Why are metals good conductors of electricity?

Metallic bonding: where the free electrons come from

Metal atoms have only a few electrons in their outer shell, and they hold them loosely. In a solid metal, the atoms pack into a regular lattice and their outer electrons leave them, becoming shared by the whole structure. What remains of each atom is a positive metal ion, surrounded by a "sea" of delocalised electrons.

The attraction between the positive ions and the negative electron sea holds the metal together. The AQA GCSE specification puts it this way: the outer electrons of metal atoms are delocalised, so they can travel anywhere in the structure, and the sharing of delocalised electrons gives rise to strong metallic bonds.

How do metals conduct electricity?

An electric current is a flow of charge. To conduct, a material needs charged particles that can move. In a metal, the delocalised electrons are exactly that. Without a voltage, they move about randomly in every direction, so there is no overall flow. Connect a battery, and they are pushed towards the positive terminal, so there is a steady drift of charge through the metal: a current.

AQA's wording to learn: metals are good conductors of electricity because the delocalised electrons in the metal carry electrical charge through the metal.

Why do metals conduct heat?

AQA states that metals are good conductors of thermal energy because energy is transferred by the delocalised electrons. When one end of a metal is heated, the electrons there gain kinetic energy, move quickly through the lattice and pass energy on to ions and electrons further along. Vibrating ions pass energy on too, but the free electrons make metals far better heat conductors than non-metals. That is why metal pan bases heat up quickly and metal feels cold to the touch: it carries heat away from your hand fast.

Why do some materials not conduct?

Our guides to ionic bonding and covalent bonding explain those structures in more detail.

Resistance: why metal wires still get warm

Metals conduct well, but not perfectly. As the electrons drift, they collide with the vibrating ions, transferring energy and heating the wire. This is resistance. Heating a metal makes its ions vibrate more, so collisions happen more often and the resistance goes up; OpenStax University Physics notes that copper's resistivity rises with temperature. Copper is used for most wiring because it conducts very well, with a resistivity of 1.68 × 10⁻⁸ Ω m at 20 °C, and is easy to draw into wires. Our guide to voltage drop shows the effect in a cable.

Try it: this question asks you to explain a property of metals from their structure.

Common mistakes

• Saying ions carry the current in a metal. It is the delocalised electrons; the ions stay fixed.
• Saying electrons are created by the battery. The electrons are already in the metal; the battery makes them drift.
• Leaving out "delocalised". Exams expect the term.
• Saying metals have no resistance. They have a small resistance, which rises as they get hotter.
• Saying ionic solids conduct. They only conduct when molten or dissolved.

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