Exam Frequency Analysis
Past paper frequency (2018 to 2024)
This topic accounts for approximately 9% of your exam marks.
stable
Medium
Stable9%
Ion formation, lattice structures and properties of ionic compounds tested consistently.
The giant ionic lattice
- In the solid state, the cations and anions arrange themselves into a regular, repeating three-dimensional pattern with positive and negative alternating in every direction
- This is called a
- Each ion is surrounded by ions of the opposite charge as its nearest neighbours
- The whole solid is one continuous structure containing many millions of alternating ions
Ionic bonding as electrostatic attraction
- An ionic bond is the electrostatic attraction between oppositely charged ions
- The forces act in all directions and extend throughout the entire lattice
- These forces are very strong
High melting and boiling points
- Ionic compounds have high melting and boiling points
- Reason: melting or boiling the solid requires overcoming the many strong electrostatic forces between every pair of opposite ions in the lattice. That takes a lot of thermal energy
- The greater the charges on the ions, the stronger the attraction and the higher the melting point
- Magnesium oxide (Mg²⁺ and O²⁻, both 2-charge) melts at about 2852 °C
- Sodium chloride (Na⁺ and Cl⁻, both 1-charge) melts at about 801 °C
Exam tip
Explaining the high melting point of an ionic compound
What comes up: "Explain why [ionic compound] has a high melting point." (3 marks, linked explanation required)
Write (three marks, must link all three): (1) There are strong electrostatic forces of attraction (2) between oppositely charged ions in the giant lattice, (3) which require a large amount of thermal energy to overcome.
Watch out: The mark scheme awards zero if you mention covalent bonds, intermolecular forces, or molecules anywhere in your answer. Do not write "break bonds" loosely — the correct phrasing is that the forces between ions are overcome, not that bonds are broken. The word "strong" is needed for M1; "electrostatic" is not required (ALLOW "strong ionic bonds") but is best practice.
Electrical conductivity
- An electric current is a flow of charged particles
- In an ionic compound the ions are the charge carriers (not free electrons)
- Solid ionic compound: poor conductor. The ions are locked in fixed positions in the lattice and cannot move
- Molten or dissolved in water (aqueous): good conductor. The ions are free to move and carry charge
Exam tip
Explaining electrical conductivity of ionic compounds
What comes up: "Explain why solid [ionic compound] does not conduct electricity." or "Explain why [ionic compound] conducts when molten but not when solid." (2 marks)
Write (two marks): (1) In the solid, ions are held in fixed positions in the lattice and cannot move. (2) When molten or dissolved, the ions are free to move and carry charge.
Watch out: The mark scheme awards no marks if you refer to electrons moving — ions are the charge carriers in ionic compounds, not free electrons. For the solid-only question, state that ions cannot move (not that there are "no free electrons", which is the explanation for ionic not metallic conduction).
Solubility in water
- Many ionic compounds are soluble in water
- Water's polar molecules pull the ions out of the lattice and surround them in solution
- e.g. NaCl, KNO₃ and Na₂SO₄ all dissolve readily in water
Brittleness
- Ionic crystals are
- A sharp blow can shift one layer of ions sideways across the next, bringing ions of the same charge next to one another. They repel, and the crystal cracks along that plane