材料学基础2.ppt

材料科学基础Fundamental of MaterialsFundamental of MaterialsProf:Tian Min Bo Tel:62795426，62772851 E-mail:Department of Material Science and EngineeringTsinghua University.Beijing 100084Lesson two1.2 Atomic bonding uu.Bonding forces and energiesFN:net forceFA:attractive forceFR:repulsive forceFor atomic systems:Bonding energies:uu.Primary interatomic bonds 1.Ionic bonding1.Ionic bonding Ionic bonding is always found in compounds that are composed of both metallic and nonmetallic elements,elements that are situated at the horizontal extremities of the periodic table.Interionic Forces Z1,Z2number of electrons removed or added from the atoms during the ion formation e electron charge a interionic separation distance 0permittivity of free space8.8510-12C2/(Nm2)Interionic EnergiesAttractive energyRepulsive energyExample problem 2.1Example problem 2.1 If the attractive force between a pair of Mg2+and S2-is 1.4910-8N and if the S2-ion has a radius of 0.184nm,calculate a value for the ionic radius of the Mg2+ion in nanometers.SolutionSolution2.Covalent bonding2.Covalent bonding Materials with covalent bonding are characterized by bonds that are formed by sharing of valence electrons among two or more atoms.3.Metallic bonding3.Metallic bonding Metallic bonding occurs in solid metals.In metals in solid state,atoms are packed relatively close together in a systematic pattern or crystal structure.uu.Secondary bonding （Van Der waals bonding)The driving force for secondary bonding is the attraction The driving force for secondary bonding is the attraction of the electric dipoles contained in atoms or molecules.of the electric dipoles contained in atoms or molecules.An electric dipole moment is created when two equal and An electric dipole moment is created when two equal and opposite charges are separated.opposite charges are separated.Neutral atomEVan Der waals bondingA dipole moment is defined as the charge value multiplied by the separation distance between positive and negative charges,orLondon forcesLondon forces If the interactions are between two dipoles that are induced in atoms or molecules,we refer to them as London forces.1.Van der waals interactionsDebye forcesDebye forces When an induced dipole interacts with a molecule that has a permanent dipole moment,we refer to this interaction as a Debye interaction.Keesom forcesKeesom forces If the interactions are between molecules that are permanently polarized,we refer to these as Keesom interaction.2.Secondary bondingFluctuating Dipole bondsFluctuating Dipole bonds These bonding forces arise because the asymmetrical distribution of electron charge distribution in these atoms creates electric dipoles.Permanent Dipole bondsPermanent Dipole bonds Permanent Dipole bonds exist between adjacent polar molecules.3.Mixed bondingIonic-Covalent Mixed bondingIonic-Covalent Mixed bonding Metallic-Covalent Mixed BondingMetallic-Covalent Mixed BondingMetallic-Ionic Mixed BondingMetallic-Ionic Mixed BondingXA and XB are the electronegativities of the atoms A and B in the compoundSummary Ionic bondingPrimary atomic bonds Covalent bonding Metallic bonding Permanent dipole bondsSecondary atomic bonds Fluctuating dipole bondsExamples and DiscussionsExerciseThank you!2