Ionic structure and conductivity
Explain bulk properties using the particles present and whether they can move.
What you will learn
- Describe a giant ionic lattice.
- Explain conduction in molten ionic compounds.
Strong attractions form a lattice
An ionic solid contains positive and negative ions in a repeating three-dimensional arrangement. Strong electrostatic attractions act throughout the lattice. A considerable energy input is needed to overcome enough of these attractions for melting, giving many ionic compounds high melting points.
Conductivity needs mobile charge
In the solid, ions are held in fixed positions and cannot carry charge through the material. When molten, ions can move and carry current. If the compound dissolves in water, the separated ions can also move through the solution.
Do not use free electrons to explain conduction in an ionic melt. Mobile electrons explain metallic conduction; mobile ions explain the ionic case.
Worked example
A substance has a high melting point and conducts when molten but not when solid. Explain the likely structure.
Show the worked solution
- A high melting point suggests strong attractions in an extended structure.
- Lack of solid conduction suggests the charged particles cannot move in the solid.
- Conduction after melting is consistent with mobile ions.
Answer A giant ionic structure is consistent with the evidence.
Common mistakes
- The ions already exist in the solid; melting makes them mobile.
- Not every ionic compound is soluble in water.
What can you explain now?
Why does solid sodium chloride not conduct electricity?
Compare with the explanation
Its ions cannot move through the lattice
There are charged particles, but they are held in fixed positions and cannot transport charge through the solid.
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