Types of Primary Chemical Bonds - PowerPoint PPT Presentation

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Types of Primary Chemical Bonds

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  1. + - + - + - + - + + + + e- e- + + + e- + + + Types of Primary Chemical Bonds Isotropic, filled outer shells • Metallic • Electropositive: give up electrons • Ionic • Electronegative/Electropositive • Colavent • Electronegative: want electrons • Shared electrons along bond direction Close-packed structures

  2. Metals • single element, fairly electropositive • elements similar in electronegativity

  3. Ionic Compounds • elements differing in electronegativity anion cation Ceramics

  4. Covalent Compounds sp3 s2p2 s2p4 s2p3 s2p1 s2 semi-conductors

  5. diamond         Hybridized Bonds • Elemental carbon (no other elements) sp3 hybridization     also methane: CH4 one s + three p orbitals  4 (x 2) electron states (resulting orbital is a combination)

  6. Covalent Structures  both species tetrahedral Recall: zinc blende ZnS: +2 -2 GaAs: +3 -3 single element: C or Si or Sn or sp3 S Zn diamond

  7. graphite                                     Another way to hybridize • Elemental carbon (no other elements) sp2 hybridization     trigonal symmetry one s + two p orbitals  3 (x 2) electron states (resulting orbital is a combination) one unchanged p orbital

  8. Forms of carbon with sp2 bonds Graphite* Nobel Prize Chemistry, 1996 Fullerene Nobel Prize Physics, 2010 Graphene Nanotube source: Wikipedia * http://www.electronics-cooling.com/assets/images/2001_August_techbrief_f1.jpg

  9. Structural Characteristics • Metals • Close-packed structures (CN = 12) • Slightly less close-packed (CN = 8) • Ionic structures • Close-packed with constraints • CN = 4 to 8, sometimes 12 • Covalent structures • Not close-packed, bonding is directional • Any can be strongly or weakly bonded (Tm)

  10. Diamond vs. CCP 8 atoms/cell, CN = 4 4 atoms/cell, CN = 12 ½ tetrahedral sites filled

  11. Cl Na Avogardo’s # Computing density • Establish unit cell contents • Compute unit cell mass • Compute unit cell volume • Unit cell constant, a, given (or a and c, etc.) • Or estimate based on atomic/ionic radii • Compute mass/volume, g/cc • Example: NaCl • Contents = 4 Na + 4 Cl • Mass = 4(atom mass Na + atomic mass Cl)/No • Vol = a3 • Units =

  12. Quartz (SiO2) Single Crystal vs. Polycrystalline Rb3H(SO4)2 Diamond Ba(Zr,Y)O3-d Regions of uninterrupted periodicity amalgamated into a larger compact Periodicity extends uninterrupted throughout entirety of the sample External habit often reflects internal symmetry = grains delineated by grain boundaries

  13. Isotropic vs. Anisotropic graphite* diamond polycrystalline averaging * http://www.electronics-cooling.com/assets/images/2001_August_techbrief_f1.jpg

  14. + - + - + - + - + + + + e- e- + + + e- + + + Types of Bonds  Types of Materials Isotropic, filled outer shells • Metallic • Electropositive: give up electrons • Ionic • Electronegative/Electropositive • Colavent • Electronegative: want electrons • Shared electrons along bond direction Metals Ceramics Semi-Conductors Close-packed structures

  15. What’s Missing? units many Polymers methane H Long chain molecules with repeated units Molecules formed by covalent bonds Secondary bonds link molecules into solids C H C H C H H http://en.wikipedia.org/wiki/File:Polyethylene-repeat-2D.png

  16. Polymer Synthesis H H C=C H H • Traditional synthesis • Initiation, using a catalyst that creates a free radical • Propagation • Termination unpaired electron  R – C – C  R  + C=C  R……C – C – C – C  R…… C – C  + C=C  R –(C-C)n– R R…… C – C  +  C – C……R

  17. width is a measure of polydispersity # of polymer chains molecular weight Polydispersity • Traditional synthesis  large variation in chain length Average chain molecular weight molecular weight number average # of polymer chains of Mi total number of chains weight average = weight fraction weight of polymer chains of Mi total weight of all chains by number • Degree of polymerization • Average # of mer units/chain mer molecular weight by weight

  18. width is a measure of polydispersity Polydispersity • Traditional synthesis  large variation in chain length Average chain molecular weight molecular weight number average # of polymer chains of Mi total number of chains # of polymer chains weight average = weight fraction weight of polymer chains of Mi total weight of all chains molecular weight by number • Degree of polymerization • Average # of mer units/chain mer molecular weight by weight

  19. New modes of synthesis • “Living polymerization” • Initiation occurs instantaneously • Chemically eliminate possibility of random termination • Polymer chains grow until monomer is consumed • Each grows for a fixed (identical) period

  20. Polymers • Homopolymer • Only one type of ‘mer’ • Copolymer • Two or more types of ‘mers’ • Block copolymer • Long regions of each type of ‘mer’ • Bifunctional mer • Can make two bonds, e.g. ethylene  linear polymer • Trifunctional mer • Can make three bonds  branched polymer

  21. Polymer Configurations H H C = C C C C C H H • Linear • Branched • Cross-linked C C C C C

  22. Polymers H H C = C H in H out H H C C C C 109.5° C C C C C R Placement of side groups is fixed once polymer is formed Example side group: styrene R =

  23. R R R R Cl H C = C C C C C H H C C C C C R R R R C C C C Syndiotactic C C C C C R R R R C C C C C C C C C Isotactic   Atactic

  24. Thermal Properties • Thermoplastics • Melt (on heating) and resolidify (on cooling) • Linear polymers • Thermosets • Soften, decompose irreversibly on heating • Crosslinked • Crystallinity • Linear: more crystalline than branched or crosslinked • Crystalline has higher density than amorphous

  25. c a b b g a Formal Crystallography • Crystalline • Periodic arrangement of atoms • Pattern is repeated by translation • Three translation vectors define: • Coordinate system • Crystal system • Unit cell shape • Lattice points • Points of identical environment • Related by translational symmetry • Lattice = array of lattice points • space filling • defined by 3 vectors • parallelipiped • arbitrary coord system • lattice pts at corners +

  26. hcp ccp (fcc) bcc Cubic unit cells Specify: a Cubic implies: |a1| = |a2| = |a3| = a a = b = g = 90° Hexagonal unit cell Specify: a, c Hexagonal implies: |a1| = |a2| = a g = 120° a = b = 90° But the two types of cubic unit cells are different!

  27. a, b, c, a, b, g – all arbitrary C or A centered for b = arbitrary a, b, c – arbitrary a= g = 90 a, b, c – arbitrary 6 or 7 crystal systems a, c – arbitrary b = a a = b = 90 14 lattices a – arbitrary; a = b = c a– arbitrary;a=b=g a, c – arbitrary a – arbitrary

  28. Centered Lattices unconventional choice conventional choice b b a a b b a a conventional choice unconventional choice unconventional is primitive conventional is centered both are primitive cells

  29. More on Lattices X

  30. More on Lattices X

  31. Lattice types of some structures

  32. Lattice types? CsCl Structure BCC Metal How many lattice points per unit cell?

  33. Lattice types? Fluorite Zinc blende (sphaelerite)

  34. Lattice types? A M O Diamond Perovskite: AMO3