Fred J. Grieman

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Fred J. Grieman
Solids (Lecture 2) Molecular Solids Nonmetallic
Network Solids Read Solids handout given on
website
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• Unit Cell
• Crystal atoms/molecules arranged in 3d repeating
  pattern
• Lattice set of points with identical
  environments
• (pick one type of atom, for example)
• 3) Connect Lattice Points generates repeating
  parallelepipeds
• 4) One parallelepiped is a UNIT CELL
• 5) Crystal is built from 3d stacking of Unit
  Cells
• 6) Crystal has symmetry of Unit Cell

Four types of crystalline solids Metals,
Nonmetallic Network, Ionic, Molecular
(Just Shown)
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Molecular Solids Individual molecules held
together by weak van der Waal Forces (London
Dispersion and dipole) Easily Melted and
Soft Examples Noble Gases Maximize
London Forces ? CLOSEST PACK STRUCTURES
hexagonal closest pack (hcp) or cubic closest
pack (ccp) He is hcp, all other noble gases
are ccp
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On dimple
hcp
Two choices
Green choice
Red choice
ccp
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hcp
6
ccp
rotate
Face centered cubic
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Coordination 12 for ccp and hcp
4r
e
8(1/8) 6(1/2) 4 atoms/unit cell Space
filled e2 e2 (4r)2 e 81/2r r
atom radius Vcube e3 83/2r3 Vatoms
4(4?/3)r3 4 atoms Space filled Vatoms/
Vcube x 100 4(4?/3)r3 / 83/2r3 x
100 74
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Other Molecular Crystals O2, N2, X2, CH4,
Al2Cl6, FeCl3 Vast majority of organic compounds
Unit cell slightly different (least
symmetrical of unit cells) Monoclinic
parallel piped w/ 2 angles 90, all sides unequal
length Triclinic all angles different, all
sides different lengths Metals (much more
later) very electron poor (not enough for
covalent bond) electron sharing ?
sea of electrons often closet packed to
increase e-/n attraction 70 ccp or hcp 25
bcc bcc body centered cubic 8 nearest
neighbors, space filled 68 Alkali, Ba, V,
Cr, Fe, Nb, Mo, Ta, W
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Metal unit cells
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Non-metallic Network Solids Whole crystal one
large covalently bonded molecule Localized
bonds poor conductor of heat electricity
. Example Carbon
C orbital rich, but enough e- to make
.
covalent bonds Three
allotropes fullerenes, diamond,
graphite Fullerenes molecular solids Diamond
network solid Graphite sheets of network solids
sp3
sp2
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(Diamond overhead)
Graphite
(overhead)
Another example ZnS Face centered cubic with
S in tetrahedral holes
(overhead)
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Next Lectures Ionic Crystals Metals Semiconduct
ors
Exam 3 Average 148.2/200 (74.1) Std.
Dev. 35.0 (17.5) High Score?
190/200