7.3 Formula of an Ionic Compound

This topic explains how to determine the formula of an ionic compound from the charges and oxidation numbers of its constituent ions.

Definition

To determine the formula of an ionic compound from the ionic charges and oxidation numbers of the constituent ions, you must determine the simplest whole-number ratio of cations (positively charged ions) to anions (negatively charged ions) that results in a neutral compound.

Simple explanation: Ionic compounds are electrically neutral — the total positive charge must equal the total negative charge. So you find the simplest ratio of ions that balances the charges.

Main Idea

Ionic compounds are electrically neutral. This means:

Total positive charge of cations = Total negative charge of anions

• Identify the ions and their charges.
• Determine the ratio needed to balance the charges.
• Write the formula using subscripts.

Simple explanation: The positive and negative charges must cancel out exactly.

Core Concept

Steps to Derive the Formula

Step 1: Identify the ions in the compound and their charges.

Step 2: Determine the ratio of charges needed to balance each other.

Step 3: Write the formula with subscripts representing the relationship defined in Step 2.

Detailed Explanation

Example 1: Calcium Chloride

• Calcium ion: Ca²⁺
• Chloride ion: Cl⁻

• Calcium has a charge of +2.
• Chloride has a charge of −1.
• You need two chloride ions for every calcium ion to balance the charges.

Step 3: Write the formula: CaCl₂

Final Answer: CaCl₂

Simple explanation: Ca²⁺ needs 2 Cl⁻ to balance the +2 charge. So the formula is CaCl₂.

Example 2: Magnesium Oxide

• Magnesium ion: Mg²⁺
• Oxide ion: O²⁻

• Magnesium has a charge of +2.
• Oxide has a charge of −2.
• One magnesium ion is required for each oxide ion.

Step 3: Write the formula: MgO

Final Answer: MgO

Simple explanation: Mg²⁺ and O²⁻ have equal but opposite charges, so they combine 1:1.

Example 3: Aluminium Sulphate

• Aluminium ion: Al³⁺
• Sulphate ion: SO₄²⁻

• Aluminium has a charge of +3.
• Sulphate has a charge of −2.
• You need two aluminium ions for every three sulphate ions to balance the charges.

Step 3: Write the formula: Al₂(SO₄)₃

Final Answer: Al₂(SO₄)₃

Simple explanation: To balance +3 and −2, use 2 Al³⁺ (total +6) and 3 SO₄²⁻ (total −6).

Important Note on Parentheses

It is important to remember that when writing a formula, parentheses must be used to add polyatomic ions when more than one is needed to balance the charges.

Example: Al₂(SO₄)₃ — the (SO₄)₃ means 3 sulphate ions.

Simple explanation: If you need more than one polyatomic ion, put it in brackets and write the number outside.

Summary of Steps

Intermediate Level

The Criss-Cross Method

A quick way to write ionic formulas:

• Write the ions with their charges.
• Criss-cross the charge numbers to become subscripts.
• Simplify if needed.

• Ca²⁺ and Cl⁻
• Criss-cross: Ca₁Cl₂
• Simplify: CaCl₂

• Al³⁺ and O²⁻
• Criss-cross: Al₂O₃
• Simplify: Al₂O₃

• Al³⁺ and SO₄²⁻
• Criss-cross: Al₂(SO₄)₃
• Simplify: Al₂(SO₄)₃

Simple explanation: The charge on one ion becomes the subscript of the other ion.

Using Oxidation Numbers

Oxidation numbers can also be used to determine the formula. The total positive oxidation number must equal the total negative oxidation number.

• Al has oxidation state +3
• SO₄ has oxidation state −2
• Need 2 Al³⁺ (+6) and 3 SO₄²⁻ (−6)
• Formula: Al₂(SO₄)₃

Common Ionic Compounds and Their Formulas