Chemical Formulae and Balancing Equations
You cannot do stoichiometry, electrolysis or organic chemistry without this. It rests on two skills: writing correct formulae from ionic charges, and balancing with coefficients only.
1. Writing formulae from ions
The common charges:
| Charge | Ions |
|---|---|
| 1+ | Group I (Li⁺, Na⁺, K⁺), H⁺, NH₄⁺ (ammonium), Ag⁺ |
| 2+ | Group II (Mg²⁺, Ca²⁺), Zn²⁺, Cu²⁺, Fe²⁺, Pb²⁺ |
| 3+ | Al³⁺, Fe³⁺ |
| 1− | Group VII (F⁻, Cl⁻, Br⁻, I⁻), OH⁻, NO₃⁻, HCO₃⁻ |
| 2− | O²⁻, S²⁻, SO₄²⁻, CO₃²⁻ |
| 3− | PO₄³⁻, N³⁻ |
Learn the polyatomic ions with their charges — nitrate NO₃⁻, sulfate SO₄²⁻, carbonate CO₃²⁻, hydroxide OH⁻, ammonium NH₄⁺. These appear constantly.
Nitrate is NO₃⁻; nitrite is NO₂⁻. Confusing them was recorded twice. Similarly sulfate SO₄²⁻ vs sulfite SO₃²⁻. The -ate ion has more oxygen than the -ite ion — that rule covers all of them.
Aluminium forms a POSITIVE ion, Al³⁺. A recorded error had it negative — aluminium is a metal, and metals lose electrons.
The method — charges must cancel:
- Write the two ions with their charges
- Swap and drop the numbers (criss-cross), or find the lowest ratio that gives a total charge of zero
- Simplify if possible
Examples:
- Sodium chloride: Na⁺ and Cl⁻ → NaCl
- Magnesium chloride: Mg²⁺ and Cl⁻ → MgCl₂
- Aluminium oxide: Al³⁺ and O²⁻ → Al₂O₃
- Aluminium bromide: Al³⁺ and Br⁻ → AlBr₃
- Calcium carbonate: Ca²⁺ and CO₃²⁻ → CaCO₃ (charges already cancel)
Check the element symbol. Recorded errors included bromine (Br) confused with boron (B), and sodium written as S instead of Na. Use the periodic table.
Brackets for polyatomic ions:
When you need more than one polyatomic ion, put it in BRACKETS.
- Calcium hydroxide: Ca²⁺ and OH⁻ → Ca(OH)₂
- Zinc nitrate: Zn²⁺ and NO₃⁻ → Zn(NO₃)₂
- Aluminium sulfate: Al³⁺ and SO₄²⁻ → Al₂(SO₄)₃
Ca(OH)₂ is not CaOH₂. Without brackets the subscript applies only to the H, which is a different (and non-existent) substance. Confusion about brackets was recorded directly.
Transition metals have variable charges, shown by Roman numerals: iron(II) is Fe²⁺, iron(III) is Fe³⁺, copper(II) is Cu²⁺.
Read the Roman numeral — it IS the charge. Confusion about transition metal charges was recorded.
2. The diatomic elements
Seven elements exist as diatomic molecules: H₂, N₂, O₂, F₂, Cl₂, Br₂, I₂.
A memory aid: Have No Fear Of Ice Cold Beer — Hydrogen, Nitrogen, Fluorine, Oxygen, Iodine, Chlorine, Bromine.
Remember oxygen is O₂ when writing equations. Tutors flagged this twice — writing “O” instead of “O₂” is one of the most common equation errors and makes balancing impossible.
3. Balancing equations
Balance using COEFFICIENTS (the big numbers in front). NEVER change the subscripts.
Changing a subscript changes the substance: H₂O is water, H₂O₂ is hydrogen peroxide.
The method:
- Write the correct formulae first — and don’t change them afterwards
- Count the atoms of each element on both sides
- Balance the metals first, then non-metals, then hydrogen, then oxygen last
- Treat polyatomic ions as a single unit if they appear unchanged on both sides
- Re-count everything at the end
Balance metals first, then treat polyatomic ions as one unit. This was tutors’ own recommended order, and it saves a great deal of trial and error.
Oxygen last — it usually appears in the most places, so leaving it until the end means fewer adjustments.
Worked example — combustion of propane:
C₃H₈ + O₂ → CO₂ + H₂O
- Carbon: 3 on the left → 3CO₂
- Hydrogen: 8 on the left → 4H₂O
- Oxygen: right side now has (3 × 2) + (4 × 1) = 10, so we need 5O₂
C₃H₈ + 5O₂ → 3CO₂ + 4H₂O
Count carefully. Miscounting oxygen atoms during balancing was the single most frequent recorded error in this topic — count the oxygens in every product, including the water.
Using fractions:
Sometimes a fraction is the easiest route — then multiply everything through to clear it.
Example: C₂H₆ + 3½O₂ → 2CO₂ + 3H₂O
- Multiply all through by 2: 2C₂H₆ + 7O₂ → 4CO₂ + 6H₂O
Fractions are a legitimate intermediate step, but clear them before giving your final answer unless the question allows them.
4. Combustion — complete and incomplete
Complete combustion (plenty of oxygen): hydrocarbon + O₂ → CO₂ + H₂O Incomplete combustion (limited oxygen): → CO (carbon monoxide) and/or C (soot) + H₂O
Confusing the products of complete and incomplete combustion was recorded. Complete gives carbon dioxide; incomplete gives carbon monoxide — a toxic gas — and soot.
5. State symbols
(s) solid, (l) liquid, (g) gas, (aq) aqueous (dissolved in water)
(l) and (aq) are different. A pure liquid is (l); something dissolved in water is (aq).
Add state symbols whenever the question shows them or asks for them.
6. Reactions worth knowing by name
| Reaction | Equation |
|---|---|
| Haber process (ammonia) | N₂ + 3H₂ ⇌ 2NH₃ |
| Photosynthesis | 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂ |
| Thermal decomposition of a carbonate | CaCO₃ → CaO + CO₂ |
| Acid + metal | acid + metal → salt + hydrogen |
| Acid + carbonate | acid + carbonate → salt + water + carbon dioxide |
| Neutralisation | acid + base → salt + water |
The industrial process for ammonia is the HABER process. A recorded attempt called it “de-unitrification” — the name is worth learning, along with its equation and conditions.
7. Marks for partial answers
You get credit for writing correct reactants and products even if the equation isn’t balanced. Tutors flagged this directly — so always attempt the equation, and write the formulae you are sure of.
Show your working on any calculation attached to an equation.
8. Mistakes that cost marks
Changing subscripts instead of using coefficients.
Writing O instead of O₂ (and the other diatomics).
Omitting brackets — CaOH₂ instead of Ca(OH)₂.
Confusing nitrate with nitrite, or sulfate with sulfite.
Wrong ionic charges, especially for aluminium and transition metals.
Confusing element symbols — Br with B, S with Na.
Miscounting oxygen when balancing.
Confusing complete and incomplete combustion products.
Leaving fractions in a final answer.
Wrong or missing state symbols.
Frequently asked questions
How do I write a formula from ions? Combine them so the charges cancel, then simplify.
What is the formula for aluminium oxide? Al₂O₃ — from Al³⁺ and O²⁻.
When do I use brackets? When you need more than one polyatomic ion — e.g. Ca(OH)₂.
What’s the difference between nitrate and nitrite? Nitrate is NO₃⁻; nitrite is NO₂⁻. The -ate ion has more oxygen.
Which elements are diatomic? H₂, N₂, O₂, F₂, Cl₂, Br₂, I₂.
How do I balance an equation? Use coefficients only — metals first, then non-metals, hydrogen, and oxygen last.
Can I change the subscripts? No — that changes the substance.
Can I use fractions? Yes, as an intermediate step — then multiply through to clear them.
What do complete and incomplete combustion produce? Complete: CO₂ + H₂O. Incomplete: CO and/or carbon (soot) + H₂O.
What does (aq) mean? Dissolved in water — different from (l), a pure liquid.
Quick revision checklist
- I know the common ionic charges, including polyatomic ions
- I know -ate has more oxygen than -ite
- I can write formulae so the charges cancel
- I use brackets for multiple polyatomic ions
- I can read Roman numerals as transition metal charges
- I know the seven diatomic elements
- I balance with coefficients, never subscripts
- I balance metals first and oxygen last
- I treat unchanged polyatomic ions as a unit
- I re-count every element at the end
- I can clear fractions
- I know the products of complete and incomplete combustion
- I use correct state symbols
- I always attempt the equation for partial credit
These notes cover chemical formulae and balancing equations in the Cambridge IGCSE Chemistry (0620) syllabus and are written for Grade 9–11 / Year 10–11 students. They are based on teaching patterns observed across a large set of one-to-one IGCSE Chemistry lessons, with particular attention to the errors students make most often and the wording examiners reward. Always check the current syllabus and data booklet for your own exam series.
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