All of the information about a crystal is held in the unit cell. The repeated stacking of a unit cell in three dimensions perfectly reproduces the large scale structure of the crystal. When all of the atoms or ions in a cell have the same radius, such as in a metal, the unit cell will be symmetrical. In other cases, such as when atoms or ions are present in different ratios, the unit cell will be asymmetrical. The structure of a lattice dictates the physical properties of the compound, including melting point, density, mechanical properties such as strength and hardness, and electrical and thermal conductivity.
In all solids, atoms or ions are configured in their least energetic and most stable positions. This energy is called lattice energy and, if it did not exist, then solids would not exist. Therefore, lattice energy is defined as the energy needed to create one mole of a crystal lattice from its constituent ions. The bonding in metals is called metallic bonding, while the bonding in non-metallic crystal lattices can be either ionic or covalent bonding. Both metallic and ionic bonds are strong, which is why metals and mineral have high melting points. Some minerals, such as silicon oxides or silicas, have covalent bonds. Silicas feature alternating silicon-oxygen bonds that are strong and is why silicas also have high melting temperatures.
Sodium chloride, or common table salt, has a simple crystalline structure, comprising equally spaced positively charged sodium ions and negatively charged chloride ions. The three dimensional structure of sodium chloride is cubic, with each of the ion at 90 degrees to each other, meaning that every sodium ion is surrounded by six chloride ions and each chloride ion surrounded by six sodium ions. The unit cell for sodium chloride is symmetrical because sodium and chloride ions are present in the same ratio.
A body centered cubic structure (bcc) comprises a cube with a seventh ion at its center. A primative cubic cell is like a bcc only without the central particle. A hexagonal close packed unit (hcp) comprises two hexagons on top of each other with three central ions interspersed between them. The structures of metals can change with temperature. At below 1,660 degrees F iron is body centered cubic. Above this temperature, iron changes to face centered cubic. And above 2,550 degrees, it reverts to being body centered cubic. Under high pressure, iron can also become hexagon close packed in structure. The ability of a solid to form different crystal structures is called a polymorphism.