Lab 7 - Solid State Modeling

Lab 7 - Solid State Modeling - Lab 7 Solid State M odeling...

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4/15/13 Lab 7 - Solid State Modeling www.webassign.net/ebooks/wsugencheml1/lab_7/manual.html 1/10 Contents > Lab 7 - Solid State Modeling Lab 7 - Solid State Modeling Purpose A To explore some simple solid state structures. B To explore unit cell stoichiometry. Goals To visualize the three-dimensional arrangement of atoms and ions in common solid state structures. To determine the stoichiometry of unit cells. To gain a working knowledge of unit cells, packing efficiency, coordination numbers and their relationship to each other. Introduction The solid state is the least energetic of the three common states of matter. Atoms and ions are packed together in a regular arrangement. The arrangement is normally one that minimizes the energy of the particles by maximizing the interaction of positive and negative charges. Maximizing the contact between atoms, which maximizes the dispersion forces holding atoms and molecules together, also minimizes energy. Thus, atoms and ions are often packed in a spatially efficient manner with alternating charges. When this ordered arrangement extends throughout the entire crystal in very well defined and repeating patterns ( a crystal lattice ), the solid is considered crystalline . If the arrangement is not as repetitive (local order), the solid is amorphous . This experiment deals only with crystalline solids. Although this description might lead you to believe that there is only one best way to arrange atoms to minimize their energies, there are actually many different ways atoms and ions can pack to achieve this. The arrangement a particular compound or material chooses depends to a large extent on the size and charge of the particles. The three simplest and common lattices are: Simple cubic or primitive in which identical particles are found on the eight corners of a cube. Body centered cubic in which identical particles are found on the eight corners plus one in the center of the cube.
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4/15/13 Lab 7 - Solid State Modeling www.webassign.net/ebooks/wsugencheml1/lab_7/manual.html 2/10 Face centered cubic in which identical particles are found on the eight corners plus the six faces of the cube. In this modeling experiment, you will build models of a few of the packing arrangements that simple materials can take. The models will focus on the unit cell of these structures. The unit cell is the smallest repeating unit in the three dimensional array. You will also explore unit cell stoichiometry . We normally think of the stoichiometry of a substance as the ratio of moles of each element. This is reflected in the chemical formula of that substance. This same ratio must also hold in the smallest repeating unit of the solid state, the unit cell. The models will help you visualize the number of each atom or ion inside a unit cell and help you determine the stoichiometry. Packing efficiency and coordination number will also be investigated. The packing efficiency
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