GCSE Chemistry revision

Ionic compounds

Chemical bonds, ionic, covalent and metallic

AQA 4.2.1.3 Foundation & Higher

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Key knowledge

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What is an Ionic Compound?

Ionic bonding involves a metal atom (e.g. sodium) transferring one or more electrons to a non-metal atom (e.g. chlorine), forming two oppositely charged ions.
The oppositely charged ions are attracted to each other by electrostatic forces, and this attraction is called an ionic bond.
The overall structure formed by ionic bonding is called an ionic compound.

1. What is an Ionic Compound? AQA

Notes

Ionic bonding involves a metal atom (e.g. sodium) transferring one or more electrons to a non-metal atom (e.g. chlorine), forming two oppositely charged ions.
The oppositely charged ions are attracted to each other by electrostatic forces, and this attraction is called an ionic bond.
The overall structure formed by ionic bonding is called an ionic compound.

The Giant Ionic Lattice Structure

In real ionic compounds, vast numbers of positive and negative ions alternate in all directions, each attracted to all surrounding ions of opposite charge.
This arrangement forms a three-dimensional regular lattice structure, sometimes called a giant ionic lattice.
Ionic compounds can be represented using ball and stick diagrams, where lines between ions represent the ionic bonds.

2. The Giant Ionic Lattice Structure AQA

Notes

In real ionic compounds, vast numbers of positive and negative ions alternate in all directions, each attracted to all surrounding ions of opposite charge.
This arrangement forms a three-dimensional regular lattice structure, sometimes called a giant ionic lattice.
Ionic compounds can be represented using ball and stick diagrams, where lines between ions represent the ionic bonds.

High Melting and Boiling Points

The melting and boiling points of a substance depend on the strength of the bonds holding it together.
Ionic compounds contain a very large number of strong ionic bonds throughout the lattice, requiring a huge amount of energy to break.
As a result, ionic compounds have very high melting and boiling points, only achievable at very high temperatures.

3. High Melting and Boiling Points AQA

Notes

The melting and boiling points of a substance depend on the strength of the bonds holding it together.
Ionic compounds contain a very large number of strong ionic bonds throughout the lattice, requiring a huge amount of energy to break.
As a result, ionic compounds have very high melting and boiling points, only achievable at very high temperatures.

Electrical Conductivity of Ionic Compounds

For a substance to conduct electricity, it must contain charged particles (ions or electrons) that are free to move.
In solid ionic compounds, all ions are fixed in place within the lattice, so they cannot conduct electricity.
When an ionic compound is melted or dissolved in water, the ions are free to move, allowing the compound to conduct electricity.

4. Electrical Conductivity of Ionic Compounds AQA

Notes

For a substance to conduct electricity, it must contain charged particles (ions or electrons) that are free to move.
In solid ionic compounds, all ions are fixed in place within the lattice, so they cannot conduct electricity.
When an ionic compound is melted or dissolved in water, the ions are free to move, allowing the compound to conduct electricity.

Writing Formulae for Simple Ionic Compounds

To write the formula of an ionic compound, the charges of the ions must balance so the overall compound is neutral.
Sodium chloride (NaCl) has a 1+ sodium ion and a 1− chloride ion, so one of each is needed to balance the charges.
Magnesium chloride has a 2+ magnesium ion and two 1− chloride ions, giving the formula MgCl₂.

5. Writing Formulae for Simple Ionic Compounds AQA

Notes

To write the formula of an ionic compound, the charges of the ions must balance so the overall compound is neutral.
Sodium chloride (NaCl) has a 1+ sodium ion and a 1− chloride ion, so one of each is needed to balance the charges.
Magnesium chloride has a 2+ magnesium ion and two 1− chloride ions, giving the formula MgCl₂.

Using Brackets in Ionic Formulae

Some ionic compounds contain polyatomic ions (ions made of more than one element), such as hydroxide (OH⁻) and sulphate (SO₄²⁻).
When more than one polyatomic ion is needed in a formula, brackets are placed around the ion with a subscript number outside, e.g. Ca(OH)₂.
The brackets ensure the subscript applies to the entire ion, not just the last element — for example, Ca(OH)₂ means two complete hydroxide ions.

6. Using Brackets in Ionic Formulae AQA

Notes

Some ionic compounds contain polyatomic ions (ions made of more than one element), such as hydroxide (OH⁻) and sulphate (SO₄²⁻).
When more than one polyatomic ion is needed in a formula, brackets are placed around the ion with a subscript number outside, e.g. Ca(OH)₂.
The brackets ensure the subscript applies to the entire ion, not just the last element — for example, Ca(OH)₂ means two complete hydroxide ions.

Balancing Charges for Complex Compounds

When the charges of two ions do not cancel simply, find the lowest common multiple (LCM) of the two charge values to balance them.
For aluminium sulphate, the aluminium ion is 3+ and sulphate is 2−; the LCM of 3 and 2 is 6, requiring two Al³⁺ ions and three SO₄²⁻ ions.
This gives the formula Al₂(SO₄)₃, where the overall charge is zero (2 × 3+ = 6+ and 3 × 2− = 6−).

7. Balancing Charges for Complex Compounds AQA

Notes

When the charges of two ions do not cancel simply, find the lowest common multiple (LCM) of the two charge values to balance them.
For aluminium sulphate, the aluminium ion is 3+ and sulphate is 2−; the LCM of 3 and 2 is 6, requiring two Al³⁺ ions and three SO₄²⁻ ions.
This gives the formula Al₂(SO₄)₃, where the overall charge is zero (2 × 3+ = 6+ and 3 × 2− = 6−).

Common Polyatomic Ions to Memorise

The hydroxide ion has the formula OH⁻ and a charge of 1−.
The sulphate ion has the formula SO₄²⁻ and a charge of 2−.
The nitrate ion is NO₃⁻, the carbonate ion is CO₃²⁻, and the ammonium ion is NH₄⁺ — these must all be memorised as they cannot be found on the periodic table.

8. Common Polyatomic Ions to Memorise AQA

Notes

The hydroxide ion has the formula OH⁻ and a charge of 1−.
The sulphate ion has the formula SO₄²⁻ and a charge of 2−.
The nitrate ion is NO₃⁻, the carbonate ion is CO₃²⁻, and the ammonium ion is NH₄⁺ — these must all be memorised as they cannot be found on the periodic table.

Summary / Key Terms

Ionic compounds have high melting and boiling points due to the large number of strong ionic bonds in the giant lattice structure.
Ionic compounds do not conduct electricity when solid, but do conduct when molten or dissolved in water because the ions become free to move.
The formula of an ionic compound is determined by balancing the charges of the positive and negative ions so the overall compound is electrically neutral.

9. Summary / Key Terms AQA

Notes

Ionic compounds have high melting and boiling points due to the large number of strong ionic bonds in the giant lattice structure.
Ionic compounds do not conduct electricity when solid, but do conduct when molten or dissolved in water because the ions become free to move.
The formula of an ionic compound is determined by balancing the charges of the positive and negative ions so the overall compound is electrically neutral.