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Ions and Ionic Formulae

A concise Cambridge International AS Level Chemistry revision guide to ionic formulae. It covers predicting the charge on a simple ion from its group, the nine named ions, formulae from Roman numerals and constructing the formula of any ionic compound.

AS Level
Topic 2: Atoms, Molecules and Stoichiometry
9701 Papers 1 and 2
Cambridge International specification11 spec points in this lesson
  • 2.3.1
  • 2.3.1(a)
  • 2.3.1(b)-i
  • 2.3.1(b)-ii
  • 2.3.1(b)-iii
  • 2.3.1(b)-iv
  • 2.3.1(b)-v
  • 2.3.1(b)-vi
  • 2.3.1(b)-vii
  • 2.3.1(b)-viii
  • 2.3.1(b)-ix
What these spec points say
  • 2.3.1 write formulas of ionic compounds from ionic charges and oxidation numbers (shown by a Roman numeral)
  • 2.3.1(a) the prediction of ionic charge from the position of an element in the Periodic Table
  • 2.3.1(b)-i recall of the name and formula for the nitrate ion, NO3–
  • 2.3.1(b)-ii recall of the name and formula for the carbonate ion, CO32–
  • 2.3.1(b)-iii recall of the name and formula for the sulfate ion, SO42–
  • 2.3.1(b)-iv recall of the name and formula for the hydroxide ion, OH–
  • 2.3.1(b)-v recall of the name and formula for the ammonium ion, NH4+
  • 2.3.1(b)-vi recall of the name and formula for the zinc ion, Zn2+
  • 2.3.1(b)-vii recall of the name and formula for the silver ion, Ag+
  • 2.3.1(b)-viii recall of the name and formula for the hydrogencarbonate ion, HCO3–
  • 2.3.1(b)-ix recall of the name and formula for the phosphate ion, PO43–
Before you start

GCSE Recap: Losing and Gaining Electrons

Before you start, check the GCSE picture of how ions are formed.

1

Predicting the Charge on a Simple Ion

A metal atom in Groups 1, 2 or 13 loses its outer electrons to reach the electron configuration of the previous noble gas. A non-metal atom in Groups 15, 16 or 17 gains electrons to reach the next.

The number of electrons lost or gained is the charge on the ion, and it can be read straight from the group number.

GroupElectrons lost or gainedIon chargeExamples
1Lose 11+Li+, Na+, K+
2Lose 22+Mg2+, Ca2+, Ba2+
13Lose 33+Al3+
15Gain 33-N3-, P3-
16Gain 22-O2-, S2-
17Gain 11-F–, Cl–, Br–, I–
Transition metalsVariableGiven in the nameFe2+ in iron(II), Fe3+ in iron(III)

Key idea: Metals in Groups 1 and 2 form ions with a charge equal to the group number, and Group 13 metals form 3+ ions. Non-metals in Groups 15 to 17 form negative ions with a charge of 18 minus the group number.

Strip of Groups 1, 2, 13, 15, 16 and 17 showing electrons lost or gained, ion charges and example ions.

Metals lose their outer electrons to form positive ions and non-metals gain electrons to form negative ions, so the charge can be read from the group.

Check your understanding

Quick Check: Charges from a Neighbour

Five rapid questions; each one names an element in the same group to work from.

2

The Compound Ions You Must Know

Some ions are groups of atoms covalently bonded together that carry an overall charge. Cambridge names five that you must be able to write from memory.

IonFormulaChargeTypical compound
SulfateSO42-2-MgSO4
HydroxideOH–1-Ca(OH)2
NitrateNO3–1-KNO3
CarbonateCO32-2-Na2CO3
AmmoniumNH4+1+NH4Cl
Displayed formulae of the sulfate, hydroxide, nitrate, carbonate and ammonium ions in brackets with lone pairs, formal charges and overall charges

The five compound ions Cambridge expects you to recall.

Exam focus: Ammonium is the only common positive compound ion. Sulfate and carbonate are 2-; hydroxide and nitrate are 1-.

Check your understanding

Quick Check: Inside a Compound Ion

Decide whether the statement about ammonium nitrate is true or false.

3

Constructing the Formula of an Ionic Compound

An ionic compound is neutral overall, so the positive and negative charges must cancel.

Use the charge-balance method: write the ions, find the lowest number of each that makes the total charge zero, and write the formula with those numbers as subscripts.

Brackets go round a compound ion when more than one is needed.

CompoundIonsBalance the chargesFormula
Magnesium chlorideMg2+, Cl–One 2+ needs two 1-MgCl2
Aluminium oxideAl3+, O2-Two 3+ (6+) balance three 2- (6-)Al2O3
Calcium nitrateCa2+, NO3–One 2+ needs two nitrates; bracket the compound ionCa(NO3)2
Ammonium sulfateNH4+, SO42-Two 1+ balance one 2-(NH4)2SO4
Iron(III) hydroxideFe3+, OH–One 3+ needs three 1-Fe(OH)3

A quick check: the total positive charge equals the total negative charge, and the subscripts are the smallest whole numbers that achieve that.

The “swap the charges” shortcut gives the same answer but simplify afterwards: Mg2+ with O2- is MgO, not Mg2O2.

Exam sentence: The formula of an ionic compound shows the simplest whole-number ratio of ions that gives no overall charge.

Charge-balance method with worked cards for MgCl₂, Al₂O₃, Ca(NO₃)₂, (NH₄)₂SO₄ and Fe(OH)₃.

Choose the smallest numbers of each ion that make the total charge zero, and bracket a compound ion whenever more than one is needed.

Check your understanding

Quick Check: Build Four Formulae

Drag the correct formula into each blank; the ion charges are given for you.

Check your understanding

Quick Check: Read a Formula Backwards

Work out the charge on the metal ion from the formula you are given.

Check your understanding

Quick Check: Which Formula Is Right?

In each round, pick the one statement that gives the correct formula.

4

Common Exam Mistakes

  • Writing NaCl2 or MgCl. Check that the charges cancel: Na+ needs one Cl–, Mg2+ needs two.
  • Leaving out brackets: CaNO32 instead of Ca(NO3)2.
  • Giving sulfate as SO4– or carbonate as CO3–. Both carry a 2- charge.
  • Writing a molecular formula for a giant lattice. NaCl is a ratio of ions, not a molecule of one sodium and one chlorine.
  • Forgetting the Roman numeral: iron chloride is ambiguous; FeCl2 is iron(II) chloride and FeCl3 is iron(III) chloride.

Exam sentence: Charges cancel, smallest ratio, brackets round repeated compound ions.

5

The Nine Named Ions and Roman-Numeral Formulae

Cambridge adds four ions to the five above and asks for formulae written from oxidation numbers.

IonFormulaExample compound
ZincZn²⁺ZnSO₄
SilverAg⁺AgNO₃
HydrogencarbonateHCO₃⁻NaHCO₃, Ca(HCO₃)₂
PhosphatePO₄³⁻Na₃PO₄, Ca₃(PO₄)₂

Roman numerals give the charge. In a name such as iron(III) chloride or copper(II) sulfate, the numeral is the oxidation number of the metal, which for a simple ion equals its charge.

Iron(III) is Fe³⁺, so iron(III) chloride is FeCl₃; iron(II) is Fe²⁺, so iron(II) chloride is FeCl₂.

Copper(II) sulfate is CuSO₄; manganese(IV) oxide is MnO₂ because Mn⁴⁺ balances two O²⁻.

NameMetal ionAnionFormula
Iron(III) sulfateFe³⁺SO₄²⁻Fe₂(SO₄)₃
Copper(I) oxideCu⁺O²⁻Cu₂O
Lead(II) nitratePb²⁺NO₃⁻Pb(NO₃)₂
Chromium(III) hydroxideCr³⁺OH⁻Cr(OH)₃

Exam sentence: The Roman numeral in a name is the oxidation number of the metal, which gives the charge on its ion; balance that charge against the anion to write the formula.

Check your understanding

Quick Check: The Extra Ions and Roman Numerals

Four rapid questions on the ions added by this card.

6

Where This Sits in the Cambridge Syllabus

Cambridge asks you to write the formulae of ionic compounds from ionic charges and from oxidation numbers shown as Roman numerals.

It also asks you to recall the nitrate, carbonate, sulfate, hydroxide, ammonium, zinc, silver, hydrogencarbonate and phosphate ions. This page covers the charges, the nine ions and the balancing method.

FAQs

These questions secure the formula-writing slips that cost easy marks.

How do I predict the charge on an ion from its group?

Groups 1, 2 and 13 form 1+, 2+ and 3+ ions. Groups 15, 16 and 17 form 3-, 2- and 1- ions: the charge is 18 minus the group number.

Why does calcium nitrate need brackets?

Two nitrate ions are needed, and the 2 must apply to the whole NO3 group, so the formula is Ca(NO3)2. Without brackets the 2 would apply to oxygen only.

Is ammonium a metal ion?

No. NH4+ is a compound ion of nitrogen and hydrogen, but it acts like a 1+ metal ion in formulae: ammonium chloride is NH4Cl.

What does the Roman numeral in iron(III) chloride tell me?

It is the oxidation number of iron, +3, so the ion is Fe3+. Three Cl– ions balance it, giving FeCl3.

What is the formula of calcium phosphate?

Ca2+ and PO43- balance as three 2+ against two 3-, so the formula is Ca3(PO4)2.

Dr. Mohammed Al-Fatah

Written by:
Dr. Mohammed Al-Fatah

Chemistry specialist revision notes for A Level Chemistry.

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