Deducing Formulae Using Common Ions

Section 1 of 6Ion charges from the periodic table

Group	Change in electrons	Ion charge	Example
1	Lose 1	1+	Na+
2	Lose 2	2+	Mg2+
3	Lose 3	3+	Al3+
5	Gain 3	3-	N3-
6	Gain 2	2-	O2-
7	Gain 1	1-	Cl-

Section 2 of 6Other ions to know

Transition metals

• Transition metals Their ions can have more than one charge, so a Roman numeral in the name gives it.
• Copper(II) oxide contains Cu²⁺, whereas copper(I) nitrate contains Cu⁺.

Non-metal names

• Non-metal names Single non-metal ions end in -ide, for example chloride, bromide and nitride.

Compound ions

• Compound ions Some ions contain several atoms: hydroxide OH^-, nitrate NO₃^-, sulfate SO₄^2-, carbonate CO₃^2- and ammonium NH₄⁺.

Section 3 of 6Working out a formula

1

Write down the symbol or formula of each ion with its charge.

2

Find how many of each ion are needed so that the total positive charge equals the total negative charge.

3

Write the simplest whole-number ratio of ions.

4

Write the formula with the positive ion first, putting the ratio as subscripts and omitting the number 1.

5

If you need more than one compound ion, put that ion in brackets before adding the subscript.

Section 4 of 6Worked examples

Compound	Ions	Ratio	Formula
Sodium bromide	Na+, Br-	1 : 1	NaBr
Aluminium fluoride	Al3+, F-	1 : 3	AlF3
Aluminium oxide	Al3+, O2-	2 : 3 (total 6+ and 6-)	Al2O3
Magnesium nitrate	Mg2+, NO3-	1 : 2	Mg(NO3)2
Ammonium sulfate	NH4+, SO42-	2 : 1	(NH4)2SO4

Section 5 of 6Key terms

Term	Meaning
Ion	An atom or group of atoms with an overall electric charge.
Compound ion	An ion made of more than one type of atom, such as sulfate.
Stock notation	Roman numerals in a name that give the charge on a metal ion, as in iron(III).
Ionic compound	A compound made of oppositely charged ions in a giant lattice that is overall neutral.
Charge balance	Choosing the numbers of ions so that positive and negative charges cancel.

Section 6 of 6Exam technique

1

Writing Mg(NO₃)2 incorrectly as MgNO₃2. Brackets are needed round the whole nitrate ion, with the subscript outside.

2

Changing the charge on an ion to make a formula work. Instead change the number of ions.

3

Leaving out the brackets when only one compound ion is needed. Brackets are used only when two or more are needed, such as Ca(OH)₂.

4

Forgetting that the Roman numeral in a name is the charge, so iron(III) chloride is FeCl₃.