A pure metal contains atoms that are all the same size, arranged in regular layers. In a model made from identical balls, the balls sit in neat rows. When a force is applied the layers can slide over each other easily, so pure metals such as gold, copper and iron are soft and can be bent or hammered into shape.
An alloy is a mixture of a metal with at least one other element, often another metal. The atoms of the added element are a different size. In the ball model, a few larger or smaller balls are mixed into the rows. These distort the regular layers, so it is harder for the layers to slide over each other. The alloy is therefore harder and stronger than the pure metal.
Brass is an alloy of copper and zinc and is harder than copper. Steel is an alloy of iron and carbon and is harder than iron. The bonding is still metallic, but atoms of different sizes stop the layers sliding.
