What is a Unit Cell in a Crystalline Solid?
The beautiful, external regularity of a crystal of salt or a metal suggests that it has an internal symmetry as well. Indeed, all crystal lattices (whether of molecules, networks, metals or ionic solids) are built of unit cells, the smallest, repeating unit that has all of the symmetry characteristics of the way the atoms are arranged. The external appearance of the solid is a reflection of the unit cell. In the figure below an unit t cells for a “two-dimensional solid” made from flat, circular objects. Here the unit cell is a square. Each circle at a comer of a square contributes one fourth of its area to the area inside the square. Thus, there is a net of one circle per unit cell. All known crystals show only seven types of unit cells. Taking spheres to represent the packing of objects one can show only seven unit cell types are possible based on geometric restrictions. Crystal Systems: Seven Types of Unit Cells Available for Crystal Packing Describe the Three types of cubic lattices
