A substance conducts electricity only if it contains charged particles that are free to move. In metals and graphite those particles are delocalised electrons. In ionic compounds they are ions, and the ions can move only when the compound is molten or dissolved.
This lesson closes the main ideas in bonding and structure. It uses the lattice ideas from giant structures and the melting behaviour from molecular and ionic melting.
What must be present for electrical conduction?
Two conditions must be met: there must be charged particles, and they must be free to move. A substance that fails either condition is an insulator.
| Substance type | Solid | Molten or dissolved | Charge carriers |
|---|---|---|---|
| Ionic compound | does not conduct | conducts | ions |
| Simple molecular | does not conduct | does not conduct | none (neutral molecules) |
| Metal | conducts | conducts (molten) | delocalised electrons |
| Graphite | conducts | not applicable at school level | delocalised electrons |
| Diamond | does not conduct | not applicable at school level | none |
The “not applicable” entries are skipped because these carbon forms do not melt in any normal school context.
How to explain a conductivity result, step by step
- State the observation: conducts or does not conduct, in which state.
- Name the structure: ionic, simple molecular, metal, graphite or diamond.
- Name the particles present: ions, molecules or delocalised electrons.
- Say whether they are free to move in that state.
- Name what carries the charge: ions or electrons.
Worked example
Explain why an ionic compound such as sodium chloride does not conduct as a solid but conducts when molten.
Solid: sodium chloride is a giant ionic lattice of Na⁺ and Cl⁻ ions. The ions are charged but are held in fixed positions by strong electrostatic attractions, and there are no free electrons. No charged particles are free to move, so it does not conduct.
Molten: when the lattice is heated above the melting point, the ions are no longer held in fixed positions. They are free to move. When a voltage is applied, Na⁺ ions move towards the negative electrode and Cl⁻ ions move towards the positive electrode, so the moving ions carry the current.
The explanation names the same particles in both states, and what changes is whether they can move.
The mistake to watch for
A frequent error is to say that electrons carry the current in molten ionic compounds, or that solid ionic compounds have no ions.
Mistaken answer: “Solid sodium chloride does not conduct because it has no ions.”
The solid does contain ions. They are not free to move.
The correction is that solid sodium chloride has ions, but they cannot move. In the molten state, ions carry the charge. Electrons carry the charge only in metals and graphite, where they are delocalised.
Check yourself
Try these without looking back, then open each answer.
1. Copper conducts when solid. What carries the charge?
Show answer
Delocalised electrons that are free to move through the lattice of copper ions.
2. Explain why a simple molecular substance such as sugar does not conduct when molten. (Think about particles only.)
Show answer
Its particles are neutral molecules. There are no ions and no delocalised electrons, so there are no charged particles free to move, and no current can flow.
3. Why does graphite conduct, but diamond does not, although both are pure carbon?
Show answer
In graphite, each carbon atom uses 3 outer electrons in bonds, so the fourth is delocalised and free to move along the layers. In diamond, all 4 outer electrons are used in bonds, so no electrons are free to move.
Where this leads next
The idea of ions moving to electrodes continues in electrolysis reasoning. Test all five lessons together with the bonding and structure practice set. The mole and equation-ratio tutor becomes useful when you later connect ions to quantities.
If explanations keep sounding right but earn partial credit, our teachers can look at your actual sentences in online one-to-one Chemistry tuition.