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 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 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 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 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