Metallic bonding

Last time

Before we start: last time, in Isotopes and relative atomic mass, you learned these points. Here they are again on one card. Keep them in mind, because today builds on them.

Why a copper wire works

A copper wire carries electricity into your home, and can be bent round a corner without snapping. Salt cannot do the first, and diamond cannot do the second. The reason lies in the way metal atoms are bonded, and it is called metallic bonding.

The structure of a metal

A metal is a giant lattice of positive ions, arranged in regular layers. Each metal atom has given up its outer electrons. These electrons are no longer held by any one atom. We say the electrons are delocalised. They move freely through the whole metal, like a sea.

The picture

Here is the structure of a metal. The positive ions stay in their places, and the delocalised electrons move between them. Every ion is strongly attracted to the sea of electrons around it.

Where do the electrons come from?

Magnesium has the configuration two, eight, two. It has two outer electrons. In the metal, each atom releases these two electrons into the sea, and becomes a Mg²⁺ ion. The ion has two, eight, with a full outer shell. The electrons now belong to the whole metal.

The metallic bond

The metallic bond is the strong electrostatic attraction between the positive ions and the delocalised electrons. It acts in all directions, and it holds the whole lattice together. There are no negative ions in a metal, and no shared pairs of electrons.

Why metals conduct

To conduct electricity, a substance needs charged particles that are free to move. In a metal, the delocalised electrons are free to move through the lattice, so they carry the current. This works in a solid, so metals conduct electricity as solids, and they conduct heat well too. It is the electrons that move. The positive ions stay where they are.

Why metals can be shaped

Metals are malleable, so they can be hammered into shape. They are also ductile, so they can be drawn into wires. When a force is applied, the layers of positive ions slide over each other. The delocalised electrons move with them, so the ions are still attracted, and the metal does not break.

Melting points

Most metals have high melting points, because the metallic bonds are strong, and a lot of energy is needed to overcome them. The more delocalised electrons per atom, and the higher the charge on the ion, the stronger the bond. So magnesium, with two electrons per atom, has a higher melting point than sodium, with one.

Properties and reasons

Here are the properties of metals, and the reason for each. Conduct electricity: delocalised electrons. Malleable and ductile: layers of ions slide. High melting point: strong metallic bonds.