Writing chemical symbols and formulae

To write an ionic formula reliably, write the ion charges, balance the total charge to zero and use brackets when a polyatomic ion repeats.

Symbols and formulae mean different things

A chemical symbol identifies an element, such as Na for sodium or Cl for chlorine. Capital letters matter: Co is cobalt, while CO represents carbon monoxide.

A molecular formula gives the actual number of each kind of atom in one molecule: H₂O has two H atoms and one O atom. An ionic formula gives the simplest whole-number ratio of ions in a neutral compound: NaCl has a 1:1 sodium-ion to chloride-ion ratio, rather than separate NaCl molecules.

For ionic compounds, the formula must have overall charge 0.

Choose the right formula-writing method

• For a neutral atom, electrons = proton number Z (see Atoms).
• Ionic compounds: write ion charges, then balance to make total charge 0 (see Ionic Bonds).
• Covalent compounds: use the stated relative numbers of atoms or a familiar syllabus formula (see Covalent Bonds).
• Do not write individual ion charges in the final formula of a neutral compound (write NaCl, not Na + Cl⁻).
• Never write a subscript 1 (write NaCl, not NaCl₁).
• Distinguish an atom ratio from the actual numbers in one molecule. Simplify ratios for an empirical formula; preserve actual atom numbers for a molecular formula.

From symbols to a neutral formula

Common ions

These are the ions that recur across K324 equations, salts, electrolysis and qualitative analysis. Learn them in useful groups, and check unfamiliar charges from the information given.

Balance charges for an ionic formula

The total positive charge must equal the total negative charge (the compound must be neutral).

A shortcut: crossing charge magnitudes

1. Write the symbols of the ions.
2. Write their charges as superscripts.
3. Cross over the numerical value of the charges to become subscripts for the opposite ion.
4. Simplify the ratio if possible.

Ratios and actual atom numbers

Read carefully whether the information gives a ratio or the actual numbers in one molecule.

• A ratio X:Y = 2:4 simplifies to 1:2, giving the empirical formula XY₂: the simplest whole-number atom ratio.
• A molecule containing two X atoms and four Y atoms has the molecular formula X₂Y₄. Do not simplify it: that would change the stated atom count.
• Ethene has molecular formula C₂H₄ and empirical formula CH₂. The empirical formula alone does not tell you the size of a molecule.

If each molecule is stated to contain one C atom and two O atoms, write CO₂. For two H atoms and one O atom per molecule, write H₂O. Do not invent ion charges for an atom-count question.

You will use composition and relative molecular mass to work out these formulae in Empirical and molecular formulae.

Roman numerals identify the metal ion charge

Some metals can form ions with different charges (e.g., iron can be Fe²⁺ or Fe³⁺). Names use Roman numerals:

• FeCl₂ is iron(II) chloride
• FeCl₃ is iron(III) chloride

Check the formula

• Including individual ion charges in the formula of a neutral compound.
• Forgetting brackets for polyatomic ions (e.g., writing CaOH₂ instead of Ca(OH)₂).
• Not simplifying after crossover (e.g., leaving Mg₂O₂).
• Mixing up subscripts and charges (subscripts count atoms or ions; superscript charges state electrical charge).
• Ignoring Roman numerals and writing “iron chloride” without the oxidation state.

Explain how you found the formula

Worked examples

Modelled example 1

Writing aluminium oxide

Core

Problem

Write the formula of aluminium oxide.

Study the worked solution

1. Write the ions

   Method

   Identify aluminium and oxide ion charges.

   Reason

   The formula ratio must balance the actual ionic charges.

   Working

   Al³⁺ and O²⁻.

2. Find the smallest balanced totals

   Method

   Use the lowest common multiple of 3 and 2.

   Reason

   Two aluminium ions give 6 + and three oxide ions give 6-.

   Working

   2(+3) + 3(-2) = 0.

3. Write the neutral formula

   Method

   Convert the ion ratio into subscripts and omit charges.

   Reason

   The final formula records the simplest neutral ratio.

   Working

   Al₂O₃.

Guided practice 2

Using brackets for calcium hydroxide

About 5 min

Problem

Write the formula of calcium hydroxide.

Balance first, then decide whether brackets are needed

Hydroxide ions required

OneTwoThree

Final formula

Ca(OH)2CaOHCaOH2

Check my attempt

Hints

Hint 1: write both charges

Calcium is Ca²⁺ and hydroxide is OH⁻.

Hint 2: preserve the whole ion

A subscript applying to more than one hydroxide ion must apply to both O and H.

View solution step by step

1. Balance the charges

   Method

   Pair one calcium ion with two hydroxide ions.

   Reason

   1(+2) + 2(-1) = 0.

   Working

   Ca²⁺:OH⁻ = 1:2.

2. Apply brackets

   Method

   Enclose the repeated polyatomic ion before writing subscript 2.

   Reason

   Brackets show that both O and H occur twice.

   Working

   Ca(OH)₂.

Common misconception 3

Find the charge before naming iron oxide

Find and correct the mistake

Learner response

A student names Fe₂O₃ “iron(II) oxide” because the formula contains two iron atoms. Locate the error and give the correct name using a Roman numeral.

Derive charge rather than copying a subscript

Charge on each iron ion

2+3+6+

Name using a Roman numeral

Iron(II) oxideIron(III) oxideIron(I) oxide

Check my attempt

View solution step by step

1. Locate the error

   Method

   Reject using the number of iron atoms as the Roman numeral.

   Reason

   The Roman numeral gives oxidation state, not formula subscript.

   Working

   Subscript 2 ≠ iron charge.

2. Calculate the iron charge

   Method

   Balance the total oxide charge.

   Reason

   Three O²⁻ ions give 6-, so two iron ions must give 6 +.

   Working

   3(-2) = -6; + 6÷2 = +3 per iron ion.

3. Write the name

   Working

   Fe₂O₃ is iron(III) oxide.

Examiner practice 4

Writing ammonium sulfate

3 marks

Examination question

Write the formula of ammonium sulfate, showing the ions and charge balance. [3 marks]

Show charges before the final formula

Ions, ratio and formula

View solution step by step

1. Write the ions

   1 mark

   Method

   Identify ammonium and sulfate.

   Reason

   Formula ratios follow their charges.

   Working

   NH₄ + and SO₄²⁻.

2. Balance charge

   1 mark

   Method

   Use two ammonium ions per sulfate ion.

   Reason

   2(+1) + 1(-2) = 0.

   Working

   Ratio NH₄ + :SO₄²⁻ = 2:1.

3. Use brackets

   1 mark

   Method

   Enclose the repeated ammonium ion.

   Reason

   The subscript 2 applies to the entire polyatomic cation.

   Working

   (NH₄)₂SO₄.

Self-mark with the mark scheme

Compare your response with each mark point. Select a point only when your response contains that evidence.

Self-mark ions, neutral ratio and bracketed formula separately.

Mark points

1. 1 mark: Writes NH4+ and SO4 2-.
2. 1 mark: Uses two ammonium ions per sulfate ion.
3. 1 mark: Gives (NH4)2SO4 with correct brackets.

Calculate my mark

Challenge 5

Balancing charges in aluminium sulfate

Minimal support

Charge-and-bracket transfer

Write the formula of aluminium sulfate. Show the smallest neutral ion ratio and explain the bracket placement.

Balance 3+ against 2-

Al3+ : SO4 2- ratio

2 : 33 : 21 : 1

Formula

Al2(SO4)3Al3(SO4)2Al2SO4

Check my attempt

Hints

Hint 1: find a common charge total

The lowest common multiple of 2 and 3 is 6.

Hint 2: preserve sulfate as one ion

The subscript 3 applies to the whole sulfate ion.

View solution step by step

1. Write the ions

   Method

   Use aluminium and sulfate charges.

   Reason

   Charge magnitudes determine the neutral ratio.

   Working

   Al³⁺ and SO₄²⁻.

2. Balance total charge

   Method

   Use two aluminium ions and three sulfate ions.

   Reason

   2(+3) + 3(-2) = 0.

   Working

   Smallest ratio = 2:3.

3. Write the bracketed formula

   Method

   Bracket the repeated sulfate ion.

   Reason

   The subscript 3 must multiply the complete polyatomic group.

   Working

   Al₂(SO₄)₃.

Next step

Try Mind Stretcher 1 independently.

Try it yourself

Mind stretcher 1: Simplify the ion ratioExtension

Question: A student writes magnesium oxide as Mg₂O₂. Correct it and explain the mistake.

Show answer

The crossover method gives a ratio, but you must simplify it.

Mg₂O₂ simplifies to MgO (divide subscripts by 2).

Mind stretcher 2: Use the Roman numeral to write a formulaExtension

Question: A compound is named iron(II) chloride. Write its formula.

Show answer

Iron(II) means Fe²⁺. Chloride is Cl⁻.

Need two Cl⁻ for one Fe²⁺.

Final: FeCl₂.

Mind stretcher 3: Should this molecular formula be simplified?Extension

One hydrogen peroxide molecule contains two hydrogen atoms and two oxygen atoms. Write its molecular formula and its empirical formula. Explain why they differ.

Show answer

The molecular formula is H₂O₂ because it records the actual atoms in one molecule. The empirical formula is HO because the 2:2 atom ratio simplifies to 1:1. Do not replace H₂O₂ with HO when asked for the molecular formula.

Practise and check