3.1.3.3 - Metallic Bonding

The Metallic Bonding Model

The metallic bonding model starts with atoms of the metal. Their outer electrons become shared across the whole structure. That leaves behind positive metal ions arranged in a regular lattice.

The phrase "sea of electrons" is often used as a model for this structure. That is useful, as long as you remember that the electrons are not attached to one particular ion and can move throughout the metal.

So a metal is best pictured as a giant lattice of positive ions surrounded by mobile electrons. The attraction acts throughout the structure, not just between one pair of particles. That is why metallic bonding gives metals a giant structure.

The diagram below shows this model in the simplest useful way: a regular lattice of positive ions with delocalised electrons moving throughout the structure.

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Ion charge, radius and attraction

The word lattice matters. It tells you that the positive ions are arranged in a regular, repeating pattern throughout the metal.

This helps avoid two common mistakes. A metal is not made of separate molecules, and it is not best described as a set of neutral atoms with their outer electrons still attached. The structure contains positive ions vibrating about fixed positions in the solid, while the delocalised electrons move throughout the whole lattice.

The strength of metallic bonding depends on the strength of the electrostatic attraction between these two parts of the structure. Higher-charge ions attract the delocalised electrons more strongly. Smaller ions also give stronger attraction because the positive charge is closer to the delocalised electrons. For a simple comparison such as sodium and magnesium, magnesium also contributes two delocalised electrons per atom instead of one, increasing the electron density. Charge, ionic radius and the number of delocalised electrons act together; melting-point trends across all metals are not determined by charge alone.

Quick recap

Now pull the whole model together. A complete description needs positive ions, a lattice, delocalised electrons, and attraction between the opposite charges.

If you can picture a lattice of positive ions, electrons moving throughout it, and electrostatic attraction between the two, you have the core model of metallic bonding.