Alloy

An alloy is a mixture of two or more elements, of which the main one is a metal. It is harder and stronger than the pure metal from which it is made. Malay: aloi · Chinese: 合金.

A pure metal is soft because it is built of identical atoms arranged in neat layers that can slide over one another when a force is applied. In an alloy, atoms of a different element and a different size are mixed among them. These foreign atoms sit between the layers and disrupt their orderly arrangement, so the layers can no longer slide easily. The result is a material that is harder, stronger and often more resistant to corrosion, and it can also be given a more attractive appearance. This is why most metals are used in everyday life as alloys rather than in their pure form.

Example. Steel is an alloy of iron with a small amount of carbon; bronze is an alloy of copper and tin; brass is an alloy of copper and zinc; and duralumin is an alloy of aluminium with copper and magnesium, light but strong enough for aircraft bodies.

Confusion to avoid. Do not confuse an alloy with a compound. An alloy is a mixture, so its components are not chemically bonded and its proportions can vary; a compound has its elements chemically combined in a fixed ratio. Also keep alloy separate from pure metal.

A helpful habit is to describe an alloy at the level of atoms. Say that the foreign atoms of a different size sit among the layers of the main metal and prevent those layers from sliding, and then name the alloy and one of its uses. That structure turns a vague answer into a full-mark one, and it works for every alloy in the chapter.

Explaining hardness in terms of atoms disrupting the sliding layers is the exact wording SPM Chemistry looks for when it asks why an alloy is harder than a pure metal.