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CHAPTER 16

CHAPTER 16. SOLIDS. CRYSTALS. Corresponding angles betw. faces on diff. crystals of the same subst. are always the same. Extensive properties of crystals vary; intensive properties remain the same ALL TRUE SOLID SUBSTS. ARE CRYSTALINE. CRYSTALS. Apparent exceptions:

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CHAPTER 16

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  1. CHAPTER 16 SOLIDS

  2. CRYSTALS • Corresponding angles betw. faces on diff. crystals of the same subst. are always the same. • Extensive properties of crystals vary; intensive properties remain the same • ALL TRUE SOLID SUBSTS. ARE CRYSTALINE.

  3. CRYSTALS • Apparent exceptions: • Some substs. seem like solids, but aren’t • Some crystals are too small to appear crystalline to the unaided eye.

  4. CRYSTALS • CRYSTALS - a rigid body in which the particles are arranged in a repeating pattern. • arrangement of units is determined by bonds betw. particles • bonding partially determines the props. of the crystal

  5. CRYSTALS • formed by repetition of identical units • There’s a relationship betw. the repeating units & the external shape of the crystal. • classified on the basis of external shape into 7 crystal systems. • may be determined by X-ray & e- diffraction patterns

  6. UNIT CELL • Simplest repeating unit in a crystal • Possible to have >1 kind of unit cell w/ the same shape • 14 diff. kinds of 3-D unit cells

  7. 3 of the Simplest Unit Cells 1.Simple Cubic - ea. particle has 6 immediate neighbors 2.Face Centered Cubic (FCC) - ea. particle has 12 immediate neighbors 3.Body Centered Cubic (BCC) - ea. particle has 8 immediate neighbors

  8. Space Lattice • 3-D arrangement of unit cells repeated over & over in a definite geometric arrangement • particles are very close together • Space lattices & unit cells are mental models • crystals are built of atoms, ions, or molecs.

  9. Compound Unit Cells • Usually makes up ionic crystals • NaCl is FCC - ea + ion surrounded by 6 - ions & ea - ions surrounded by 6 + ions • Structure is determined by the ratio of the radii of the ions.

  10. Compound Unit Cells • Simple Salts - formed by Group 1 metals and Group 17 nonmetals • Except for Cs, always have structures based on FCC lattice

  11. Closest Packing • Spheres layered & packed as close together as possible will make a lattice that is either hexagonal or face-centered cubic in the closest arrangement.

  12. Closest Packing • Hexagonal Closest Packing (HCP) - most frequently found in metals @ room temp. • Cubic Closest Packing (CCP) - same as FCC

  13. Closest Packing • Another structure - Body-Centered Cubic (BCC) - not as closely packed as the other 2 - openings betw. atoms are smaller, but more openings • found particularly in Group 1 metals • Packing can be changed by a change in temp.

  14. Elementary Crystals • Crystals made of atoms or molecules of the same element • 2 structures of Carbon - Diamond and Graphite • Graphite - atoms w/in ea. layer have hexagonal arrange. & held close by strong covalent bonds • layers are far apart & held by van der Waals forces

  15. Elementary Crystals • Diamond - ea. C atom is bonded covalently to 4 other C atoms • atoms are @ vertices of tetrahedron • diamond crystal is a single molecule

  16. Elementary Crystals • Nonmetals have low melting pts. • lattice positions are occupied by molecs. • atoms w/in molec are held together by covalent bonds • molecs are held to ea. other by weak van der Waals forces

  17. Network Crystals or Macromolecules • Consists of a single molec. w/ all component atoms bonded in a network fashion. • Very high melting pt. • To melt subst., covalent bonds must be broken • 10 X stronger than van der Waals forces

  18. ISOMORPHISM AND POLYMORPHISM • ISOMORPHOUS - crystals of diff. solids w/ the same structure and shape • POLYMORPHOUS - a single subst. having 2 or more crystalline shapes

  19. CRYSTAL DEFECTS • Perfect crystals are rare - most contain defects • 2 Basic types of defects • 1st type occurs w/in the unit cell • Ex. - in an ionic crystal, an ion may be missing or out of place • foreign subst. may be present

  20. CRYSTAL DEFECTS • 2nd type concerns manner in which unit cells are joined • Dislocations • Edge Dislocation - an extra layer of atoms extends part way into a crystal • Screw Dislocation- particles are slightly out of position • due to unequal growth of crystal

  21. SEMICONDUCTORS • Defects in crystals can be valuable • Doped crystals are used in semiconductor materials to make transistors • impurities are added deliberately • READ THIS SECTION!!!!

  22. Hydrated Crystals • When solids crystallize from water solns., molecs. of water may be incorporated into crystal • Hydrated Ions - ions which are chemically bonded to water molecs. • Hydrated Crystals - crystals containing hydrated ions

  23. Hydrated Crystals • Water can be removed by heating or lowering press. or both • Anhydrous - resulting comp. w/out water molecs. • some gain water easily & can be used to dry other substs. • drying agents - desiccants

  24. Hydrated Crystals • Hygroscopic - substs. which can pick up water from the air • ions have high attraction for water • dehydrated crystals will capture & hold water molecs

  25. Hydrated Crystals • Deliquescent - a hygroscopic subst. that takes up enough water from the air to dissolve & form a liquid soln. • Efflorescent - a subst. which releases water molecs. to the air from the crystal

  26. Liquid Crystals • Crystalline solids are highly ordered in 3 dimensions • Have sharp melting pt. @ specific temp • same as freezing pt. of liquid • Liquids are disordered in all 3 dimensions

  27. Liquid Crystals • Some solids lose order in only 1 or 2 dimensions @ melting pt. • @ higher temps., remaining order is destroyed • betw. these 2 transition temps., these substs. retain some degree of order • LIQUID CRYSTALS

  28. Liquid Crystals • Smectic Substs. - retains 2 dimensional order • Nematic Substs. - retains 1 dimensional order

  29. Liquid Crystals • Formed by long, rod-like molecs. arranged in a parallel manner • when attr. force betw. layers is overcome by energy, smectic material results • layers are still intact

  30. Liquid Crystals • When energy then overcomes end-to-end attracton, only parallel order remains • - nematic material

  31. Liquid Crystals • Mesomorphic Subst. - subst. in liquid crystal form • - betw. solid & liquid • Mesomorphic substs. exhibit anisotropy • Anisotropic Materials - show diff. props. in diff. directions • Graphite is anisotropic bec. of its bonding structure - layers

  32. Amorphous Materials • W/out crystalline form • Substs. which appear to be solids, but are not crystalline • disordered arrangement of particles • - glass is good example • no fixed melting pt. • softens & melts gradually

  33. Viscosity • The resistance of a liquid to flow • Glass & cold molasses - viscous materials • Water and alcohol - nonviscous liquid • Glass has no specific freezing pt - becomes more viscous & flows more slowly as it cools • Supercooled liquid

  34. Metastable • Substs. which can occur in long-lasting amorphous form • not most stable form, but not likely to change on its own • Ex. glass

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