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Put a copper wire, a crystal of salt, a glass of salty water, a glass of sugary water and a pencil lead into a simple circuit with a bulb, one at a time. The bulb lights for the copper, the salty water and the pencil lead. It stays dark for the dry salt crystal and the sugary water.
That result puzzles many students. Salt is in both the crystal and the salty water, so why does only one of them conduct? And why would a pencil conduct when it isn’t a metal? The answer to every one of these questions is the same short rule.
The one rule
A substance conducts electricity if it contains charged particles that are free to move.
Both parts matter:
- Charged. An electric current is a flow of charge. Neutral particles, such as whole molecules, carry no charge, so moving them carries no current.
- Free to move. Charged particles locked in fixed positions cannot flow, however many there are.
The charged particles can be electrons or ions. Which one depends on the type of bonding.
Here is an analogy. Think of a car park and a motorway. Both can be full of cars, but only on the motorway are the cars actually going somewhere. A current is like traffic flow. You need vehicles (charged particles) and you need them to be able to travel (freedom to move). A car park full of parked cars gives no traffic at all.
Metals: a sea of moving electrons
In a metal, each atom gives up one or more of its outer electrons to a shared pool. These delocalised electrons do not belong to any single atom. They drift through the whole structure, between a regular arrangement of positive metal ions. You can read more in metallic bonding.
When you connect a battery, these electrons drift towards the positive terminal, and new electrons enter from the negative terminal. That is the current. Because the electrons are already free, metals conduct as solids and as liquids. Molten iron conducts, and so does mercury, a liquid metal at room temperature.
Some metals conduct better than others. Silver is the best, with copper close behind, which is why copper is the everyday choice for wiring. See the best conductor of electricity for a ranking and the reasons behind it.
Ionic compounds: it depends on the state
Sodium chloride is made of Na⁺ and Cl⁻ ions. So it has charged particles. But in a solid crystal each ion is held tightly in place by its neighbours. The ions can vibrate but cannot travel. That is the car park.
Now melt it, or dissolve it in water. The ions break free of the lattice and can wander. Connect a battery and:
- positive ions (Na⁺) drift towards the negative electrode;
- negative ions (Cl⁻) drift towards the positive electrode.
That movement of ions is the current. So:
| State of an ionic compound | Conducts? | Why |
|---|---|---|
| Solid | No | Ions fixed in the lattice |
| Molten (liquid) | Yes | Ions free to move |
| Dissolved in water | Yes | Ions free to move |
A liquid or solution that conducts because it contains moving ions is called an electrolyte. When current passes through it, chemical changes happen at the electrodes. That process is electrolysis, and it is how aluminium and chlorine are made industrially. The electrolysis calculator shows how much product a given current can make.
Simple molecular substances: no charged particles
Sugar, wax, ethanol, sulfur and oxygen are made of neutral molecules. The electrons are all tied up in covalent bonds inside each molecule or in lone pairs, and the molecules have no overall charge. Solid, liquid or dissolved, nothing charged is moving, so they do not conduct.
That explains the sugary water. Sugar dissolves by splitting into separate sugar molecules, not ions. The bulb stays dark.
There is one important twist. A few molecular substances react with water to make ions. Hydrogen chloride is a gas made of HCl molecules and does not conduct as a pure liquid. In water, though, it forms H⁺ (as H₃O⁺) and Cl⁻ ions, and hydrochloric acid conducts well. The conductivity comes from the ions made by the reaction, not from the molecules.
What about pure water? Water contains a tiny number of ions formed when water molecules split, so it conducts very, very slightly. Tap water conducts better because of the dissolved salts in it, which is part of why electricity and bathrooms are a dangerous mix.
Giant covalent structures: mostly no, with one star exception
In diamond, every carbon atom uses all four of its outer electrons in strong covalent bonds to four neighbours. No electrons are left over to move, and there are no ions. Diamond does not conduct. Neither does silicon dioxide (sand, quartz).
Graphite is different. Each carbon atom bonds to only three neighbours within flat layers. The fourth outer electron from each atom becomes delocalised along the layer, much as in a metal. These electrons can move, so graphite conducts, especially along the layers. That is why the pencil lead lit the bulb, and why graphite is used for electrodes. Read more in giant covalent structures.
Silicon sits in between. It has the same network as diamond but its bonds are weaker, and a few electrons can escape at room temperature. It conducts a little, and more as it gets warmer, which makes it a semiconductor. See semiconductors explained.
The whole picture
| Substance type | Example | Solid | Liquid | In water | Moving charge carriers |
|---|---|---|---|---|---|
| Metal | Copper | Yes | Yes | Does not dissolve | Electrons |
| Ionic | Sodium chloride | No | Yes | Yes | Ions |
| Simple molecular | Sugar | No | No | No | None |
| Molecular that makes ions in water | Hydrogen chloride | No | No | Yes | Ions (after reaction) |
| Giant covalent | Diamond | No | Does not melt normally | Insoluble | None |
| Giant covalent, layered | Graphite | Yes | Does not melt normally | Insoluble | Electrons |
Common mistakes
“Electrons flow through salt solution.” They do not. In a solution or molten salt, the current is carried by ions moving. Electrons flow in the wires, and swap over to ions at the electrodes.
“Solid ionic compounds conduct because they contain ions.” Having ions is not enough. They must be free to move. Solid salt does not conduct.
“All covalent substances are insulators.” Graphite is covalent and conducts. Silicon is covalent and is a semiconductor.
“Sugar solution conducts because it’s a solution.” Only solutions containing ions conduct. Sugar dissolves as molecules.
“Pure water is a good conductor.” Pure water is a very poor conductor. It is the dissolved salts in ordinary water that make it conduct well enough to be dangerous.
Saying “free electrons” for ionic compounds. In ionic compounds, say “ions free to move”. Save “delocalised electrons” for metals and graphite.
Key takeaways
- To conduct, a substance needs charged particles that are free to move.
- Metals conduct as solids and liquids because of delocalised electrons.
- Ionic compounds conduct only when molten or dissolved, because only then can their ions move.
- Simple molecular substances do not conduct in any state, unless they react with water to form ions.
- Giant covalent substances do not conduct, except graphite (delocalised electrons in its layers); silicon is a semiconductor.
- Current is carried by electrons in metals and graphite, and by ions in electrolytes. For related structure patterns, see how bonding type predicts melting point.
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