Metallic bondingEdexcel International A Level Chemistry: Revision notes

Metals exist as giant metallic lattices: a regular arrangement of positive metal ions surrounded by a sea of delocalised electrons. The outer electrons of each atom are released from the atom and are free to move throughout the whole structure; they are not attached to any one ion.

For example:

• sodium: Na⁺ ions and one delocalised electron per atom
• magnesium: Mg²⁺ ions and two delocalised electrons per atom
• aluminium: Al³⁺ ions and three delocalised electrons per atom

The lattice as a whole is electrically neutral, because the charge of the ions is balanced by the charge of the delocalised electrons.

Metallic bonding is the strong electrostatic attraction between the positive metal ions and the delocalised electrons. It acts in all directions, so it is non-directional.

The strength of metallic bonding increases when:

• the charge on the ion is larger (more delocalised electrons per atom)
• the ionic radius is smaller, so the charge density of the ion is greater
• the sea of delocalised electrons is denser

This is why magnesium (Mg²⁺) is bonded much more strongly than sodium (Na⁺).

Metals conduct electricity in both the solid and liquid states, because the delocalised electrons are free to move through the lattice. When a potential difference is applied, the electrons drift towards the positive terminal, carrying charge as a current. The ions stay in their positions in the solid lattice.

Compare this with a covalent substance such as sulfur, which has no delocalised electrons and no ions, so it has no mobile charge carriers and does not conduct.

More delocalised electrons per atom generally means a better conductor, so magnesium conducts better than sodium.

Most metals have high melting temperatures because a large amount of energy is needed to overcome the strong electrostatic attraction between the positive ions and the delocalised electrons and break down the lattice.

Trends:

• Down Group 1 (Li 181 °C to K 63 °C): the ions are the same charge but bigger, so the charge density falls, the attraction weakens and the melting temperature falls.
• Across Na, Mg, Al (98, 650, 660 °C): ionic charge rises from 1+ to 3+, the sea of electrons is denser and the ions are smaller, so the attraction strengthens and melting temperature rises.
• Calcium (842 °C) is much higher than potassium (63 °C) because it has 2+ ions and two delocalised electrons per atom.

When a metal melts, the regular lattice breaks down and the ions can move past one another, but metallic bonding is still present in the liquid.

Metals can be hammered into shape (malleable) and drawn into wires (ductile). When a force is applied, layers of ions slide over one another. The delocalised electrons still attract the ions in their new positions, because the bonding is non-directional, so the metal changes shape without breaking.