Does It Conduct Electricity?
Now that we have met every type of structure — ionic, simple molecular, giant covalent and metallic — we can pull the ideas together and answer one of the most common questions in this topic: will a given substance conduct electricity? You do not need to memorise a list. You can reason it out from first principles every time.
An electric current is a flow of charge. For a substance to conduct, it must contain charged particles that are free to move to carry that charge from one place to another.
There are only two kinds of mobile charge carrier you need to know:
- Delocalised electrons — negatively charged electrons that are not held in fixed positions and are free to move.
- Free-moving ions — charged ions that are able to move, rather than locked in place in a lattice.
So the question always reduces to one thing: does this substance contain delocalised electrons, or free-moving ions? If it has neither, it cannot conduct.
Substances That Conduct
| Substance | Charge carrier | Why it conducts |
|---|---|---|
| Metals (solid or molten) | Delocalised electrons | Metallic bonding leaves a ‘sea’ of delocalised electrons free to move throughout the lattice. |
| Graphite, graphene & carbon nanotubes | Delocalised electrons | Each carbon atom uses only 3 of its 4 outer electrons for bonds; the fourth is delocalised and free to move. |
| Ionic compounds — molten (liquid) | Free-moving ions | Melting breaks up the lattice, so the ions are free to move and carry charge. |
| Ionic compounds — dissolved in water | Free-moving ions | Dissolving breaks up the lattice, freeing the ions to move — this is why salt solution, and even tap water, conducts. |
Substances That Do Not Conduct
Everything else fails the test — it has no delocalised electrons and no free-moving ions, so there is nothing to carry the charge:
| Substance | Why it does not conduct |
|---|---|
| Ionic compounds — solid | The ions are present, but locked in fixed positions in the lattice — they cannot move. |
| Simple molecular substances (including pure water) | Molecules have no overall charge, no delocalised electrons and no free ions. |
| Giant covalent — diamond & silicon dioxide | All outer electrons are held in covalent bonds; none are delocalised. (Graphite is the exception.) |
| Polymers | Large covalent molecules — no delocalised electrons and no free ions. |
The classic trap: a solid ionic compound does not conduct, but the same compound conducts as soon as it is molten or dissolved. The bonding has not changed — what changes is whether the ions are free to move. Always check the state before you answer.
For each substance, state whether it conducts electricity and name the charge carrier responsible:
(a) copper wire (b) solid potassium chloride (c) potassium chloride dissolved in water (d) diamond
Show answer
- (a) Conducts — copper is a metal, so it has delocalised electrons that are free to move.
- (b) Does not conduct — the ions are locked in fixed positions in the solid lattice and cannot move.
- (c) Conducts — dissolving frees the ions, so the free-moving ions carry the charge.
- (d) Does not conduct — all of carbon’s outer electrons are held in covalent bonds; there are no delocalised electrons and no free ions.
🧪 Exam-style questions
Which substance conducts electricity when molten but not when solid? Tick (✓) one box.
Which substance does not conduct electricity in any state? Tick (✓) one box.
Both graphite and diamond are made only of carbon, yet graphite conducts electricity and diamond does not. Explain why.
Show answer
- In graphite each carbon forms only 3 bonds, leaving one delocalised electron per atom that is free to move and carry charge 1 mark
- In diamond each carbon forms 4 bonds, so there are no delocalised electrons free to carry charge 1 mark