The simplest portion of the lattice that makes up the repeating unit is called the unit cell. The pattern of the array is called the crystal lattice, and the individual positions are called lattice sites. Crystalline solids are orderly, repeating, 3-D arrays of particles, which can be atoms, ions, or groups of atoms, such as polyatomic ions or molecules. The unit cell is the smallest repeat unit of the crystalline lattice that generates the entire lattice with translation. In this section, we define the unit cell and discuss how it is packed with atoms. Our study is simplified because, instead of studying the positions of the enormous number of particles that constitute the entire crystal, we need study only the small number of particles that comprise a unit cell. 8.1 Unit Cells Introduction However, the long range order that characterizes crystalline solids means that there is a small repeat unit, called the unit cell, that can be used to generate the entire crystal. Thus, studying the solid state could be a formidable task. Even a small crystal contains millions and millions of particles. This chapter is devoted to the study of crystalline solids. Table salt and sugar are two common examples of crystalline solids. If the order exists throughout the entire solid (long range order), the solid is said to be a crystalline solid. If the order is over short distances only (local order), the solid is an amorphous solid. Chapter 8 – Solids Introduction Solids are characterized by an orderly arrangement of their particles.
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