In the solid phase, a substance maintains its form regardless of the form of the container. Any motion exhibited by the individual molecules is vibrational in nature. Solids can be either amorphous or crystalline in structure. The structures of amorphous solids are irregular or random, while the structures of crystalline solids are formed of repeating, regular units called unit cells. Solids can be classified as metals, ionic solids, covalent network solids, or molecular solids according to the nature of the bonds which hold them together. Metals are composed of a series of positively charged ions (cations) surrounded by a layer of electrons. These electrons are not limited in movement to the proximity of one bond with another atom, but are delocalized throughout the whole solid. Ionic solids are a regular alternation of cations and anions (negatively charged ions) held together by ionic bonds. In covalent network solids, the atoms of the molecules are covalently bonded in large networks throughout the solid. Molecular solids are made up of atoms or molecules held together through the weak interactions: dipole-dipole, hydrogen bonding and Van der Waal interactions.
The simplest crystals have structures that are simple cubic, body-centered cubic, or face-centered cubic. In a simple cubic form, one atom (or ion) occupies each corner of a cube. The crystal is then formed by layers of these cubes. Another way of viewing the cubic unit is that each atom is a member of eight cubes simultaneously.
