Chapter 11 Lecture 2--Solids
A. Metallic Solid Structure
1. Closest Packing = arrangement of atoms to occupy the least space
a. Atoms are spheres that are bonded to each other in all directions
b. There are multiple arrangements to achieve closest packing
2. Hexagonal Closest Packed (hcp)
a. Also called “aba” packing
b. Every other layer is in the identical position
c. Results in a hexagonal unit cell
Hexagonal Closest Packed
Cubic Closest Packed
3. Cubic Closest Packed (ccp)
a. Also called “abc” packing
b. Every fourth layer is in the identical position
c. Results in a face-centered cubical unit cell
4. Counting atoms
a. Both hcp and ccp result in each atom having 12 “nearest neighbors”
i. Six atoms surrounding each atom in the same layer
ii. Three atoms in the layer above and three in the layer below
b. Counting atoms within the unit cell is important for some applications
i. For face-centered cubic unit cell (ccp), there are 4 atoms in unit cell
ii. Eight corners, each containing 1/8 of an atom = 1 atom
iii. Six faces, each containing 1/2 of an atom = 3 atoms
iv. Example: Density of
ccp
Ag with
r = 144 pm
2
2
2
L  L  ( 4r )
2 L2  16r 2
L2  8r 2
L  8r  8 (144 pm)  407 pm
4r
L
m (4atoms)(107.9 g / mol )(1mol / 6.022 x10 23 atoms)
d 
 10.6 g / cm3
3
10
V
cm 
3  1x10

(407 pm) 
pm


C. Network Atomic Solids: Carbon and Silicon
1. Properties: directional, covalent bonds, “giant molecules”, brittle, insulators
2. Carbon Allotropes = different forms of an element in the same state (solid)
a. Diamond
i. All carbon atoms sp3 hybridized and tetrahedral
ii. Hardest natural substance known
iii. Insulator: MO diagram has large gap between filled/unfilled MO’s
Diamond
Metal
b. Graphite
i. Slippery, black, conductor
ii. All carbons sp2 hybridized and planar
iii. Three directional, covalent s-bonds
iv. Extra p-orbitals form “clouds” above/below carbon planes; this allows
electrons to “flow” making graphite a conductor
v. Lubricant: strong bonding within layers, weak bonding between them
3. Silicon is a very important component of rocks and minerals
a. Silica = SiO2 = quartz, sand
b. CO2 is a gas: sigma and pi bonds between carbon and oxygens
c. SiO2 is a network solid: Si 3p orbitals too large to overlap with O 2p
No pi bonds, so Si forms two more sigma bonds with other O’s
Quartz = tetrahedral Si bonded to 4 O’s, empirical formula = SiO2
d. Silicates = tetrahedral SiO4 units with O:Si ratios greater than 2:1
Other cations needed to balance the resulting negative charge (Al3+)
4. Glass = rapidly cooled molten silica (>1600 oC)
a. Amorphous solid: silica crystal structure doesn’t re-form on cooling
b. Result is an amorphous solid (glass) rather than crystalline (quartz)
c. Common Glass: Na2CO3 added to silica
d. Lab Glass (Pyrex): B2O3 added to silica = doesn’t expand/contract
e. Lenses: K2O added to silica = harder, maintains precise shape
Melt,
Rapidly
Cool
Quartz Crystal
Quartz Glass
5. Ceramics = nonmetallic, silicate clays that are fire hardened
a. Brittle, strong, chemical and heat resistant
b. Glass = homogeneous frozen solution (1 phase)
c. Ceramic = heterogeneous frozen solution (2 phases)
i. Glassy “cementing” phase
ii. Suspended silicate crystals
d. Fire hardening removes water, which had allowed clays to “flow”
e. New “high tech” materials are often ceramics
6. Semiconductors
a. Si solid has a structure like diamond, but a smaller MO energy gap
i. Some electrons can cross the gap at 25 oC = semiconductor
ii. Increasing the temperature increases how conducting it is
b. Doped Si includes some other element impurities = more conducting
i. N-type semiconductor = dopant has 1 more electron than Si (As)
ii. P-type semiconductor = dopant has 1 less electron than Si (B)
N-type
Extra e-
P-type
Extra “holes”
7. Semiconductors and Electronics
a. Both n-type and p-type semiconductors are better conductors than Si
i. Extra electrons make it easier for electrons to cross gap
ii. Extra “holes” mean some unpaired electrons, which conduct better
Holes are regarded as positive charge carrier
b. P-N Junction
i. Reverse Bias = no current flows = p-type connected to negative
terminal and n-type connected to positive terminal
ii. Forward Bias = current flows = p-type connected to positive terminal
and n-type connected to negative terminal
Electrons can flow from n-type to p-type (extra e- to extra holes)
iii. AC-DC Rectifier: alternating current changed into direct current
because electrons only flow during Forward Bias phase
N-type and P-type
semiconductors
Reverse Bias
And
Forward Bias
In p-n junctions