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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 ce ll of these structures. The unit cell is the smallest repeating unit in
the three dimensional array. You will also explore unit ce ll s toichiome try. 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
Packing efficiency and coordination number will also be investigated. The packing e fficie ncy
of a unit cell is the fraction of the volume of the unit cell that is occupied by atoms, ions or molecules (commonly
expressed as the percent). Thus, as more atoms are packed into a volume, the packing efficiency increases. The
coordination numbe r is the number of atoms surrounding another atom in a crystal lattice. The coordination
number can also predict trends in packing efficiency. As the coordination number increases, the packing
efficiency increases as well. The space in a unit cell that is not occupied by an atom is referred to as the void
volume . As void volume increases, packing efficiency decreases.
In Part A of the experiment, you will build
some simple structures that metallic elements pack in. You may have encountered these in the lecture class. Part
B is an exercise in exploring unit cell stoichiometry; finding the ratio of two different ions inside a few unit cells. Equipment
1 Solid- State Model Kit
1 ruler About the Model Kit
This model kit was developed by the Institute for Chemical Education at the University of Wisconsin Madison. The instruction pages that follow are reprinted with permission from the Institute for Chemical
Education (ICE), John W. Moore, director. The Chemistry Department of North Carolina State University
wishes to express thanks to ICE for permission to reproduce th...
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This document was uploaded on 01/22/2014.
- Spring '14