Why Metals Conduct and Plastics Do Not

Copper carries electricity so readily that we wrap it around the planet. The plastic sheath on that same cable carries essentially none. Both are made of atoms, both have electrons, and yet their behaviour differs by a factor of about ten thousand million million million. The reason is not how many electrons each material has, but how free those electrons are to move.

Electrons that belong to everyone

In a metal, atoms give up their outermost electrons to a shared pool. Picture a lattice of positively charged atomic cores sitting in a sea of electrons that belong to no particular atom — sometimes described as a sea of delocalised electrons. Apply a voltage, and that sea drifts. Because the electrons were never tied to individual atoms, almost nothing has to happen for current to flow.

This single arrangement explains most of what we associate with metals. They conduct electricity, because the electron sea moves. They conduct heat, because those same mobile electrons carry thermal energy quickly through the material. They are shiny, because a free electron sea reflects light efficiently. And they bend rather than shatter, because the atomic cores can slide past one another while the shared electrons hold everything together.

Electrons with nowhere to go

Plastics are built on covalent bonds, where atoms share electrons in pairs, each pair firmly localised between two specific atoms. There is no shared pool. Applying a modest voltage does not move these electrons anywhere, because every one of them is committed to holding a particular bond together.

Prising an electron loose in an insulator requires a large amount of energy — in the language of physics, a wide band gap between the states electrons occupy and the states in which they could travel. At everyday voltages, nothing crosses that gap, so no current flows. Push hard enough, though, and the gap can be overcome violently: that is dielectric breakdown, the arc that jumps through insulation when voltage exceeds what the material can withstand.

The interesting middle

Between conductors and insulators sit semiconductors like silicon, whose band gap is narrow enough that a modest nudge — heat, light, or a deliberately added impurity — can push electrons into the travelling states. This controllability is the entire foundation of modern electronics. A conductor always conducts and an insulator never does, but a semiconductor can be switched, and a switch is what a transistor is.

Where conduction happens without electrons

Not all conduction is electronic. In saltwater, molten salts and batteries, charge is carried by ions — whole charged atoms physically moving through a liquid. It is slower and messier than electrons drifting through copper, but it is how nerve signals travel in your body and how every battery in your house delivers power.

One idea, many properties

The neat thing about this explanation is its economy. Ask why copper conducts, why it is shiny, why it can be drawn into wire, and why it feels cold to touch, and the answer to all four is the same: its electrons are shared and free to move. Ask the same questions of polyethylene and the answer is again the same, in reverse. Bonding determines almost everything about how a material behaves — which is why materials engineers spend their careers thinking about it, and why choosing between two superficially similar plastics can decide whether an engineered part survives. Our sponsor Kinsoft has a practical illustration of that trade-off in its guide to selecting materials for functional 3D-printed parts.