Metallic bonding and alloys

Metals are giant structures of atoms arranged in a regular pattern (layers). The outer electrons leave their atoms and become delocalised — free to move through the whole structure, like a "sea" of electrons. This leaves positive metal ions.

The metallic bond is the strong electrostatic attraction between the positive ions and the delocalised electrons. It acts throughout the structure.

• High melting and boiling points. Metallic bonds are strong, so a lot of energy is needed to break them.
• Good conductors of electricity. The delocalised electrons move through the metal, carrying charge.
• Good conductors of heat. The delocalised electrons transfer energy quickly.
• Malleable (can be bent and shaped). In a pure metal, the atoms are all the same size and arranged in layers, so the layers can slide over each other.

Most pure metals are too soft for everyday use, so they are mixed with other elements to make alloys. An alloy is a mixture of a metal with at least one other element, usually another metal.

The atoms of the added element are a different size. They distort the layers, so the layers can't slide over each other as easily. That makes alloys harder and stronger than pure metals.

Examples: steel (iron with carbon), brass (copper and zinc) and bronze (copper and tin). Pure gold is very soft, so jewellery is made from gold alloys.