Solids

The solid state is a state of matter characterized by the absence of freedom between molecules or ions (metals for example).

The macroscopic criteria of matter in the solid state are:

• the solid has its own form;
• the solid has its own volume.

Types of bonds

If a solid object is definite, it is thanks to the bonds between the atoms, ions or molecules that make up the solid.

(Face-centered cubic crystal structure of NaCl.)

• For example, covalent bonds bind the carbon atoms of a diamond. They are normally difficult to deform. Purely covalent solids are normally quite hard.
• There are also ionic bonds, between ions of opposite charges. They ensure the cohesion of a grain of salt, for example, thanks to the bond between positive sodium ions and negative chloride ions. Alumina is an example of a very strong ionically bonded material. However, ions of the same sign repel each other. So if a shock or a deformation brings the ions of the same sign together, the ionic solid can break.
• Metal bonds also make matter solid at normal temperature except for mercury. They form between any metal atom and in any relative position of the atoms. So metallic solids are more malleable.
• Weaker bonds such as hydrogen bonds make matter solid at low temperatures (eg ice).
• Some even weaker bonds, such as van der Waals bonds, only make matter solid at very low temperatures (eg, dioxygen).

Kinds of bonds

Some solids result from several kinds of bonds.

Some examples, pyrite has partially covalent and partially metallic bonds between iron and sulfur.

In many rocks and glasses, there are partially covalent and partially ionic bonds, as in silicates, phosphates or sulfates, with ionic bonds between anions and the sodium, potassium, magnesium and calcium cations.

The structure of paper results from the existence of covalent bonds in cellulose fibers and hydrogen bonds between the fibers. These fibers can separate in water because these hydrogen bonds form preferentially with water rather than with neighboring fibers.

Many polymers like polyethylene or polytetrafluoroethylene have covalent bonds between carbon atoms in their chains, but van der Waals bonds between the chains. Polyethylene is a thermoplastic material because during hot molding, the chains slide easily between them in the mold.

However, one should not consider the solid as a frozen state of matter because the atoms vibrate around their position of equilibrium. With temperature, these vibrations increase to the point of breaking the bonds during the fusion, sublimation or pyrolysis of the solid.

The solids have low expansion and low compressibility.

Most solids are crystals. The atoms of a crystal are arranged in space in a regular and orderly fashion. The interatomic distances remain constant, we speak of order at great distance. Conversely, some solids like glass are amorphous and have no order at great distance.