chapter2

chapter2 - CHAPTER 2C HAPTER 2 CHAPTER 2 CHAPTER 2...

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Unformatted text preview: CHAPTER 2C HAPTER 2 CHAPTER 2 CHAPTER 2 INTERATOMIC FORCESI NTERATOMIC FORCES INTERATOMIC FORCES INTERATOMIC FORCES What kind of force holds the atoms together in a solid? Interatomic Binding r Allofthemechanismswhichcausebondingbetweentheatoms derivefromelectrostaticinteractionbetweennucleiandelectrons. r Thedifferingstrengthsanddifferingtypesofbondaredeterminedby theparticularelectronicstructuresoftheatomsinvolved. r Theexistenceofastablebondingarrangementimpliesthatthe spatialconfigurationofpositiveioncoresandouterelectronshasless totalenergythananyotherconfiguration(includinginfiniteseparation oftherespectiveatoms). r Theenergydeficienceoftheconfigurationcomparedwithisolated atomsisknownascohesiveenergy,andrangesinvaluefrom0.1 eV/atomforsolidswhichcanmusteronlytheweakvanderWaalsto 7ev/atomormoreinsomecovalentandioniccompoundsandsome metals. Energies of Interactions E nergies of Interactions Energies of Interactions Energies of Interactions Between AtomsB etween Atoms Between Atoms Between Atoms r Theenergyofthecrystalislowerthanthatofthefree atomsbyanamountequaltotheenergyrequiredto pullthecrystalapartintoasetoffreeatoms.Thisis calledthebinding(cohesive)energyofthecrystal. aCl is more stable than a collection of free Na and Cl. c NaCl is more stable than a collection of free Na and Cl. c Ge crystal is more stable than a collection of free Ge. Cl Na NaCl r This typical curve has a minimum at equilibrium distance R r R > R 0 ; c the potential increases gradually, approaching 0 as R b e force is attractive V(R) Repulsive c the force is attractive r R < R ; c the potential increases very rapidly, approaching at small separation. c the force is repulsive R r R R Attractive r Forcebetweentheatomsisthenegativeoftheslopeofthiscurve. Atequlibrium,repulsiveforcebecomesequalstotheattractivepart. The potential energy of either atom will be given by: or simply: V=decreaseinpotentialenergy+increaseinpotentialenergy (duetoattraction) (duetorepulsion) () m n a b Vr r r = + V(r): thenetpotentialenergyofinteractionasfunctionofr r: thedistancebetweenatoms,ions,ormolecules a,b: proportionalityconstantofattractionandrepulsion, respectively m , n: constantcharacteristicsofeachtypeofbondandtype ofstructure Types of Bonding Mechanisms Itisconventionaltoclassifythebondsbetween atomsintodifferenttypesas r Ionic, r Covalent, r Metallic, r VanderWaals, r Hydrogen. All bonding is a consequence of the electrostatic interaction between nucleiand electronsobeyingSchrdingersequation. 1 1 1 1 1 1 1 1 -IONIC BONDING IONIC BONDINGIONIC BONDING IONIC BONDINGIONIC BONDINGI ONIC BONDING...
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chapter2 - CHAPTER 2C HAPTER 2 CHAPTER 2 CHAPTER 2...

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