Ionic Bonding and Ionic CompoundsAQA GCSE Chemistry: Revision notes

Section 1

What is ionic bonding?

Ionic bonding is the electrostatic attraction between oppositely charged ions. It forms when a metal transfers electrons to a non-metal.

The metal loses electrons → becomes a cation (positive ion)
The non-metal gains those electrons → becomes an anion (negative ion)
Both ions end up with a full outer shell (stable noble gas configuration)
The opposite charges attract each other — this attraction is the ionic bond
There are no individual molecules — just a giant lattice of ions

Key termsioncationanionelectrostatic attraction

Think of it like this

The metal is like someone donating something they do not need. The non-metal desperately needs it to fill its outer shell. After the transfer, both are stable.

Section 2

Working out ion charges

The charge on an ion tells you how many electrons were transferred:

Group	Action	Charge	Examples
Group 1 metals	Lose 1 electron	+1	Na⁺, K⁺, Li⁺
Group 2 metals	Lose 2 electrons	+2	Mg²⁺, Ca²⁺
Group 3 metals	Lose 3 electrons	+3	Al³⁺
Group 6 non-metals	Gain 2 electrons	-2	O²⁻, S²⁻
Group 7 non-metals	Gain 1 electron	-1	Cl⁻, Br⁻, F⁻

Exam tip

The charge number matches the group number — Group 2 metals always form 2+ ions. For non-metals, count back from Group 8: Group 7 is 1 away → -1, Group 6 is 2 away → -2.

Section 3

Writing ionic formulas

To write the correct formula for any ionic compound:

Write the symbol and charge of each ion
Balance the charges so the total = zero
The ratio needed gives you the formula — do not include charges in the final answer

Example

Magnesium Chloride: Step 1: Mg²⁺ and Cl⁻ Step 2: Need 2 × Cl⁻ to cancel 1 × Mg²⁺ (2+ and 2− = zero) Result: MgCl₂

Aluminium Oxide: Step 1: Al³⁺ and O²⁻ Step 2: Need 2 × Al³⁺ (total 6+) and 3 × O²⁻ (total 6−) = zero Result: Al₂O₃

Common mistake

Writing MgCl instead of MgCl₂. Never assume one of each ion — always balance the charges to zero first.

Section 4

Structure — the giant ionic lattice

All ionic compounds form a giant ionic lattice — a regular, repeating 3D arrangement of alternating positive and negative ions.

Millions of ions held together by strong electrostatic forces in all directions
No individual molecules — the lattice extends throughout the entire solid
In NaCl: each Na⁺ is surrounded by 6 Cl⁻ ions, and each Cl⁻ by 6 Na⁺ ions
The pattern repeats identically in three dimensions

Key termsgiant ionic lattice

Think of it like this

Imagine a 3D chessboard extending in every direction, black and white squares alternating perfectly. That is the lattice — except the forces between the pieces hold the whole structure rigid.

Section 5

Properties of ionic compounds

Property	Behaviour	Reason
Melting point	Very high	Strong electrostatic forces across the whole lattice need huge energy to break
Conductivity (solid)	Does not conduct	Ions are fixed in the lattice — cannot move
Conductivity (molten/dissolved)	Conducts electricity	Ions are free to move and carry charge
Solubility	Many dissolve in water	Water molecules pull individual ions away from the lattice
Brittleness	Shatters when struck	Layers shift, like charges align and repel, lattice splits apart

Exam tip

"Why does it conduct electricity?" is one of the most common exam questions. You must write that ions are free to move — saying "it has ions" or "it has charges" alone scores zero marks.

Common mistake

Saying ionic compounds conduct because electrons can flow. Ionic compounds conduct via ion movement, not electrons. Free electrons are a feature of metals, not ionic compounds.

Exam tip

For melting point questions, never just say "strong bonds." Write: strong electrostatic forces between oppositely charged ions throughout the giant ionic lattice — that is the mark scheme language.

Must Know

Ionic bonding = metal + non-metal, electron transfer, electrostatic attraction
Metal loses electrons → cation (positive) · Non-metal gains electrons → anion (negative)
Both ions end up with a full outer shell
Ionic compounds form a giant ionic lattice — no individual molecules
High melting points — strong electrostatic forces throughout the lattice
Conducts only when molten or dissolved — ions must be free to move
Conducts via ion movement, not electron movement
Higher ion charges = stronger forces = higher melting point
Balance charges to zero when writing formulas

That's the notes covered.

Carry on to the next subtopic.