Chem 115 December 3 2015 Lecture 27 Crystalline Solids have a repeating pattern of molecules When most liquids are cooled they eventually freeze and form crystalline solids Solids in which the atoms ions or molecules are arranged in a definite repeating pattern solids may be single crystal polycrystalline or amorphous Single Crystals have ONE LARGE ordered domain diamonds sugar grains salt grains Polycrystalline Solids have MULTIPLE SMALLER ordered domains chunks of ice metals granite Amorphous Solids don t have a highly ordered structure glass wax Large single Crystals are stable but hard to form Single Crystal atoms or molecules form ordered arrays to MAXIMIZE BONDING or INTERMOLECULAR INTERACTIONS Single Crystal silicon is used for circuits and solar cells Computer chips and many solar cells electronic properties Amorphous Solids lack a regular patter have poorly defined shapes because their particles lack an orderly arrangement throughout the shape Result when molecules in the liquid can t move easily often when they re very large As the temperature is lowered their molecules stop in random positions Ex candle wax butter glass many plastics Bonding in Solids Influences their physical Properties How strong are bonds IMF How are electrons shared How easily do bonds IMF Which type of crystalline is CaF2 shift Ionic Which type of crystalline solid is solid CO2 Molecular Solid dry ice Unit Cell repeat unit to build the crystal The collection of all of the points within a crystal that have identical environments is called a SPACE LATTICE These points are also arranged in a definite repeating patter A volume of a space lattice that will generate the entire lattice if repeated in three dimensions is called a UNIT CELL Atoms Pack in a Lattice to Maximize their interactions Interactions are stronger when the atoms are closer together Hexagonal Structure packs the atoms closer together stronger interactions found more often in nature Simple Cubic large diamond space second layer directly over first 52 effective Body Centered Cubic seconds layer of diamond spaces in first third layer over diamond spaces in second 68 effective 3D Hexagonal Lattice only one choice for the second layer third layer over orange spaces hexagonal close packing third layer over white spaces Cubic close packing Metals based of their Group 1 Unit cell generate atomic adopt different lattice structures partly on the configuration valence shell Body Centered Cubic can the lattice if repeated in 3D Simple Cubic 1 Atom per unit cell 8 x 1 8 1 Body Centered Cubic 2 atoms per unit cell 8 x 1 8 1 in the middle 2 Face Centered Cubic 4 atoms per unit cell 8 x 1 8 6 x 4 One crystalline form of Iron has a density of 7 93 g cm edge length of 0 286 nm How many iron atoms are in the unit cell 7 93 g x 1mol 55 85 g 1 42 mol x 6 022 1023 8 55 x 1022 atoms cu volume 286 1 107 nm 3 and a cubic unit Aluminum crystallizes in a face centered cubic FCC structure If the edge of the unit cell is 404 94 pm then what is the radius of an Al atom A2 B2 C2 C 4R R a2 b2 4 R 143 17 pm Material Structure Describes Packing of Atoms or Molecules Crystalline solids are ordered amorphous solids are not Difference in bonding in solids impact physical properties including melting point and conductivity