Learning focus
Develop precise chemical language, connect particle and bonding models to observations, interpret diagrams and data, and apply ideas to unfamiliar examples.
Giant lattice
An ionic compound forms a giant three-dimensional lattice of alternating positive and negative ions. Each ion is attracted to several oppositely charged neighbours. A solid such as sodium chloride does not consist of separate NaCl molecules.
High melting point
Strong electrostatic attractions act throughout the lattice. A large amount of energy is required to separate the ions, so ionic compounds usually have high melting and boiling points.
Solid conductivity
The ions in a solid are charged but fixed in position. They cannot carry charge through the material, so the solid does not conduct electricity.
Molten and aqueous conductivity
When molten, the lattice is broken and ions can move. When dissolved, water separates and hydrates the ions. Mobile ions carry current, so the melt and solution conduct.
Brittleness
If lattice layers shift, like charges may become adjacent. Strong repulsion then splits the crystal. The compound is strongly bonded yet brittle because of its ordered charge arrangement.
Practical or data skill
Compare a solid ionic compound, its aqueous solution and distilled water using a safe low-voltage conductivity circuit.
Examination tip
Current in an ionic melt or solution is carried by ions, not electrons.
Review questions and suggested answers
Question 1
Why does solid NaCl not conduct?
Suggested answer
Its ions are fixed.
Question 2
Why is melting point high?
Suggested answer
Strong attractions throughout the lattice require much energy to overcome.
Question 3
What carries charge in molten NaCl?
Suggested answer
Mobile Na+ and Cl- ions.
