Ions & Chemical Formulae

Video Lesson

Introduction

An important part of GCSE Chemistry is using ions to work out chemical formulae. Before we learn how to do this, we need to understand the different types of ions. There are three groups to get to grips with:

1. The ions of elements that can be worked out from the element’s group number
2. The ions of transition metal elements
3. The molecular ions and hydrogen (these have to be learnt)

Let’s look at each group in turn.

1. Ions from Group Numbers

The periodic table can tell us the charge on many ions. Metals lose electrons to form positive ions, and non-metals gain electrons to form negative ions. The number of electrons lost or gained depends on the element’s group number:

Group	Ion Charge	Examples
Group 1	+1	Li+, Na+, K+
Group 2	+2	Be2+, Mg2+, Ca2+
Group 3	+3	Al3+, Ga3+
Group 5	−3	N3−, P3−
Group 6	−2	O2−, S2−
Group 7	−1	F−, Cl−, Br−

2. Transition Metal Ions

The transition metals form positive ions just like the other metals, but the periodic table will not tell us the charge on these ions. Instead, we find the charge from roman numerals in the compound’s name:

Name	Ion
Iron (II)	Fe2+
Iron (III)	Fe3+
Chromium (III)	Cr3+
Copper (II)	Cu2+
Silver (I)	Ag+

Notice that the same transition metal can form ions with different charges. Iron, for example, can be Fe²⁺ or Fe³⁺. The roman numeral in the name tells you which one is being used. If you see “iron (II)”, the iron ion has a 2+ charge. If you see “iron (III)”, it has a 3+ charge.

3. Molecular Ions and Hydrogen

There are some ions that are made from more than one element. Most of these are negative. These are called molecular ions and they just have to be learnt:

Name	Formula	Charge
Phosphate	PO43−	3−
Sulfate	SO42−	2−
Carbonate	CO32−	2−
Hydroxide	OH−	1−
Nitrate	NO3−	1−
Hydrogen	H+	1+
Ammonium	NH4+	1+

You will need to know these ions and their charges. A helpful way to remember them is to practise writing them out until they become second nature.

4. Writing Chemical Formulae from Ions

Ionic compounds are made up of positive ions and negative ions. The key rule is that the charges must cancel out to ensure the ionic compound is electrically neutral. This means the total positive charge must equal the total negative charge.

Worked Example 1: Magnesium Chloride

Step 1: Write down the ions. Magnesium is in Group 2, so it forms Mg²⁺. Chlorine is in Group 7, so it forms Cl⁻.

Step 2: Balance the charges. Magnesium has a 2+ charge, and each chloride ion has a 1− charge. We need two chloride ions to cancel the 2+ charge: (+2) + (2 × −1) = 0.

Answer: MgCl₂

Worked Example 2: Aluminium Oxide

Step 1: Write down the ions. Aluminium is in Group 3, so it forms Al³⁺. Oxygen is in Group 6, so it forms O²⁻.

Step 2: Balance the charges. We need the total positive and negative charges to cancel. The smallest numbers that work are 2 aluminium ions and 3 oxide ions: (2 × +3) + (3 × −2) = +6 − 6 = 0.

Answer: Al₂O₃

Worked Example 3: Sodium Sulfate

Step 1: Write down the ions. Sodium is in Group 1, so it forms Na⁺. Sulfate is a molecular ion: SO₄²⁻.

Step 2: Balance the charges. Each sodium ion has a 1+ charge, and sulfate has a 2− charge. We need two sodium ions: (2 × +1) + (−2) = 0.

Answer: Na₂SO₄

Worked Example 4: Iron (II) Oxide vs Iron (III) Oxide

This example shows why the roman numeral matters. The same two elements give different formulae depending on the charge of the iron ion:

• Iron (II) oxide: Fe²⁺ and O²⁻. The charges are already equal and opposite, so the formula is simply FeO.
• Iron (III) oxide: Fe³⁺ and O²⁻. We need 2 iron ions and 3 oxide ions: (2 × +3) + (3 × −2) = 0. The formula is Fe₂O₃.

Worked Example 5: Calcium Nitrate

Step 1: Write down the ions. Calcium is in Group 2, so it forms Ca²⁺. Nitrate is a molecular ion: NO₃⁻.

Step 2: Balance the charges. We need two nitrate ions to cancel the 2+ charge of calcium: (+2) + (2 × −1) = 0.

Answer: Ca(NO₃)₂

Notice the brackets around the nitrate ion. When you need more than one of a molecular ion, you put the ion in brackets and write the number outside. This tells us there are two complete NO₃ groups, not just two oxygen atoms at the end.

Worked Example 6: Aluminium Sulfate

Step 1: Write down the ions. Aluminium forms Al³⁺. Sulfate is SO₄²⁻.

Step 2: Balance the charges. We need 2 aluminium ions and 3 sulfate ions: (2 × +3) + (3 × −2) = +6 − 6 = 0.

Answer: Al₂(SO₄)₃

5. Practice Questions

Now it’s your turn! Can you work out the chemical formulae for these compounds? Try them yourself before checking the answers below.

1. Lithium carbonate
2. Copper (II) oxide
3. Silver nitrate
4. Potassium sulfate
5. Aluminium bromide
6. Sodium oxide
7. Magnesium hydroxide
8. Hydrogen sulfate (sulfuric acid)
9. Barium chloride
10. Beryllium nitrate

Answers

Compound	Positive Ion	Negative Ion	Formula
Lithium carbonate	Li+	CO32−	Li2CO3
Copper (II) oxide	Cu2+	O2−	CuO
Silver nitrate	Ag+	NO3−	AgNO3
Potassium sulfate	K+	SO42−	K2SO4
Aluminium bromide	Al3+	Br−	AlBr3
Sodium oxide	Na+	O2−	Na2O
Magnesium hydroxide	Mg2+	OH−	Mg(OH)2
Hydrogen sulfate (sulfuric acid)	H+	SO42−	H2SO4
Barium chloride	Ba2+	Cl−	BaCl2
Beryllium nitrate	Be2+	NO3−	Be(NO3)2

Summary

• Ions from Groups 1, 2 and 3 form positive ions with a charge equal to the group number. Groups 5, 6 and 7 form negative ions.
• Transition metal ion charges are shown by roman numerals in the compound name.
• Molecular ions (sulfate, carbonate, hydroxide, nitrate, phosphate, ammonium) must be memorised.
• To write a chemical formula, the positive and negative charges must cancel to give a neutral compound.
• Use brackets around molecular ions when more than one is needed in the formula.

Related Lessons

• Atomic Structure – revise how the periodic table determines the number of protons, neutrons, and electrons.
• Balancing Symbol Equations – put your formulae skills into practice by balancing chemical equations.