Solid State Modeling CH102, Lab 6 Goals : Explore unit cell stoichiometry to visualize the arrangement of atoms and ions in common solid state structures. To gain a working knowledge of unit cells, packing efficiency and coordination numbers. Safety : No chemicals will be used. Please wear proper attire and safety glasses. Waste : No waste will be generated in this lab. X O X O X O X O O X O X O X O X X O X O X O X O O X O X O X O X Prelab Assistance Crystalline solids have long range order in the crystal lattice. O X X O Unit cell : smallest unit that represents the crystal lattice and empirical formula. Repetition of the unit cell will generate the crystalline solid. Position of Atoms in Cubic Unit Cells center face edge corner 1 atom Unit cell 1 atom ? ( 1 atom inside cell) = 6 atom ? ( ? atom inside cell) = 12 atom ? ( ? atom inside cell) = 8 atom ? ( ?atom inside cell) = 3 atoms Unit cell 3 atoms Unit cell 1 atom Unit cell Types of Unit Cells The SAME type of atom can be packed in ! Simple cubic ! Body centered cubic ! Face centered cubic unit cells. DIFFERENT types of atoms can be packed in ! NaCl structure ! CsCl structure ! Fluorite structure When comparing the size of cations and anions, remember that cations are smaller than their neutral element, and anions are larger than their neutral element. Part A Experimental Overview Simple Cubic (primitive) Body-Centered Cubic Face-Centered Cubic Students will construct unit cells composed of the same type of atom 8 atoms on corners 8 atoms on corner 1 atom in center 8 atoms on corner 6 atoms on face Part B Experimental Overview NaCl Students will construct unit cells composed of cations and anions. Consider NaCl. 8 Cl 1- ions on corners 6 Cl 1- ions on faces 12 Na 1+ ions on edges 1 Na 1+ ion in center + + = 8 Cl 1- ions on corners 6 Cl 1- ions on faces 12 Na 1+ ions on edges 1 Na 1+ ion in center = + + Part B Experimental Overview CsCl Students will construct unit cells composed of cations and anions. Consider CsCl. 8 Cs 1+ ions on corners 1Cl 1- ion in center 8 Cs 1+ ions on corners 1Cl 1- ion in center + = + = Part B Experimental Overview Fluorite Students will construct unit cells composed of cations and anions. Consider fluorite (calcium fluoride). 8 F 1- ions in center 8 Ca 2+ ions on corners 6 Ca 2+ ions on faces + = + 8 Ca 2+ ions on corners 6 Ca 2+ ions on faces 8 F 1- ions in center + = + 1 4 Postlab Assistance Example : A solid was found to have atoms ?E? on the edges and atoms ?F? in simple cubic packing. How many ?E? are there in a unit cell? 12 edge ?E?s ? ( atoms inside cell) = 3 ?E?s in unit cell How many ?F?s are there in a unit cell? Simple cubic has 8 atoms on corner sites 8 corner ?F?s ? ( atoms inside cell) = 1 ?F? in unit cell What is the chemical formula? E 3 F 1 8 Postlab Assistance Example : A solid crystalized in a face centered cubic cell that is 5.42 ? on a side. The atoms have a radius of 1.73 ?. a) How many atoms are there in each unit cell? 4 for face centered cubic b) What is the volume of one unit cell? V = length 3 = (5.42 ?) 3 = 1.59 ? 3 c) What is the volume of 1 atom? V = ? radius 3 = x (3.14159)(1.73 ?) 3 = 21.7 ? 3 d) What is the total volume of atoms in 1 unit cell? 4 atoms x 21.7 ? 3 = 86.8 ? 3 / unit cell unit cell atom e) What is the packing efficiency? 4 3 4 3 86.8 ? 3 atoms x 100 = 54.6% 159 ? 3 volume
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