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1 Intermolecular Forces / Liquids and SolidsV. Solids A. Crystal Structures 1. Introduction A crystalline solidis a solid whose component particles are arranged in an orderly, three-dimensional repeating structural pattern. rigid long-range order Positions occupied in a crystalline solid are called lattice points. The array of lattice points is called a (crystal) lattice. A unit cellis the basic repeating unit of a lattice. The lattice is composed of repeating unit cells. Below is an example of a two-dimensional lattice. ••••••••••••••••A two-dimensional lattice with the unit cell indicated by the dashed lines. The black dots represent either atoms or molecules. For a two-dimensional lattice, two axes and angle between the axes define a unit cell. Note that axes are vectors – need length and direction to define an axis. See Fig 11.14, Chang, 9thedition for a three-dimensional lattice and its associated unit cell. For a three-dimensional lattice, three axes and three angles define a unit cell. 2. Types of unit cells There are a total of seven types of unit cells (see Fig 11.15, Chang, 9thedition) We will cover only the cubic unit cell all sides same length and all angles 90°
2 ••••••••The cubic unit cell There are three types of cubic cells. FOR NOW, ASSUME LATTICE POINTS ARE OCCUPIED BY ATOMS. a. simple cubic (sc) or simple cubic cell (scc) This is, as implied, the simplest arrangement. This is easily pictured by imagining a layer of spheres (baseballs, ping-pong balls, etc.) arranged first in a single layer. These spheres in a single layer are arranged as shown below. Then an identical layer is placed above the first layer with the spheres directly above one another. This