Pure metals consist of a lattice of positive ions, arranged in regular pattern, surrounded by a sea of delocalised electrons. When a force is applied, the layers of positive ions slide over one another easily, without disrupting the metallic bonding. This makes the pure metal soft and malleable.

An alloy is a mixture of a pure metal with other elements. The atoms of the added element have a different size from those of the pure metal. This breaks up the regular arrangement of atoms in the pure metal. When a force is applied, the atoms of different sizes cannot slide over one another easily. This makes the alloy stronger and harder.