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Metallic Solid Structure Closest Packing = arrangement of atoms to occupy the least space PowerPoint Presentation
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Metallic Solid Structure Closest Packing = arrangement of atoms to occupy the least space

Metallic Solid Structure Closest Packing = arrangement of atoms to occupy the least space

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Metallic Solid Structure Closest Packing = arrangement of atoms to occupy the least space

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  1. Chapter 11 Lecture 2--Solids • Metallic Solid Structure • Closest Packing = arrangement of atoms to occupy the least space • Atoms are spheres that are bonded to each other in all directions • There are multiple arrangements to achieve closest packing • Hexagonal Closest Packed (hcp) • Also called “aba” packing • Every other layer is in the identical position • Results in a hexagonal unit cell

  2. Cubic Closest Packed Hexagonal Closest Packed

  3. Cubic Closest Packed (ccp) • Also called “abc” packing • Every fourth layer is in the identical position • Results in a face-centered cubical unit cell • Counting atoms • Both hcp and ccp result in each atom having 12 “nearest neighbors” • Six atoms surrounding each atom in the same layer • Three atoms in the layer above and three in the layer below

  4. Counting atoms within the unit cell is important for some applications • For face-centered cubic unit cell (ccp), there are 4 atoms in unit cell • Eight corners, each containing 1/8 of an atom = 1 atom • Six faces, each containing 1/2 of an atom = 3 atoms • Example: Density of ccp Ag with r = 144 pm 4r L

  5. Diamond Metal • Network Atomic Solids: Carbon and Silicon • Properties: directional, covalent bonds, “giant molecules”, brittle, insulators • Carbon Allotropes = different forms of an element in the same state (solid) • Diamond • All carbon atoms sp3 hybridized and tetrahedral • Hardest natural substance known • Insulator: MO diagram has large gap between filled/unfilled MO’s

  6. Graphite • Slippery, black, conductor • All carbons sp2 hybridized and planar • Three directional, covalent s-bonds • Extra p-orbitals form “clouds” above/below carbon planes; this allows electrons to “flow” making graphite a conductor • Lubricant: strong bonding within layers, weak bonding between them

  7. Silicon is a very important component of rocks and minerals • Silica = SiO2 = quartz, sand • CO2 is a gas: sigma and pi bonds between carbon and oxygens • 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 • Silicates = tetrahedral SiO4 units with O:Si ratios greater than 2:1 Other cations needed to balance the resulting negative charge (Al3+)

  8. Glass = rapidly cooled molten silica (>1600 oC) • Amorphous solid: silica crystal structure doesn’t re-form on cooling • Result is an amorphous solid (glass) rather than crystalline (quartz) • Common Glass: Na2CO3 added to silica • Lab Glass (Pyrex): B2O3 added to silica = doesn’t expand/contract • Lenses: K2O added to silica = harder, maintains precise shape Melt, Rapidly Cool Quartz Glass Quartz Crystal

  9. Ceramics = nonmetallic, silicate clays that are fire hardened • Brittle, strong, chemical and heat resistant • Glass = homogeneous frozen solution (1 phase) • Ceramic = heterogeneous frozen solution (2 phases) • Glassy “cementing” phase • Suspended silicate crystals • Fire hardening removes water, which had allowed clays to “flow” • New “high tech” materials are often ceramics • Semiconductors • Si solid has a structure like diamond, but a smaller MO energy gap • Some electrons can cross the gap at 25 oC = semiconductor • Increasing the temperature increases how conducting it is • Doped Si includes some other element impurities = more conducting • N-type semiconductor = dopant has 1 more electron than Si (As) • P-type semiconductor = dopant has 1 less electron than Si (B) P-type Extra “holes” N-type Extra e-

  10. 7. Semiconductors and Electronics • Both n-type and p-type semiconductors are better conductors than Si • Extra electrons make it easier for electrons to cross gap • Extra “holes” mean some unpaired electrons, which conduct better Holes are regarded as positive charge carrier • P-N Junction • Reverse Bias = no current flows = p-type connected to negative terminal and n-type connected to positive terminal • 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) • AC-DC Rectifier: alternating current changed into direct current because electrons only flow during Forward Bias phase

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