1 / 25

Solids

Solids. Ch.13. Solids. Fixed, immobile (so to speak) Symmetry Crystals So what’s the inner order?. Unit Cells. Unit cell = smallest repeating unit containing all symmetry characteristics Unit cell reflects stoichiometry of solid

talbot
Télécharger la présentation

Solids

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. Solids Ch.13

  2. Solids • Fixed, immobile (so to speak) • Symmetry • Crystals • So what’s the inner order?

  3. Unit Cells • Unit cell = smallest repeating unit containing all symmetry characteristics • Unit cell reflects stoichiometry of solid • Several unit cell types possible, but atoms or ions placed at lattice points or corners of geometric object

  4. Crystal Lattices • 3D unit cells built like legos  • Crystal Lattice = arrangement of units cells • seven 3D units cells found • Simplest = Cubic Unit Cell (equal length edges meeting at 90° angles) • Each face part of 2 cubes • Each edge part of 4 cubes • Each corner part of 8 cubes

  5. Cubic Unit Cell • 3 types: • 1)Primitive or Simple Cubic (SC) • 2) Body-Centered Cubic (BCC) • 3) Face-Centered Cubic (FCC)

  6. Cubic Unit Cell (cont.) • Similarity: • Same ions/atoms/molecules at each corner • Difference: • BCC/FCC have more items at other locations • BCC has same item in center of cube • FCC has same item centered on each side of cube

  7. SC: What do they look like? • BCC: • FCC:

  8. Which metals have which crystal lattices? • Simple cubic: Po • BCC: GI, 3B, 4B, Ba, Ra, Fe • FCC: VIIIB, IB, Al, In, Pb

  9. How many atoms per unit cell? • SC: each atom shared by 8 cubes • 8 corners of cube  1/8 of each corner atom w/in unit cell = 1 net atom/unit cell

  10. More on SC • Each atom touches one another along edge • Thus, each edge = 2r • Coordination number (# of atoms with which each atom is in direct contact) = 6 • Packing efficiency = fraction of volume occupied = 52%

  11. How many atoms per unit cell? (cont.) • BCC: 2 net atoms w/in unit cell (SC + 1 in center) • FCC: 6 faces of cube  ½ atom w/in unit cell = 3 atoms + 1 atom (SC) = 4 net

  12. More on BCC • Each atom does not touch another along edge • However, atoms touch along internal diagonal • Thus, each edge length = 4r/3 • Let’s derive this… • Coordination number (# of atoms with which each atom is in direct contact) = 8 • Central atom touches 8 atoms • Packing efficiency = fraction of volume occupied = 68%

  13. More on FCC • Each atom does not touch another along edge • However, atoms touch along face diagonal • Thus, each edge length = (22)r • Let’s derive this… • Coordination number (# of atoms with which each atom is in direct contact) = 12 • Packing efficiency = fraction of volume occupied = 74%

  14. Problems • Eu is used in TV screens. Eu has a BCC structure. Calculate the radius of a europium atom given a MW = 151.964 g/mol, a density of 5.264 g/cm3. • Iron has a BCC unit cell with a cell dimension of 286.65 pm. The density of iron is 7.874 g/cm3 and its MW = 55.847 g/mol. Calculate Avogadro’s number.

  15. CCP and HCP: Efficiency in Stacking • CCP = Cubic Close-Packing (it’s FCC) • HCP = Hexagonal Close-Packing • 74% packing efficiency

  16. Structures of ionic solids • Take a SC or FCC lattice of larger ions • Place smaller ions in holes w/in lattice • Smallest repeating unit = unit cell

  17. CsCl • SC unit cell • Cs+ in center of cube  Cubic hole • Surrounded by 1 Cl- (in 8 parts) • 1 Cs+ : 1 Cl- • Coordination # = 8 • Why SC and not BCC? • Because ion in center different from lattice pt ions

  18. LiCl • Notice: Li+ has octahedral geometry • Thus, cation in octahedral hole (between 6 ions) • Coordination # = 6 • FCC

  19. NaCl • FCC • Lattice has net 4 Cl-/unit cell • (8x1/8)+(6x1/2) = 4 • 1 Na+ in center of unit cell • 3 Na+ along edges of unit cell • (12x1/4) = 3 • Thus, net total of 4 Na+ ions • Total 4 Cl- : 4 Na+  1:1

  20. Tetrahedral holes • Each ion surrounded by 4 other oppositely-charged ions • Unit cell: 4 of each ion  total 8 ions • Coordination # = 4 • 8 tetrahedral holes in FCC unit cell • 4 by Zn2+ and 4 by S2- • Zn2+ occupies ½ of tetrahedral holes and surrounded by 4 S2- • S2- forms FCC unit cell

  21. ZnS

  22. ZnS

  23. Other Types of Solids: Network Solids • Array of covalently bonded atoms • Graphite, diamond, and silicon • The latter two  sturdy, hard, & high m.p.’s

  24. Graphite and diamond

  25. Other Types of Solids: Amorphous Solids • Glass & plastics • No regular structure • Break in all sorts of shapes • Long range of m.p.’s

More Related