12 Lecture 8, February 1, 2008 B. Atomic arrangements 1. How are atoms typically arranged within a solid? a. amorphous (glassy)—completely disordered; no atom-to-atom arrangment b. crystalline—ordered, repeating pattern; treats atoms like hard spheres (i.e., marbles). This means that only certain arrangements are possible, based on the shape and size of the atoms. 2. A unit cell is the smallest piece of the solid that retains the overall symmetry of the crystal. It is not necessary that the unit cell only contains one atom, but all corners of the unit cell must be in identical environments. The most important feature of the unit cell is its overall geometry, not the types or numbers of atoms that are located within it. 3. The term "lattice" refers to the underlying geometric relationships that generate the overall repeating pattern within the crystal. Atoms, molecules, and ions are "placed on the lattice". C. Types of unit cells
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