Metallic Bonding and Structures

From separate atoms to a metallic structure

A piece of metal is not one enormous atom, nor is it made from separate metal molecules. It contains an immense number of particles joined in a giant three-dimensional structure.

In the metallic-bonding model, the outer-shell electrons of the metal atoms become delocalised. This means that they are no longer associated with one particular atom or one bond: they can move throughout the whole structure. After losing these outer electrons, the remaining particles are positive metal ions. The nuclei have not changed; only the arrangement of the outer electrons has changed.

The positive ions are closely packed in a regular, repeating lattice. The delocalised electrons occupy the spaces throughout this lattice. Although the model contains charged particles, the metal as a whole is electrically neutral: the total positive charge of the ions is balanced by the total negative charge of the electrons.

What the metallic bond is

Opposite charges attract. A metallic bond is the strong electrostatic attraction between the positive metal ions and the delocalised electrons. The attraction acts throughout the giant structure, binding all its particles together.

A bond is therefore not a tiny rod or a separate piece of matter between particles. The lines or sticks sometimes used in models merely represent forces of attraction. Nor should a metallic bond be described as just the delocalised electrons: the bond is the attraction between those electrons and the positive ions.

Unlike a simple molecular substance, a metal does not consist of separate, fixed-size molecules. Its lattice can extend through an entire sample, so the chemical formula of a pure metal is simply its element symbol—for example, copper is written as $\mathrm{Cu}$ rather than as a molecular formula.

How metallic bonding differs from other bonding

The essential distinction is the arrangement of the electrons and charged particles:

• In a metal, positive metal ions form a giant lattice and are attracted to electrons delocalised across the whole structure.
• In an ionic compound, both positive and negative ions occupy a giant lattice; there is not a sea of delocalised electrons in the solid.
• In a covalent structure, atoms are joined by shared pairs of electrons. In metallic bonding, the outer electrons are delocalised throughout the metal rather than confined to particular pairs of atoms.

This is why it is misleading to draw a metal as a collection of independent pairs of atoms. Metallic bonding is a property of the extended structure.

Connecting structure to metallic properties

The model also helps explain familiar properties of metals. Delocalised electrons can move through the lattice and carry charge, so metals conduct electricity. Their movement also transfers thermal energy efficiently, making metals good thermal conductors.

Strong attraction between the ions and delocalised electrons requires substantial energy to overcome, so many metals have high melting and boiling points. This is a general pattern rather than a rule without exceptions: mercury, for example, is liquid at room temperature.

Pure metals are usually malleable and ductile. Layers of ions can slide past one another, but the ions remain attracted to the mobile electrons, so the structure can change shape without the bonding simply disappearing. This allows metals to be hammered into sheets or drawn into wires.