Explain ionic bonding and ionic lattices
Describe ionic bonding as electrostatic attraction between opposite ions and relate formula ratios to three-dimensional lattices.
AS Level; Cambridge International Chemistry 9701, 2025–2027 syllabus. Use the supplied Pauling values when given, show particle and bond reasoning, and distinguish intramolecular bonds from intermolecular forces. Written practice is self-checked
Before you begin
- Read element symbols and simple molecular formulae; use the Periodic Table.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
What particles attract in ionic bonding?
What you will learn
- Define ionic bonding
- Describe ionic bonding in sodium chloride, magnesium oxide and calcium fluoride
- Distinguish an ionic formula unit from an isolated molecule
Opposite charges attract throughout an ionic lattice
Ionic bonding is the electrostatic attraction between oppositely charged ions: positive cations and negative anions. The attraction acts in all directions through a giant ionic lattice, rather than only between one isolated pair of ions.
In sodium chloride, Na⁺ and Cl⁻ ions repeat in a 1:1 ratio. In magnesium oxide, Mg²⁺ and O²⁻ repeat in a 1:1 ratio. In calcium fluoride, each Ca²⁺ is balanced by two F⁻ ions, so the simplest formula unit is CaF₂.
The formula records the simplest ratio
An ionic formula gives the simplest whole-number ratio of ions needed for overall electrical neutrality. It does not show a separate molecule. The lattice contains very many ions in a repeating arrangement.
The strength of ionic attraction depends on charge and separation; use the named ions and lattice context without treating a formula as a picture of one molecule.
Pause and explain
Why is the simplest formula for calcium fluoride CaF₂?
Worked example
Explain why magnesium oxide has formula MgO and describe the bonding.
Show the worked solution
- Magnesium forms Mg²⁺ and oxygen forms O²⁻.
- One ion of each gives total charge zero, so the ratio is 1:1.
- Opposite ions attract in all directions in a giant lattice.
Answer MgO is the simplest neutral ion ratio. Ionic bonding is electrostatic attraction between Mg²⁺ and O²⁻ ions throughout the lattice.
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Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
Describe the bonding and simplest ion ratio in sodium chloride.
Give me a hint
Name the ions, then explain how the attraction extends beyond one pair.
Compare my reasoning
- Sodium forms Na⁺ and chlorine forms Cl⁻.
- Equal numbers give a neutral 1:1 ratio.
- Opposite ions attract in all directions through a giant lattice.
NaCl contains Na⁺ and Cl⁻ in a 1:1 giant lattice; ionic bonding is electrostatic attraction between the oppositely charged ions.
Look for these in your work
- Ions identified
- Ratio stated
- Lattice-wide attraction described
For calcium fluoride, state the ions, simplest formula ratio and why the formula does not represent a molecule.
Give me a hint
Balance total positive and negative charge.
Compare my reasoning
- Calcium is Ca²⁺ and fluoride is F⁻.
- Two fluoride ions balance one calcium ion.
- An ionic solid is a lattice of repeating ions, not separate CaF₂ molecules.
Ca²⁺:F⁻ = 1:2, giving CaF₂. The formula is a formula-unit ratio in a giant ionic lattice, not an isolated molecule.
Look for these in your work
- Charges identified
- Neutral ratio derived
- Lattice and molecule distinguished
Compare the charge pattern in MgO with NaCl and explain why both formulae are 1:1.
Give me a hint
Check each cation and anion charge pair.
Compare my reasoning
- Mg²⁺ and O²⁻ balance one-to-one.
- Na⁺ and Cl⁻ also balance one-to-one.
- The charge magnitudes differ, but each pair sums to zero.
Both have a 1:1 formula-unit ratio because Mg²⁺/O²⁻ and Na⁺/Cl⁻ each give zero net charge for one cation plus one anion.
Look for these in your work
- Both ion pairs identified
- Ratios derived from charge
- Different charge magnitudes acknowledged
Common mistakes
- Calling NaCl a molecule
- Describing ionic bonding as electron sharing
- Forgetting that attraction extends through the lattice
- Writing CaF instead of balancing Ca²⁺ with two F⁻ ions
What can you explain now?
What particles attract in ionic bonding?
Compare with the explanation
Oppositely charged ions
Cations and anions attract electrostatically.
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Make it stick
How does an ionic formula follow from charge balance, and why is it not a molecular formula?
If you needed the solution, close it and solve the problem again from a blank page. A correct answer is stronger when you can explain the reason for each step.