Ions and Ionic Formulae
A concise OCR A Level Chemistry A revision guide to predicting the charge on a simple ion from its group, the formulae of the nitrate, carbonate, sulfate, hydroxide, ammonium, zinc and silver ions, and constructing the formula of any ionic compound by balancing charges.
- 2.1.2(a)
- 2.1.2(a)(i)
- 2.1.2(a)(ii)-a
- 2.1.2(a)(ii)-b
- 2.1.2(a)(ii)-c
- 2.1.2(a)(ii)-d
- 2.1.2(a)(ii)-e
- 2.1.2(a)(ii)-f
- 2.1.2(a)(ii)-g
What these spec points say
- 2.1.2(a) the writing of formulae of ionic compounds from ionic charges
- 2.1.2(a)(i) prediction of ionic charge from the position of an element in the periodic table
- 2.1.2(a)(ii)-a recall of the name and formula for the nitrate ion, NO3–
- 2.1.2(a)(ii)-b recall of the name and formula for the carbonate ion, CO32–
- 2.1.2(a)(ii)-c recall of the name and formula for the sulfate ion, SO42–
- 2.1.2(a)(ii)-d recall of the name and formula for the hydroxide ion, OH–
- 2.1.2(a)(ii)-e recall of the name and formula for the ammonium ion, NH4+
- 2.1.2(a)(ii)-f recall of the name and formula for the zinc ion, Zn2+
- 2.1.2(a)(ii)-g recall of the name and formula for the silver ion, Ag+
GCSE Recap: Losing and Gaining Electrons
Before you start, check the GCSE picture of how ions are formed.
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.
| Group | Electrons lost or gained | Ion charge | Examples |
|---|---|---|---|
| 1 | Lose 1 | 1+ | Li+, Na+, K+ |
| 2 | Lose 2 | 2+ | Mg2+, Ca2+, Ba2+ |
| 13 | Lose 3 | 3+ | Al3+ |
| 15 | Gain 3 | 3- | N3-, P3- |
| 16 | Gain 2 | 2- | O2-, S2- |
| 17 | Gain 1 | 1- | F–, Cl–, Br–, I– |
| Transition metals | Variable | Given in the name | Fe2+ 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.
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.
Quick Check: Charges from a Neighbour
Five rapid questions; each one names an element in the same group to work from.
The Compound Ions You Must Know
Some ions are groups of atoms covalently bonded together that carry an overall charge. OCR names five that you must be able to write from memory.
| Ion | Formula | Charge | Typical compound |
|---|---|---|---|
| Sulfate | SO42- | 2- | MgSO4 |
| Hydroxide | OH– | 1- | Ca(OH)2 |
| Nitrate | NO3– | 1- | KNO3 |
| Carbonate | CO32- | 2- | Na2CO3 |
| Ammonium | NH4+ | 1+ | NH4Cl |
The five compound ions OCR expects you to recall.
Exam focus: Ammonium is the only common positive compound ion. Sulfate and carbonate are 2-; hydroxide and nitrate are 1-.
Quick Check: Inside a Compound Ion
Decide whether the statement about ammonium nitrate is true or false.
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.
| Compound | Ions | Balance the charges | Formula |
|---|---|---|---|
| Magnesium chloride | Mg2+, Cl– | One 2+ needs two 1- | MgCl2 |
| Aluminium oxide | Al3+, O2- | Two 3+ (6+) balance three 2- (6-) | Al2O3 |
| Calcium nitrate | Ca2+, NO3– | One 2+ needs two nitrates; bracket the compound ion | Ca(NO3)2 |
| Ammonium sulfate | NH4+, SO42- | Two 1+ balance one 2- | (NH4)2SO4 |
| Iron(III) hydroxide | Fe3+, 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.
Choose the smallest numbers of each ion that make the total charge zero, and bracket a compound ion whenever more than one is needed.
Quick Check: Build Four Formulae
Drag the correct formula into each blank; the ion charges are given for you.
Quick Check: Read a Formula Backwards
Work out the charge on the metal ion from the formula you are given.
Quick Check: Which Formula Is Right?
In each round, pick the one statement that gives the correct formula.
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.
Two More Ions to Know: Zinc and Silver
Zinc and silver are transition-block metals, but unlike most of them they form only one ion each, so their charges must be learned.
| Ion | Formula | Typical compounds |
|---|---|---|
| Zinc | Zn²⁺ | ZnO, ZnCl₂, ZnSO₄, Zn(NO₃)₂ |
| Silver | Ag⁺ | AgNO₃, AgCl, Ag₂O, Ag₂SO₄ |
Balance them exactly as for Group 2 and Group 1 ions: zinc nitrate needs two nitrate ions for one Zn²⁺, Zn(NO₃)₂; silver sulfate needs two Ag⁺ for one SO₄²⁻, Ag₂SO₄.
Key idea: Zn²⁺ behaves like a Group 2 ion and Ag⁺ like a Group 1 ion when you write formulae.
Quick Check: Zinc and Silver
Only one of these four formulae is written correctly.
Where This Sits in the OCR Specification
OCR asks you to write the formulae of ionic compounds from ionic charges, predicting the charge from the position of the element in the Periodic Table.
It also asks you to recall the formulae of the nitrate, carbonate, sulfate, hydroxide, ammonium, zinc and silver ions.
The ions and the balancing method on this page cover all of it. The same skill is used every time you write an equation.
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 charges do zinc and silver ions have?
Zinc forms only Zn2+ and silver forms only Ag+. Their names therefore need no Roman numeral.
Why is magnesium oxide MgO and not Mg2O2?
A formula shows the smallest whole-number ratio of ions. Mg2+ and O2- cancel one to one, so the formula is MgO.
Copyright notice: This OLS revision content, including the explanations, layout, diagrams, tables and embedded learning structure, is authored for Online Learning System by Dr. Mohammed Al-Fatah. It may not be copied, reproduced, redistributed or adapted without written permission.
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