Instructor: Dr. Upali Siriwardanee-mail:upali@latech.eduOffice:CTH 311 Phone 257-4941Office Hours:M,W 8:00-9:00 & 11:00-12:00 am;Tu,Th, F9:30-11:30a.m.April 4 , 2017: Test 1 (Chapters 1, 2, 3, 4)April 27, 2017:Test 2 (Chapters(6& 7)May 16, 2016:Test 3 (Chapters. 19 & 20)May 17,Make Up: Comprehensive covering all Chapters
Chemistry 481(01) Spring 2017
Chapter3. Structuresof simple solids
Crystalline solids: The atoms, molecules or ions pack together in an ordered arrangementAmorphous solids:No ordered structure to the particles of the solid. No well defined faces, angles or shapesPolymeric Solids: Mostly amorphous but some have localcrystiallnity. Examples would include glass and rubber.
The Fundamental types of Crystals
Metallic: metalcationsheld together by a sea of electronsIonic:cationsand anions held together by predominantly electrostatic attractionsNetwork: atoms bonded together covalently throughout the solid (also known as covalent crystal or covalent network).Covalent or Molecular: collections of individual molecules; each lattice point in the crystal is a molecule
Metallic Structures
Metallic Bonding in the Solid State:Metalsthe atoms have lowelectronegativities; therefore the electrons are delocalized over all the atoms.Wecan think of the structure of a metal as an arrangement of positive atom cores in a sea of electrons. For a more detailed picture see "Conductivity of Solids".Metallic:Metalcationsheld together by a sea ofvalaneceelectrons
Packing andGeometry
Close packingABC.ABC...cubic close-packed CCPgives face centered cubic or FCC(74.05% packed)AB.AB... or AC.AC... (these are equivalent). This is called hexagonal close-packing HCP
CCP
HCP
Loose packingSimple cube SCBody-centered cubic BCC
Packing and Geometry
The Unit Cell
The basic repeat unit that build up the whole solid
Unit Cell Dimensions
The unit cell angles are defined as:a, the angle formed by the b and c cell edgesb, the angle formed by the a and c cell edgesg, the angle formed by the a and b cell edgesa,b,c is x,y,z in right handed cartesian coordinates
agbacba
BravaisLattices & Seven Crystals Systems
In the 1840’sBravaisshowed that there are onlyfourteendifferent space lattices.Taking into account the geometrical properties of the basis there are 230 different repetitive patterns in which atomic elements can be arranged to form crystal structures.
FourteenBraviasUnit Cells
Seven Crystal Systems
Number of Atoms in the Cubic Unit Cell
Coner- 1/8Edge- 1/4Body- 1Face-1/2FCC = 4 ( 8 coners, 6 faces)SC = 1 (8 coners)BCC = 2 (8 coners, 1 body)
Face-1/2
Coner- 1/8
Edge - 1/4
Body- 1
Close Pack UnitCells
CCP
HCP
FCC = 4 ( 8 coners, 6 faces)
Simple cube SC Body-centered cubic BCC
Unit Cells from Loose Packing
SC = 1 (8 coners)
BCC = 2 (8 coners, 1 body)
Coordination Number
The number of nearest particles surrounding a particle in the crystal structure.Simple Cube:a particle in the crystal has a coordination number of6BodyCenterdCube: a particle in the crystal has a coordination number of8Hexagonal Close Pack &Cubic Close Pack:a particle in the crystal has a coordination number of12
Holes in FCC Unit Cells
Tetrahedral Hole(8 holes)Eight holes are inside a face centered cube.Octahedral Hole(4 holes)One hole in the middle and 12 holes along theedges( contributing 1/4) of the face centered cube
Holes in SC Unit Cells
Cubic Hole
OctahedralHole in FCC
Octahedral Hole
TetrahedralHole in FCC
Tetrahedral Hole
Structure of Metals
Crystal LatticesA crystal is a repeating array made out of metals. In describing this structure we must distinguish between the pattern of repetition (the lattice type) and what is repeated (the unit cell) described above.
Polymorphism
Metals are capable of existing in more than one form at a timePolymorphism is the property or ability of a metal to exist in two or more crystalline forms depending upon temperature and composition. Most metals and metal alloys exhibit this property.
Uranium is a good example of a metal that exhibits polymorphism.
Alloys
SubstitutionalSecond metal replaces the metalatoms in the latticeInterstitialSecond metal occupiesinterstitial space (holes)in the lattice
Properties of Alloys
Alloying substances are usually metals or metalloids. The properties of an alloy differ from the properties of the pure metals or metalloids that make up the alloy and this difference is whatcreates theusefulness of alloys. By combining metals andmetalloids, manufacturers can develop alloys that have the particular properties required for a given use.
Structure of Ionic Solids
Crystal LatticesA crystal is a repeating array made out of ions. In describing this structure we must distinguish between the pattern of repetition (the lattice type) and what is repeated (the unit cell) described above.Cationsfit into the holes in theanioniclattice since anions are lager thancations.Incases wherecationsare bigger thananionslattice is considered to be made up ofcationiclattice with smaller anions filling the holes
Basic Ionic Crystal Unit Cells
Radius Ratio Rules
r+/r- Coordination Holes in WhichRatio NumberPositiveIons Pack0.225 - 0.414 4tetrahedral holes FCC0.414 - 0.732 6 octahedralholes FCC0.732 - 1 8 cubicholes BCC
Cesium ChlorideStructure (CsCl)
Rock Salt (NaCl)
© 1995 by the Division of Chemical Education, Inc., American Chemical Society.Reproduced with permission from Soli-State Resources.
Sodium ChlorideLattice (NaCl)
NaClLattice Calculations
CaF2
Calcium Fluoride
© 1995 by the Division of Chemical Education, Inc., American Chemical Society.Reproduced with permission from Solid-State Resources.
ZincBlendeStructure (ZnS)
Lead Sulfide
© 1995 by the Division of Chemical Education, Inc., American Chemical Society.Reproduced with permission from Solid-State Resources.
WurtziteStructure (ZnS)
Summary of UnitCells
Volume of a sphere = 4/3pr3
Volume of sphere in SC = 4/3p(½)3= 0.52Volume of sphere in BCC = 4/3p((3)½/4)3= 0.34Volume of sphere in FCC = 4/3p( 1/(2(2)½))3= 0.185
Density Calculations
Aluminumhas accp(fcc)arrangement of atoms. The radius of Al = 1.423Å ( = 143.2pm). Calculate the lattice parameter of the unit cell and the density of solid Al (atomic weight = 26.98).Solution:4 atoms/cell [8 at corners (each 1/8), 6 in faces (each 1/2)]Lattice parameter:a/r(Al) =2(2)1/2a =2(2)1/2(1.432Å)=4.050Å= 4.050 x 10-8cmDensity= 2.698g/cm3
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