Names and formulae of ionic compounds

Naming Ionic Compounds Made from Two Elements

When an ionic compound contains just two elements (a metal and a non-metal):

• The metal’s name comes first (it stays the same).

• The non-metal’s name comes second, but its ending changes to –ide.

Examples – Determine the name from the formula

1. Formula: LiBr

• Metal: lithium

• Non-metal: bromine → change ending to –ide

• Name: lithium bromide

2. Formula: MgO

• Metal: magnesium

• Non-metal: oxygen → –ide

• Name: magnesium oxide

3. Formula: AlN

• Metal: aluminium

• Non-metal: nitrogen → –ide

• Name: aluminium nitride

Naming Ionic Compounds with Three or More Elements

If a compound contains more than two elements, one of them is usually oxygen. These compounds often contain a compound/polyatomic ion (a small cluster of atoms that stay together and carry a charge). You have to remember these!

Naming Compounds with Transition Metals

Some metals (like iron, copper, lead) can form more than one type of ion (e.g. Fe²⁺ and Fe³⁺). We show which ion is used with Roman numerals in the name.

Ionic Compounds Are Always Charge Neutral

Every ionic compound must have no overall charge. That means the total positive charge from the metal ions must balance the total negative charge from the non-metal ions.

Think of it like a seesaw — both sides must balance.

Example 1 – Magnesium Chloride

• Mg forms Mg²⁺ ions

• Cl forms Cl⁻ ions

To balance the +2 charge on magnesium, you need two chloride ions (each –1).

Formula: MgCl₂

Check:

(+2) + (2 × –1) = 0 ✔️

Method 1: Lowest Common Multiple (LCM) Method

This method uses the lowest common multiple of the ion charges.

Steps:

1. Write the ions with their charges.

2. Find the lowest common multiple (LCM) of the charges.

3. Work out how many of each ion you need so the total charge = 0.

4. Write the formula without charges.

Example 2 – Calcium Fluoride

• Ca²⁺ and F⁻

• LCM of 2 and 1 is 2

  → Need 2 × F⁻ to balance 1 × Ca²⁺

Formula: CaF₂

Example 3 – Aluminium Oxide

• Al³⁺ and O²⁻

• LCM of 3 and 2 is 6

  → Need 2 × Al³⁺ (+6 total) and 3 × O²⁻ (–6 total)

Formula: Al₂O₃

Example 4 – Iron(III) Chloride

• Fe³⁺ and Cl⁻

• LCM of 3 and 1 is 3

  → Need 1 × Fe³⁺ and 3 × Cl⁻

Formula: FeCl₃

Method 2: The “Drop and Swap” Shortcut

This is a quick method to get the same result as the LCM method.

Steps:

1. Write the ion symbols with their charges.

2. Drop the number of each charge.

3. Swap them to become the number of the other ion.

4. Remove the charges and simplify if needed.

Example 5 – Magnesium Nitrate

Brackets are needed around the nitrate ion because there’s more than one of that group ion.

Mg(NO₃)₂

Example 6 – Aluminium Sulphate

Al₂(SO₄)₃

Example 7 – Ammonium Phosphate

So (NH₄)₃PO₄

When the Drop-and-Swap Method Doesn’t Change Anything

Sometimes, the charges on the ions already balance perfectly 1:1 — so when you “drop and swap,” you might end up writing unnecessary numbers.

Example – Magnesium Oxide

Ions:

Mg²⁺  O²⁻

If you drop and swap:

This give

That looks wrong — because Mg₂O₂ simplifies to MgO.

You don’t need two of each; one magnesium ion and one oxide ion already balance the charges.

Correct formula: MgO

Check:

(+2) + (–2) = 0 ✔️

1️. Name these ionic compounds:

2️. Now name these compounds that contain oxygen:

Challenge (name these):