So, you think you know what a solid is?

Defining a solid is easy, surely (touch wood)? Something that keeps its shape - not like those liquids and gases that go flowing around heaven knows where and can't be trusted to come back home in one piece. But consider these examples:

1. A raindrop keeps the same shape to a reasonable approximation all the time it is falling.
2. A piece of chewing gum changes shape a couple of times a second.
Clearly we will have to work harder to get a watertight definition. Suppose you say, the forces on the raindrop were essentially zero until it hit the ground - and the gum is only changing shape because of the forces on it. Does this get us anywhere? Let's try this:
• A solid is something that doesn't change shape under a small force, only under a big force.
This ain't very scientific. Consider two more examples: a thin twig of charcoal and a dollop of molasses or any very viscous liquid. The charcoal falls apart under less force than it takes to push the molasses anywhere much different. Still not much good.

Besides, even if you apply the smallest force, any solid does change shape; otherwise it would be infinitely stiff and unphysical. But when you remove the force, it returns more or less exactly to the shape it had before. This notion of elastic deformation is crucial to the definition; even solids which exhibit an unusual relation between force and deformation (like rubber) have this property. By contrast, a viscous liquid, when you push it, stays pushed. This takes care of the raindrop and the molasses, but leaves the gum and charcoal to be explained.

All we need now is to remember how things break: either suddenly snapping, brittle fracture, or through inelastic extension (necking) followed by fracture. So, the final definition:

• A solid is defined by its ability to recover its shape after the application and removal of a force (more correctly, a stress) below a certain breaking point; above this ultimate stress it changes shape irreversibly, either gradually or suddenly.

Down among the atoms

These behaviours can be more easily explained at the atomic or molecular level. The shape is defined by the fact that the atoms or molecules in the solid oscillate (with thermal energy) about positions fixed with respect to each other (at least in equilibrium). These relative positions are fixed by the balance of forces between constituents, so when an external force is applied, the positions shift slightly in elastic deformation. The molecules or atoms do not freely rearrange themselves (as in liquids and gases) because the potential barriers, or energy barriers, (see potential difference and infinite potential well, then scratch 'infinite') are too high. Above the breaking point, the external stress overcomes the internal forces at certain points within the material and the structure changes to a topologically different one. If a new configuration with strong attractive forces can be found, the solid deforms continuously; without strong attractive forces, a catastrophic fracture occurs.

Classification and properties

Solids are classified by the geometry of their crystalline structure - or lack of structure, in the case of glasses - and by the bonding, the forces that keep the solid together - which means no more than the distribution of electrons and electric charges between the constituents. Covalent bonds, ionic bonds, metallic bonds, hydrogen bonds, van der Waals forces (a.k.a. dispersion forces or London forces) can all play roles - not all within the same solid! - (see also graphite for the strangest bonding of the lot) and explain the very different properties of different solid materials.

For example, metallic and covalent bonds are the strongest measured in terms of energy per mole: hence metals and covalently-bonded solids (such as diamond) have the highest melting points and the highest Young's modulus and tensile strength. In contrast, diatomic elements such as nitrogen, indeed most light covalently-bonded molecules, form solids with very low melting points and low resistance to deformation: although the molecules themselves are very tightly bound, the forces between them are very weak van der Waals forces.

Ionic solids have intermediate properties: they also tend to be brittle, which may be explained by the fact that they have alternating positively- and negatively-charged ions. Normally, positive and negative are aligned next to each other, resulting in strong electrostatic attraction. When the layers of an ionic crystal slip past each other under stress, positive will become adjacent to positive and negative with negative, resulting in strong repulsion at that point. So the solid suddenly becomes much easier to break along the the surface where this misalignment occurs.

The electrical properties of solids can also be explained by looking at the arrangement of their electrons, determined by the Pauli exclusion principle and the distribution of electric charge. The essential difference is how easily the electrons can be excited into states in which they can move freely around the material. In metals, it takes practically no energy to accomplish this; in semiconductors and insulators a finite small or large amount respectively is required, explaining the difficulty of driving a current through these materials (see Fermi level).