GCSE Chemistry revision

Properties of ionic compounds

How bonding and structure are related to the properties of substances

AQA 4.2.2.3 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, not just in pairs.
Each ion is attracted to all the surrounding oppositely charged ions, creating a three dimensional arrangement called a regular lattice structure.
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, not just in pairs.
Each ion is attracted to all the surrounding oppositely charged ions, creating a three dimensional arrangement called a regular lattice structure.
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 great deal of energy to break.
Because so much energy is needed to overcome all these bonds, ionic compounds have very high melting and boiling points.

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 great deal of energy to break.
Because so much energy is needed to overcome all these bonds, ionic compounds have very high melting and boiling points.

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 1− chloride ions, so two chloride ions are needed, 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 1− chloride ions, so two chloride ions are needed, giving the formula MgCl₂.

Compound Ions You Must Memorise

Some ions are made of more than one element and cannot be found directly on the periodic table, so their formulae and charges must be memorised.
The hydroxide ion is OH⁻, the sulphate ion is SO₄²⁻, and the carbonate ion is CO₃²⁻.
The nitrate ion is NO₃⁻ and the ammonium ion is NH₄⁺.

6. Compound Ions You Must Memorise AQA

Notes

Some ions are made of more than one element and cannot be found directly on the periodic table, so their formulae and charges must be memorised.
The hydroxide ion is OH⁻, the sulphate ion is SO₄²⁻, and the carbonate ion is CO₃²⁻.
The nitrate ion is NO₃⁻ and the ammonium ion is NH₄⁺.

Formulae Involving Compound Ions

Calcium is in Group 2, giving a 2+ ion, and hydroxide is 1−, so two hydroxide ions are needed per calcium atom.
Brackets are placed around a compound ion when a subscript is needed, e.g. Ca(OH)₂, to show that the entire ion is repeated, not just one element within it.
For aluminium sulphate, the 3+ aluminium ion and 2− sulphate ion require finding the lowest common multiple of 3 and 2, which is 6.
Two aluminium ions (2 × 3+ = 6+) and three sulphate ions (3 × 2− = 6−) balance the charges, giving the formula Al₂(SO₄)₃.

7. Formulae Involving Compound Ions AQA

Notes

Calcium is in Group 2, giving a 2+ ion, and hydroxide is 1−, so two hydroxide ions are needed per calcium atom.
Brackets are placed around a compound ion when a subscript is needed, e.g. Ca(OH)₂, to show that the entire ion is repeated, not just one element within it.
For aluminium sulphate, the 3+ aluminium ion and 2− sulphate ion require finding the lowest common multiple of 3 and 2, which is 6.
Two aluminium ions (2 × 3+ = 6+) and three sulphate ions (3 × 2− = 6−) balance the charges, giving the formula Al₂(SO₄)₃.

Using Ion Charges from the Periodic Table

The charge of a metal ion can often be determined from its group number; Group 1 metals form 1+ ions, Group 2 metals form 2+ ions, and Group 3 metals form 3+ ions.
Non-metal ions typically have negative charges; for example, Group 7 elements form 1− ions.
When the positive and negative charges do not cancel with a simple 1:1 ratio, find the lowest common multiple of the two charges to determine the correct number of each ion.

8. Using Ion Charges from the Periodic Table AQA

Notes

The charge of a metal ion can often be determined from its group number; Group 1 metals form 1+ ions, Group 2 metals form 2+ ions, and Group 3 metals form 3+ ions.
Non-metal ions typically have negative charges; for example, Group 7 elements form 1− ions.
When the positive and negative charges do not cancel with a simple 1:1 ratio, find the lowest common multiple of the two charges to determine the correct number of each ion.

Summary: Key Rules for Ionic Formulae

The overall charge of any ionic compound must equal zero — positive and negative charges must always balance.
Use brackets around compound ions (e.g. hydroxide, sulphate) whenever a subscript greater than 1 is needed.
Memorising the formulae and charges of common compound ions such as OH⁻, SO₄²⁻, NO₃⁻, CO₃²⁻, and NH₄⁺ is essential for GCSE Chemistry.

9. Summary: Key Rules for Ionic Formulae AQA

Notes

The overall charge of any ionic compound must equal zero — positive and negative charges must always balance.
Use brackets around compound ions (e.g. hydroxide, sulphate) whenever a subscript greater than 1 is needed.
Memorising the formulae and charges of common compound ions such as OH⁻, SO₄²⁻, NO₃⁻, CO₃²⁻, and NH₄⁺ is essential for GCSE Chemistry.