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Intermolecular Forces / Liquids and Solids
V. Solids
A. Crystal Structures
1. Introduction
A
crystalline solid
is a solid whose component particles are arranged in an orderly, threedimensional
repeating structural pattern.
rigid
longrange order
Positions occupied in a crystalline solid are called
lattice points
.
The array of lattice points is called a (
crystal
)
lattice
.
A
unit cell
is the basic repeating unit of a lattice. The lattice is composed of repeating unit cells.
Below is an example of a twodimensional lattice.
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A twodimensional lattice with the unit cell indicated by the
dashed lines. The black dots represent either atoms or molecules.
For a twodimensional lattice, two axes and angle between the axes define a unit cell.
Note that axes are vectors – need length and direction to define an axis.
See Fig 11.14, Chang, 9
th
edition
for a threedimensional lattice and its associated unit cell.
For a threedimensional lattice, three axes and three angles define a unit cell.
2. Types of unit cells
There are a total of seven types of unit cells (
see Fig 11.15, Chang, 9
th
edition
)
We will cover only the cubic unit cell
all sides same length and all angles 90°
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The cubic unit cell
There are three types of cubic cells.
FOR NOW, ASSUME LATTICE POINTS ARE OCCUPIED BY ATOMS.
a.
simple cubic (sc) or simple cubic cell (scc)
This is, as implied, the simplest arrangement. This is easily pictured by imagining a layer of spheres (baseballs,
pingpong balls, etc.) arranged first in a single layer. These spheres in a single layer are arranged as shown
below. Then an identical layer is placed above the first layer with the spheres directly above one another. This
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 Fall '08
 Maguire
 Mole, Crystal, Cubic crystal system, Crystal system, Atomic packing factor

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