Mini Chemistry
Part of Mini EducationSwitch subject

How To Write Chemical Symbols & Formulas

Formula writing: use ion charges (crossover), brackets for polyatomic ions, and key K324 / 6092 ions to write correct chemical formulas fast.

  • SEC G3 Pure Chemistry 2027
On this page

Learning objectives

  • state the symbols of the elements and formulae of the compounds mentioned in the syllabus
  • deduce the formulae of simple compounds from the relative numbers of atoms present and vice versa
  • deduce the formulae of ionic compounds from the charges on the ions present and vice versa

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

1. Definition

Chemical symbols represent elements (e.g., Na, Cl). A chemical formula shows the types and numbers of atoms/ions in a substance (e.g., NaCl, H₂O).

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

2. Key Ideas

  • 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 charges in the final formula (write NaCl, not Na + Cl⁻).
  • Never write a subscript 1 (write NaCl, not NaCl₁).
  • If you are given a relative number of atoms (e.g., C:O = 1:2), write it as a formula (CO₂), then simplify if needed (this is the same “ratio idea” used in empirical formulas).

3. Detailed Explanations

Quick Recall (formula writing)
  • Write ions with charges, then balance to make total charge 0.
  • Use brackets for polyatomic ions when needed, e.g. Ca(OH)₂.
  • Simplify ratios, e.g. Mg₂O₂ → MgO.
  • Final formulas have no charges and no subscript 1.

A. Core Ions for This Course

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.

CationSymbolCationSymbol
HydrogenH⁺AmmoniumNH₄ +
SodiumNa⁺PotassiumK⁺
SilverAg⁺CalciumCa²⁺
MagnesiumMg²⁺ZincZn²⁺
BariumBa²⁺Lead(II)Pb²⁺
Copper(II)Cu²⁺Iron(II)Fe²⁺
Iron(III)Fe³⁺AluminiumAl³⁺
AnionSymbolAnionSymbol
ChlorideCl⁻BromideBr⁻
IodideI⁻HydroxideOH⁻
NitrateNO₃-OxideO²⁻
SulfateSO₄²⁻CarbonateCO₃²⁻
Use ion charges precisely

Write the actual ion charge when balancing an ionic formula (for example, Mg²⁺ and Cl⁻). Do not treat “valency” and “charge” as interchangeable in an explanation.

B. Writing Formulas for Ionic Compounds (Balance to Neutral)

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

The “Crossover” Method (Use It Carefully)

  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.
Crossover method traps

Always simplify (e.g., Mg₂O₂ → MgO). Use brackets for polyatomic ions (e.g., Ca(OH)₂).

Writing neutral ionic formulae from ion chargesAluminium ions with charge three plus and oxide ions with charge two minus combine in a two to three ratio to form aluminium oxide, Al2O3. Calcium ions with charge two plus require two hydroxide ions, so brackets are used in Ca(OH)2.Example 1 · balance total chargeAl³⁺ + O²⁻LCM of 3 and 2 = 62(Al³⁺) + 3(O²⁻)Al₂O₃2(+3) + 3(−2) = 0Example 2 · keep a polyatomic ion togetherCa²⁺ + OH⁻two hydroxide ions neededCa²⁺ + 2(OH⁻)Ca(OH)₂brackets preserve OH as a group
Balance ion charges to obtain the simplest neutral formula; use brackets when more than one polyatomic ion is needed.

C. Writing Formulas From Relative Numbers of Atoms

When a question gives the relative numbers of atoms, use those numbers as the formula ratio and simplify if necessary.

  • C:O = 1:2 gives CO₂.
  • H:O = 2:1 gives H₂O.
  • X:Y = 2:4 simplifies to 1:2, so the formula is XY₂, not X₂Y₄.

This is an atom-ratio question, so do not invent ion charges. Use charge balance only when the particles are ions.

D. Naming Transition Metal Compounds (Roman Numerals)

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

4. Common Mistakes

  • Writing ionic formulas with charges included (wrong final format).
  • 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 are part of the formula; charges are not).
  • Ignoring Roman numerals and writing “iron chloride” without the oxidation state.

5. Exam Tips

Ionic formula method that scores
  1. Write ions with charges. 2) Balance total charge to 0. 3) Use brackets for polyatomic ions. 4) Simplify.
Name-from-formula method that scores

Use anion charge (e.g., O²⁻, Cl⁻) to find the metal ion charge, then write the Roman numeral.

6. Worked Examples

Modelled example 1

Ionic Formula (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

Brackets (Calcium Hydroxide)

About 5 min

Problem

Write the formula of calcium hydroxide.

Balance first, then decide whether brackets are needed

Hydroxide ions required
Final formula

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

Name From Formula (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 IUPAC name.

Derive charge rather than copying a subscript

Charge on each iron ion
Correct name

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

Formula From Name (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

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.

Challenge 5

Charge Balance and Brackets (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
Formula

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
A subscript applying to three sulfate ions must apply to the whole SO₄ group.
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.

7. Mind Stretchers

Mind stretcher 1: Simplify the 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: Reverse the Roman NumeralExtension

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.

8. Quiz

Quiz time

Ready to check your understanding? Try the interactive quiz, then review any questions you missed.

Go to quiz