The properties of a metal are changed by including other elements, such as carbon.
A mixture of two or more elements, where at least one element is a metal, is called an alloy. Alloys contain atoms of different sizes, which distort the regular arrangements of atoms. This makes it more difficult for the layers to slide over each other, so alloys are harder than the pure metal.
Copper, aluminium and gold are too soft for many uses. They are mixed with other metals to make them harder for everyday use. For example:
Smart alloys can return to their original shape after being bent. They are useful for spectacle frames and dental braces.
[ This page has been adapted from www.bbc.co.uk/schools/gcsebitesize/science