In metals the packing of ions is as close as possible and this explains why they usually have a high density. Some metals, for example alkali metals, are less closely packed and so have a lower density.

In pure metals, some layers of identical ions can be moved one over another without breaking the structure: thanks to this flexibility pure metals can be beaten or rolled into sheets. Another characteristic of these metals is their strenght, that is given by their metallic bonds.

Metals are also good conductors, because in their structures electrons are free to move.

Making alloys is the most common way to change the properties of metals, in fact they are formed by mixing the molten metals together and allowing them to cool and form a solid. Usually alloys are made to obtain a metal stronger than the original ones.

There is a change in properties because the presence of impurities makes it more difficult for the metal ions to slip over each other. This makes the alloy stronger but also more brittle.

In an ionic lattice the nearest neighbours of an ion are always of the opposite charge: the number of positive and negative ions is the same, so the charge will be balanced. It is important to remember that all ionic compounds are electrically neutral.

Ionic crystals are hard, but much more brittle than metallic crystals. This is a result of the structure of the layers: pushing one layer against another in an ionic crystal brings ions of the same charge next to each other, causing a repulsion that force the layers apart.

Ionic compounds can conduct electricity only when melted or in solution, because these are the only two situations where the electrons are able to move.

These crystals are held together by strong covalent bonds and the most known examples are diamond and graphite.

The properties of diamond are due to the fact that the strong covalent bonds extend in all directions. Diamond has a very high melting point and it is very hard, but it is much more brittle than giant metallic lattices. All the outer electrons are used to form covalent bonds, therefore diamond is a non-metallic element that doesn't conduct electricity.

Graphite, instead, does conduct electricity because carbon atoms are arranged in a different way in the molecular structure: within the layers of graphite each carbon atom is bonded to the others by strong covalent bonds and some weak forces of attraction are present. In this way electrons are free to move between different layers and so can conduct electricity.

Some non-metals and some covalently bonded compounds exist as solid with low melting points. Molecules of these elements or compounds form crystals that can be easily broken down by heat. These molecules are then free to move, but they have no charge: neither the liquid nor the solid forms of these substances conduct electricity.