Metals have giant structures of atoms held together by strong metallic bonding. A lot of energy is needed to overcome these bonds, so most metals have high melting points and high boiling points.
In a pure metal, the atoms are arranged in regular layers. The layers can slide over each other when a force is applied, so a pure metal can be bent and shaped. This also means pure metals are often too soft for many uses.
To make a metal harder, it is mixed with other elements, usually other metals, to make an alloy. The atoms of the added element are a different size from the atoms of the pure metal. They distort the regular layers, so the layers cannot slide over each other easily. This makes an alloy harder than the pure metal.
