our GCSE Chemistry Percentage Yield guide.
Why Is Atom Economy Important?
Chemists try to use reactions with high atom economy because they:
- produce less waste
- use raw materials more efficiently
- reduce environmental damage
- can save companies money
High versus low Atom Economy
- High atom economy → most reactant atoms become the desired product, so less waste is produced.
- Low atom economy → many atoms form unwanted by-products, so more waste is produced.
Atom Economy Formula
The formula for atom economy is:
Atom economy = (Mr of desired product ÷ total Mr of all reactants) × 100
How To Calculate Atom Economy
Follow these steps:
- Write the balanced symbol equation.
- Identify the desired product.
- Calculate the relative formula mass (Mr) of the desired product/s.
- Calculate the total Mr of all reactants.
- Use the atom economy formula.
- Multiply by 100 to get a percentage.
Worked Example
Question: Calculate the atom economy for the production of magnesium oxide.
2Mg + O₂ → 2MgO
Step 1: Find the Mr of the desired product.
Mr of MgO = 24 + 16 = 40
There are 2MgO molecules, so total Mr = 80
Step 2: Find the total Mr of all Reactants.
2Mg = 2×24 = 48
O2 = 2×16 = 32
Total Mr of reactants = 48+32 = 80
Step 3: Use the formula.
Atom economy = (80 ÷ 80) × 100
Atom economy = 100%
This reaction has a 100% atom economy because all atoms from the reactants become part of the desired product.
Second Worked Example
Question: Calculate the atom economy for the production of Calcium Oxide in the reaction below.
CaCO₃ → CaO + CO₂
Step 1: Find the Mr of the desired product (CaO)
Mr of CaO = 40 + 16 = 56
Step 2: Find the total Mr of all the reactants
Mr of CaCO3 = 40+12+(3×16) = 100
Total Mr of reactants = 100
Step 3: Use the formula
Atom economy = (56 ÷ 100) × 100
Atom economy = 56%
This reaction has a lower atom economy because carbon dioxide is produced as an unwanted by-product.
Common GCSE Exam Mistakes
- Using all of the products instead of just the desired ones
- Not using the balanced equation
- Forgetting to multiply the Mr by the balancing number in the equation
- Forgetting to multiply by 100
- Mixing up percentage yield and atom economy
Exam Tips
Always identify the desired product before starting the calculation.
Carefully calculate relative formula masses using the periodic table.
Check whether the equation is balanced before calculating.
When calculating atom economy, remember to include the balancing numbers from the equation when calculating total Mr values.
Remember that a higher atom economy means less waste is produced.
How Atom Economy Links To Other GCSE Chemistry Topics
Atom economy links closely with several other GCSE Chemistry calculation topics including:
Final Thoughts
Atom economy is an important GCSE Chemistry calculation because it links chemistry with sustainability and industrial efficiency. Once you understand how to identify the desired product and calculate relative formula masses, the method becomes very straightforward. Practising several worked examples is one of the best revision techniques for GCSE Chemistry calculations. For a complete overview of all topics, see our GCSE Chemistry revision guide.