1. 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 Chapter 11 Lecture 2--Solids

2. Hexagonal Closest Packed Cubic Closest Packed

3. 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

5. 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 4r L

6. 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 sp 3 hybridized and tetrahedral ii.Hardest natural substance known iii.Insulator: MO diagram has large gap between filled/unfilled MO’s DiamondMetal

7. b.Graphite i.Slippery, black, conductor ii.All carbons sp 2 hybridized and planar iii.Three directional, covalent  -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

8. 3.Silicon is a very important component of rocks and minerals a.Silica = SiO 2 = quartz, sand b.CO 2 is a gas: sigma and pi bonds between carbon and oxygens c.SiO 2 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 = SiO 2 d.Silicates = tetrahedral SiO 4 units with O:Si ratios greater than 2:1 Other cations needed to balance the resulting negative charge (Al 3+ )

9. 4.Glass = rapidly cooled molten silica (>1600 o C) 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: Na 2 CO 3 added to silica d.Lab Glass (Pyrex): B 2 O 3 added to silica = doesn’t expand/contract e.Lenses: K 2 O added to silica = harder, maintains precise shape Quartz Crystal Melt, Rapidly Cool Quartz Glass

10. 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 o C = 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”

11. 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

12. N-type and P-type semiconductors Reverse Bias And Forward Bias In p-n junctions