BONDING AND PROPERTIES

1 Chapter 2 -1 ISSUES TO What promotes BONDING ? What types of bonds are there? What PROPERTIES are inferred from BONDING ? BONDING AND PROPERTIESC hapter 2 -2 Atomic Structure (Freshman Chem.) atom electrons x 10-31kgprotonsneutrons atomic number= # of protons in nucleus of atom= # of electrons of neutral species A [=] atomic mass unit= amu = 1/12 mass of 12 CAtomic wt= wt of x 1023molecules or atoms1 amu/atom = 1g/molC etc.} x 10-27kgChapter 2 -3 Atomic Structure Valence electrons determine all of the following properties1)Chemical2)Electrical 3)Thermal4)OpticalChapter 2 -4 Electron Configurations Valence electrons those in unfilled shells Filled shells more stable Valence electrons are most available for BONDING and tend to control the chemical PROPERTIES example: C (atomic number = 6)1s22s22p2valence electronsChapter 2 -5 The Periodic Table Columns:Similar ValenceStructureAdapted from Fig.

2 , Callister elements:Readily give up electronsto become + elements:Readily acquire electronsto become - up 1egive up 2egive up 3einert gasesaccept 1eaccept 2eOSeTePoAtIBrHeNeArKrXeRnFClSLiBeHNaMgB aCsRaFrCaKScSrRbYChapter 2 -6 Ionic bond metal+ nonmetaldonates acceptselectrons electronsDissimilar electronegativities ex: MgOMg1s22s22p63s2O1s22s22p4[Ne] 3s2Mg2+1s22s22p6O2-1s22s22p6[Ne] [Ne] Chapter 2 -7 Occurs between + and - ions. Requires electron transfer. Large difference in electronegativity required. Example: NaClIonic BondingNa (metal) unstableCl (nonmetal) unstableelectron + -CoulombicAttractionNa (cation) stableCl (anion) stableChapter 2 -8 Ionic BONDING Energy minimum energy most stable Energy balance of attractiveand repulsivetermsAttractive energy EANet energy ENRepulsive energy ERInteratomic separation rrAnrBEN= EA+ ER= Adapted from Fig.

3 (b), Callister 2 -9 Predominant BONDING in CeramicsAdapted from Fig. , Callister 7e.(Fig. is adapted from Linus Pauling, The Nature of the Chemical Bond, 3rd edition, Copyright 1939 and 1940, 3rd edition. Copyright 1960 by Cornell : Ionic BondingGive up electronsAcquire electronsNaClMgOCaF2 CsClChapter 2 -10C: has 4 valence e-,needs 4 moreH: has 1 valence e-,needs 1 moreElectronegativitiesare from Fig. , Callister BONDING similar electronegativity share electrons bonds determined by valence s& porbitals dominate BONDING Example: CH4shared electrons from carbon atomshared electrons from hydrogen atomsHHHHCCH4 Chapter 2 -11 Adapted from Fig. , Callister BONDING Occurs between Metallic elements Ion Cores (+) surrounded by delocalized electrons (-) High electrical and thermal conductivity from free electrons Chapter 2 -12 Primary BONDING Metallic Bond--delocalized as electron cloud Ionic- covalent Mixed BONDING % ionic character= where XA& XBare Pauling electronegativities%)100(x 1 e (XA XB)24 ionic (100%) x e1 characterionic %4) ( 2= = Ex: MgOXMg= 2 -13 Arises from interaction between dipoles Permanent dipoles-molecule induced Fluctuating dipoles-general case:-ex: liquid HCl-ex: polymerAdapted from Fig.)

4 , Callister from Fig. ,Callister BONDING asymmetric electronclouds+-+-secondary bondingHHHHH2H2secondary bondingex: liquid H2 HClHClsecondary bondingsecondary BONDING +-+-secondary bondingChapter 2 -14 TypeIonicCovalentMetallicSecondaryBond EnergyLarge!Variablelarge-Diamondsmall-B ismuthVariablelarge-Tungstensmall-Mercur ysmallestCommentsNondirectional (ceramics)Directional(semiconductors, ceramicspolymer chains)Nondirectional (metals)Directionalinter-chain (polymer)inter-molecularSummary: BondingChapter 2 -15 Bond length, r Bond energy, Eo Melting Temperature, TmTmis larger if Eois From BONDING : TmrorEnergyrlarger Tmsmaller TmEo= bond energy Energyrorunstretched lengthChapter 2 -16 Coefficient of thermal expansion, ~ symmetry at ro is larger if Eois From BONDING : = (T2-T1) LLocoeff.

5 Thermal expansion Llength, Lounheated, T1heated, T2rorlarger smaller Energyunstretched lengthEoEoChapter 2 -17 Ceramics(Ionic & covalent BONDING ):Metals(Metallic BONDING ):Polymers( covalent & Secondary):Large bond energylarge Tmlarge Esmall Variable bond energymoderate Tmmoderate Emoderate Directional PropertiesSecondary BONDING dominatessmall Tmsmall Elarge Summary: Primary BondsChapter 3 -18 ISSUES TO How do atoms assemble into solid structures?(for now, focus on metals) How does the density of a material depend onits structure? When do material PROPERTIES vary with thesample ( , part) orientation?The Structure of Crystalline SolidsChapter 3 -19 Non dense, randompacking Dense, orderedpackingDense, ordered packed structures tend to havelower and PackingEnergy rtypical neighborbond lengthtypical neighborbond energyEnergyrtypical neighborbond lengthtypical neighborbond energyChapter 3 -20 atoms pack in periodic, 3D ceramics-some polymers atoms have no periodic structures-rapid coolingcrystalline SiO2noncrystalline SiO2"Amorphous" = NoncrystallineAdapted from Fig.

6 (b),Callister from Fig. (a),Callister and PackingSiOxygen typical of: occurs for:Chapter 3 -21 Crystal Systems7 crystal systems14 crystal latticesFig. , Callister cell:smallest repetitive volume which contains the complete lattice pattern of a , b, and care the lattice constantsChapter 3 -22 Metallic Crystal Structures How can we stack metal atoms to minimize empty space? stack these 2-D layers to make 3-D structuresChapter 3 -23 Tend to be densely packed. Reasons for dense packing:- Typically, only one element is present, so all atomicradii are the Metallic BONDING is not Nearest neighbor distances tend to be small inorder to lower bond Electron cloud shields cores from each other Have the simplest crystal will examine three such Crystal StructuresChapter 3 -24 Rare due to low packing denisty(only Po has this structure) Close-packed directionsare cube edges.

7 Coordination #= 6(# nearest neighbors)(Courtesy Anderson)Simple Cubic Structure (SC)Chapter 3 -25 APF for a simple cubic structure = = a343 ( )31atomsunit cellatomvolumeunit cellvolumeAtomic Packing Factor (APF)APF = Volume of atoms in unit cell*Volume of unit cell*assume hard spheresAdapted from Fig. ,Callister directionsaR= 8 x 1/8 = 1 atom/unit cellChapter 3 -26 Coordination # = 8 Adapted from Fig. ,Callister 7e.(Courtesy Anderson) Atoms touch each other along cube : All atoms are identical; the center atom is shadeddifferently only for ease of Centered Cubic Structure (BCC)ex: Cr, W, Fe ( ), Tantalum, Molybdenum2 atoms/unit cell: 1 center + 8 corners x 1/8 Chapter 3 -27 Atomic Packing Factor: BCCaAPF = 43 ( 3a/4)32atomsunit cellatomvolumea3unit cellvolumelength = 4R=Close-packed directions:3 a APF for a body-centered cubic structure = from Fig.

8 (a), Callister 3 -28 Coordination # = 12 Adapted from Fig. , Callister 7e.(Courtesy Anderson) Atoms touch each other along face : All atoms are identical; the face-centered atoms are shadeddifferently only for ease of Centered Cubic Structure (FCC)ex: Al, Cu, Au, Pb, Ni, Pt, Ag4 atoms/unit cell: 6 face x 1/2 + 8 corners x 1/8 Chapter 3 -29 APF for a face-centered cubic structure = Packing Factor: FCCmaximum achievable APFAPF = 43 ( 2a/4)34atomsunit cellatomvolumea3unit cellvolumeClose-packed directions: length = 4R=2 aUnit cell contains:6 x1/2 + 8 x1/8 = 4 atoms/unit cella2 aAdapted fromFig. (a),Callister 3 -30 Coordination # = 12 Stacking Sequence APF = 3D Projection 2D ProjectionAdapted from Fig. (a),Callister Close-Packed Structure (HCP)6 atoms/unit cellex: Cd, Mg, Ti, Zn c/a= sitesB sitesA sitesBottom layerMiddle layerTo playerChapter 3 -31 Theoretical Density, wheren= number of atoms/unit cellA= atomic weight VC= Volume of unit cell = a3for cubicNA= Avogadro s number= x 1023atoms/molDensity = =VCNAn A =CellUnitofVolumeTotalCellUnitinAtomsofM assChapter 3 -32 Ex: Cr (BCC) A= g/molR= nmn= 2 theoreticala= 4R/ 3 = nm actualaR = cellmolgunit Density, = g/cm3= g/cm3 Chapter 3 -33 Densities of Material Classes metals> ceramics> polymersWhy?

9 Data from Table B1, Callister 7e. (g/cm )3 Graphite/ Ceramics/ SemicondMetals/ AlloysComposites/ fibersPolymers122030 Based on data in Table B1, Callister *GFRE, CFRE, & AFRE are Glass,Carbon, & Aramid Fiber-ReinforcedEpoxy composites (values based on60% volume fraction of aligned fibersin an epoxy matrix). ,NiTin, ZincSilver, MoTantalumGold, WPlatinumGraphiteSiliconGlass-sodaConcre teSi nitrideDiamondAl oxideZirconiaHDPE, PSPP, LDPEPCPTFEPETPVCS iliconeWoodAFRE*CFRE*GFRE*Glass fibersCarbon fibersAramid close-packing(metallic BONDING ) often large atomic less dense packing often lighter low packing density(often amorphous) lighter elements (C,H,O) intermediate valuesIn generalChapter 3 -34 Someengineering applications require single crystals: PROPERTIES of crystalline materials often related to crystal structure.

10 (Courtesy Anderson)--Ex: Quartz fractures more easily along some crystal planes than singlecrystals for abrasives--turbine bladesFig. (c), Callister 7e.(Fig. (c) courtesyof Pratt and Whitney).(Courtesy Martin Deakins,GE Superabrasives, Worthington, OH. Used with permission.)Crystals as Building BlocksChapter 3 -35 Mostengineering materials are polycrystals. Nb-Hf-W plate with an electron beam weld. Each "grain" is a single crystal. If grains are randomly oriented,overall component PROPERTIES are not directional. Grain sizes typ. range from 1 nm to 2 cm( , from a few to millions of atomic layers).Adapted from Fig. K, color inset pages of Callister 5e.(Fig. K is courtesy of Paul E. Danielson, Teledyne Wah Chang Albany)1 mmPolycrystalsIsotropicAnisotropicChapte r 3 -36 Single Crystals- PROPERTIES vary withdirection: : the modulusof elasticity (E) in BCC iron: Polycrystals- PROPERTIES may/may notvary with grains are randomlyoriented: isotropic.