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Solids
Classified into two general types:
a. Crystalline
b. amorphorous
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Amorphous Solids
An "amorphous solid" is a solid in which there is no long-range order of the positions of the atoms.
Most classes of solid materials can be found or prepared in an amorphous form.
Prepared by fast cooling. Molecules are frozen in place when the phase changes.
Glass is the most famous example.
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Crystalline Solids
When cooled slowly, atomic and molecular builidng blocks assembled in well ordered, low energy structures called crystals.
Examples
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Types of CrystalsType Bonding Characteristic
sExamples
Metallic Metallic bonds Excellent conductor, high melting point
Silver
Copper
Ionic Electrostatic Ionic bonds
Brittle, poor conductors
NaCl
CuSO4
Molecular Intermolecular Forces
Soft, low melting point, poor conductors
H2O
Cholesterol
Network Covalent network
Hard, brittle, high melting point
Diamond
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Crystal Vocabulary
• Lattice- a three dimensional system of points designating the position of the components (ions, atoms or molecules) that make up the substance.
• Unit Cell- The smallest repeating unit of the lattice.
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IONIC CRYSTALS• In ionic crystals,
ions pack themselves so as to maximize the attractions and minimize repulsions between the ions.
7-17-1 SolidsSolids
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CUBIC CRYSTAL
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BODY-CENTERED CUBIC CRYSTAL
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FACE-CENTERED CUBIC CRYSTAL
7-17-1 SolidsSolids
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8–10
Packing of Spheres and the Structures of Metals
• Arrays of atoms act as if they are spheres. Two or more layers produce 3-D structure.
• Two cubic arrays one directly on top of the other produces simple cubic (primitive) unit cell.
• Offset layers produces a-b-a-b arrangement since it takes two layers to define arrangement of atoms.
• Called cubic closest packed. • Makes a body centered cubic unit cell.
– .
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8–11
Packing of Spheres and the Structures of Metals
• Hexagonal closest packed requires three layers to make a repeating pattern (abc, abc, …). It forms a face centered cubic unit cell.
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Determining number of spheres in a unit cell.
Corner = 1/8 sphere, each unit cell contains 8 corners
Face = 1/2 sphere, each face centered unit cell 6 corners
Body = 1 sphere, each body centered unit cell contains 1 sphere
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Determine the number of spheres in a face centered cubic unit cell.
There are 8 corners.
There are 6 faces.
There are no body spheres.
(8 X 1/8) + (6 X ½) = 4 spheres
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8–14
Cubic Unit Cells in Crystalline Solids
• Primitive-cubic shared atoms are located only at each of the corners. 1 atom per unit cell.
• Body-centered cubic 1 atom in center and the corner atoms give a net of 2 atoms per unit cell.
• Face-centered cubic corner atoms plus half-atoms in each face give 4 atoms per unit cell.
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8–15
Calculations involving the Unit Cell
• The density of a metal can be calculated if we know the length of the side of a unit cell.
l4
2r
Name # atoms Length of side (l)
Volume
Simple Cubic 8 corners X 1/8 =1 2r = l l3
Body Centered Cubic
8 corners X 1/8 =11 body atom =1 2
l3
Face Centered Cubic
8 corners X 1/8 =16 faces X ½ =3 4
l3
l43
r
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8–16
Polonium crystallizes according to the simple cubic structure. Determine its density if the atomic radius
is 167 pm.
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8–17
Calculate the radius of potassium if its density is 0.8560 g/cm3 and it has a BCC crystal structure