Metals have a giant structure of positive metal ions in a regular arrangement. Outer electrons become delocalised: free to move throughout the structure. Metallic bonding is the strong electrostatic attraction between positive ions and delocalised electrons.
Delocalised electrons move and carry charge, so metals conduct electricity. They also transfer energy through the metal, helping it conduct heat. Most metals have high melting and boiling points because strong metallic attractions need a lot of energy to overcome. There are exceptions, such as mercury being liquid at room temperature.
In a pure metal, equal-sized atoms form regular layers. The layers can slide over one another while metallic bonding holds the structure together. This makes metals malleable (can be shaped or hammered into sheets).
An alloy is a mixture of a metal with other elements. Different-sized atoms distort the regular layers, making them harder to slide. Alloys are usually harder than the pure metal. Steel is an alloy based on iron; brass contains copper and zinc. The drawing highlights sizes; electrons are omitted.
Metallic bonds attract positive ions to delocalised electrons. Alloys have distorted layers that slide less easily.
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