Lesson focus: subscriptA small low number in a formula that shows how many atoms there are, such as the 2 in H₂O. define a substance; coefficientA big number in front of a formula in an equation that tells you how many of that particle there are, such as the 2 in 2H₂O. count whole formula units or moleculeTwo or more atoms joined together, such as a water molecule (H₂O)..
Read formulae, count atomThe tiny particle that everything is made of; each element has its own kind of atom. and balance simple equations without changing substances.
Build on earlier learning
Compounds and molecules: modelsTranslate molecular models into formulae before applying coefficients.
You will be successful when you can…
- Interpret subscripts.
- Apply coefficients to whole formulae.
- Use atom inventories.
- Balance by coefficients only.
Words that carry the science
- subscript
- Small number showing atom count or ratio within a formula.
- coefficient
- Number before a formula multiplying the whole formula.
- reactant
- Starting substance in a chemical reaction.
- product
- Substance formed in a reaction.
- balanced equation
- Equation with equal counts of each atom type on both sides.
Conserve atoms without rewriting chemical identity
01Subscripts belong to identity
H₂O has two H atoms for each O atom. NaCl states a 1:1 ion ratio rather than a molecule.
02Coefficients multiply everything
2H₂O represents four H atoms and two O atoms. Inventory each element separately.
03Balance, do not redesign
2H₂ + O₂ → 2H₂O conserves H and O. Changing H₂O to H₂O₂ would name a different substance.
Balancing equations
Change the big numbers in front of each formula until there are the same number of each type of atom on both sides. You cannot change the small numbers: that would make a different substance. Play tries numbers in order until it balances.
Controls
Balance water formation
Balance H₂ + O₂ → H₂O.
- Count O: left 2, right 1, so place 2 before H₂O.
- Right now has 4 H atoms.
- Place 2 before H₂ to give 4 H atoms left.
- Check both sides: H = 4 and O = 2.
How to make the reasoning visible: Final equation: 2H₂ + O₂ → 2H₂O.
Atom inventory
Count every atom after applying coefficients.
| Expression | H atoms | O atoms | Balanced partner |
|---|---|---|---|
| 2H₂ | 4 | 0 | 2H₂O with O₂ |
| O₂ | 0 | 2 | 2H₂O with 2H₂ |
| 2H₂O | 4 | 2 | 2H₂ + O₂ |
| 3CO₂ | 0 | 6 | 3 C and 6 O atoms |
Why does 2H₂O contain four hydrogen atoms?
Why not change H₂O to H₂O₂?
How many oxygen atoms are in 3CO₂?
Check your understanding
Answer from memory, then check against the suggested answer.
What is subscript?
What is coefficient?
What is reactant?
What is a product?
Why must a chemical equation be balanced?
How many atoms are in 3CO₂?
Why can a formula represent a ratio rather than a molecule?
Balance this equation: Mg + O₂ → MgO.
Exam Question Practice
Exam Question Practice
Mark points shown after marking
Balance the equation. Write 1 where no number is needed.
(3)H₂ + O₂ → H₂O
Explain how you know your equation is balanced.
(1)Total for this question: 4 marks
Command word: Show an atom count; the check earns confidence and often marks.