工學(xué)材料科學(xué)基礎(chǔ)英文版2課件

1、Chapter 1 Atomic Structure and Interatomic Bonding 11. IntroductionSome of the important properties of solid materials depend on geometrical atomic arrangements.The properties of materials are controllable and can be tailored to the needs of a given application by controlling their structure and com

2、position.We can examine and describe the structure of materials at different levels.2Subatomic level: Electronic structure of individual atoms that defines interaction among atoms (interatomic bonding).Atomic level: Arrangement of atoms in materials.Nanostructure: the structure of material at a leng

3、th-scale of 100nm.Microstructure: the structure of material at a length-scale of 10 to 1000nm. Macrostructure: the structure of a material at a macroscopic level where the length-scale is 1000,000 nm. 3Length-scalesAngstrom = 1 = 1/10,000,000,000 meter = 10-10 mNanometer = 10 nm = 1/1,000,000,000 me

4、ter = 10-9 mMicrometer = 1m = 1/1,000,000 meter = 10-6 mMillimeter = 1mm = 1/1,000 meter = 10-3 mInteratomic distance a few A human hair is 50 mElongated bumps that make up the data track on CD are 0.5 m wide, minimum 0.83 m long, and 125 nm high45This micrograph, which represents the surface of a g

5、old specimen, was taken with a atomic force microscope (AFM). Individual atoms for this (111) crystallographic surface plane are resolved. 6Amorphous: lack a long-range ordering of atoms or ions.Crystalline: exhibit periodic arrangements of atoms or ions. The long-range atomic order is in the form o

6、f atoms or ions arranged in a three dimensional pattern that repeats over much larger distances (from 100 nm to up to few cm).Short-range atomic arrangements: the atoms of ions show a particular order only over relatively short distances.72. Atomic Structure Atomic structure influences how atoms are

7、 bonded together. An understanding of this helps categorize material as metals, semiconductors, ceramics, or polymers. Atoms = nucleus (protons and neutrons) + electrons8Charges: Electrons and protons have negative and positive charges of the same magnitude, 1.6 10-19 Coulombs.Neutrons are electrica

8、lly neutral.Masses: Protons and Neutrons have the same mass, 1.67 10-27 kg.Mass of an electron is much smaller, 9.11 10-31 kg and can be neglected in calculation of atomic mass.The atomic mass (A) = mass of protons + mass of neutrons# protons gives chemical identification of the element# protons = a

9、tomic number (Z)# neutrons defines isotope number9There are 92 naturally occurring elements, each identified by the atomic number (number of protons) and atomic weight (which includes the weight of the neutrons) and represents an average over the various isotopes that may exist). The atomic weight h

10、as units of grams per mole. A mole is the amount of material that corresponds to the atomic weight. A mole is the amount of matter that has a mass in grams equal to the atomic mass in amu of the atoms (A mole of carbon has a mass of 12 grams).The number of atoms in a mole is called the Avogadro numb

11、er, Nav = 6.023 1023.10THE PERIODIC TABLE113. Electronic in AtomsBohr atomic modelOrbital electron12Quantum numbers are the numbers in an atom that assign electrons to discrete energy levels.Principal quantum number n: is assigned integral values 1,2,3,4,5that refer to the quantum shell to which the

12、 electron belongs.Orbital (Azimuthal) quantum number l: describe the energy levels in each quantum shell. l=0,1,2,n-1. Magnetic quantum number ml: describes the number of energy levels for each orbital quantum number. l, +lSpin quantum number ms: assigned values +1/2 and -1/2 and reflects the differ

13、ent electronic spinsThe number of possible energy levels is determined by the first three quantum numbers.13The complete set of quantum numbers for each of the 11 electrons in sodium. 3s1 electron 11 n=3, l=0, ml=0, ms=+1/2 or -1/2 electron 10 n=2, l=1, ml=+1, ms= -1/2 electron 9 n=2, l=1, ml=+1, ms

14、=+1/2 2p6 electron 8 n=2, l=1, ml=0, ms= -1/2 electron 7 n=2, l=1, ml=0, ms=+1/2 electron 6 n=2, l=1, ml=-1, ms= -1/2 electron 5 n=2, l=1, ml=-1, ms=+1/2 2s2 electron 4 n=2, l=0, ml=0, ms= -1/2 electron 3 n=2, l=0, ml=0, ms=+1/2 1s2 electron 2 n=1, l=0, ml=0, ms= -1/2 electron 1 n=1, l=0, ml=0, ms=+

15、1/214The pattern used to assign electrons to energy levelsl=0(s)l=1(p)l=2(d)l=3(f)l=4(g)l=5(h)n=1(K)2n=2(L)26n=3(M)2610n=4(N)261014n=5(O)26101418n=6(P)2610141822Note: 2,6,10,14,refer to the number of electrons in the energy level.15This sequence emphasizes the relative energy levels of the shells so

16、 that the outer, higher energy and more asymmetric d levels may fill after the inner s level of the next shell and more asymmetric levels may fill after the inner p and even the d level of the next shell. 16 have complete s and p subshells tend to be unreactive.Stable electron configurations.Stable

17、Electron Configurations17Valence: The number of electrons in an atom that participate in bonding or chemical reactions. Usually, the valence is the number of electrons in the outer s and p energy levels. Mg: 1s22s22p6 valence=2Al: 1s22s22p6 valence=3Ge: 1s22s22p63s23p63d10 valence=4If an atom has a

18、valence of zero, the element is inert (non-reactive). 1s22s22p6 Valence also depends on the immediate environment surrounding the atom or the neighboring atoms available for bonding. For Example, P2O3, PH3.3s23s23p14s24p23s23p618Electronegativity is the quantitative description of an atoms desire to

19、 gain or lose an electron. It is a function of the number of electrons in an atoms valence shell, and the distance of the shell from the nucleus. For example, chlorine, with 7 valence electrons, is very eager to gain an electron to fill its outer shell, while sodium will easily give up its 1 valence

20、 electron.4. The Periodic Table19The Periodic Table20Electronegativity increases as you go right and up on the periodic table.21 Columns: Similar Valence StructureElectropositive elements:Readily give up electronsto become + ions.Electronegative elements:Readily acquire electronsto become - ions.The

21、 Periodic TableHe Ne Ar Kr Xe Rn inert gases accept 1e accept 2e give up 1e give up 2e give up 3e F Li Be Metal Nonmetal Intermediate H Na Cl Br I At O S Mg Ca Sr Ba Ra K Rb Cs Fr Sc Y Se Te Po 225. Atomic Bonding in solidThe attractive force varies inversely with the square of the distance between

22、the atoms for all of the various types of bonds. This force pulls the atoms together with a greater force as they get closer together. A repulsive force arises when the electron clouds of negative charge meet. This force increases much faster with distance, with an exponent in the range 6-9 that dep

23、ends on the particular atom and its electron shells. When the sum of these forces is zero, the distance between the atoms is at the equilibrium value 23Both the attractive and repulsive forces increase as the atoms are brought closer together, and the sum of the two is zero at the equilibrium point.

24、 The slope of the force vs. distance curve at the equilibrium point defines the force needed to pull the atoms slightly apart and is the slope of the stress vs. strain curve. This slope is the modulus of elasticity.24The Bond energy vs. distance curve is the integral of the bond force curve. The low

25、est energy is the equilibrium point. 25The depth of the minimum is the total bond strength which reflects the energy required to pull the atoms completely apart. The deeper the potential minimum, the higher the melting temperature. The bond energy curve is asymmetric. This is why most materials expa

26、nd when heated. Because of the asymmetry of the bond energy curve, the average distance between atoms increases with temperature. The narrower the potential minimum, the lower the expansion coefficient. 26Atomic Bonding in solidThere are four principal kinds of bonds that form between atoms: Metalli

27、c bond Ionic bond Covalent bond Van der Waals bondStrong BondWeak BondThe forces between atoms are electrostatic dependent directly on the electrons that surround the atoms.The different bond types are characterized by how electrons are shared which controls the geometry of the atom packing, and by

28、the relative strength of the bond .about 1/100th as strong27 Arises from a sea of donated valence electrons (1, 2, or 3 from each atom). Primary bond for metals and their alloysMetallic Bonding28The Metallic Bond: shared valence electrons forming a highly mobile electron sea. The metallic bond is fo

29、rmed between atoms that have a low value of electronegativity and easily give up their outer (valence) electrons. These form an electron sea that glues the positive ion cores (nuclei and inner electrons) together and shields the positive cores from each other.29The metallic bond has the following ch

30、aracteristics:Electrons shared among all atomsNo directionality - desire for the largest number of nearest neighborsHigh strength (slightly less than covalent or ionic; 25-200 kcal/mol)Forms between atoms with low electronegativity30 Requires shared electrons Example: CH4C: has 4 valence e, needs 4

31、moreH: has 1 valence e, needs 1 moreElectronegativities are comparable.Covalent Bonding31The Covalent bond: is formed by a sharing of valence electrons between two adjacent atoms.Convention shows this as discrete electrons in their respective orbits located between the atoms.A more realistic model s

32、hows the electrons as a cloud of negative charge between the atoms. 32 Molecules with nonmetals Molecules with metals and nonmetals Elemental solids (RHS of Periodic Table) Compound solids (about column IVA)Adapted from Fig. 2.7, Callister 6e. (Fig. 2.7 isadapted from Linus Pauling, The Nature of th

33、e Chemical Bond, 3rd edition, Copyright 1939 and 1940, 3rd edition. Copyright 1960 by Cornell University.Examples: Covalent Bonding33Since they are negative, the Covalent bonds repel each other, and this results in their staying as far apart as possible, producing the characteristic dihedral angle i

34、n 3D.Dihedral Angle: Because the electron clouds localized in the covalent bonds repel each other instead of lying in a plane, they can get farther apart in three dimensions by pointing to opposite corners of a cube. 34The length of the face diagonal in this cube is . In the diagonal plane through t

35、he cube, the sides of the right triangle define the angle as shown. Consequently, the dihedral angle between the bonds is 109.5 degrees. 35Covalent bonds are very strong. As a result, covalently bonded materials are very strong and hard.The electrical conductivity of many covalently bonded materials

36、 (i.e., silicon, diamond, and many ceramics) is not high since the valence electrons are locked in bonds between atoms and are not readily available for conduction.36The covalent bond has the following characteristics:Electrons shared among two adjacent atoms Strong directionality - number of neighb

37、ors limited High strength (slightly less than ionic; 125-300 kcal/mol) Forms between atoms with high electronegativity37 Occurs between + and - ions. Requires electron transfer. Large difference in electronegativity required. Example: NaClIonic Bonding38The Ionic Bond: valence electrons transferred

38、between two atoms. The Ionic bond is an electrostatic attraction between positively and negatively charged ions. These bonds are formed by the transfer of electrons from one atom with a low electronegativity to a different atom with a high electronegativity.39The example shows sodium (Na11) and chlo

39、rine (Cl17). Transferring the electron from Na to Cl produces positive and negatively charged ions which are smaller and larger, respectively, than the original neutral atoms. Na40 Predominant bonding in CeramicsGive up electronsAcquire electronsExamples: Ionic Bonding41The electrostatic attractive

40、force between the atoms is non-directional, and so the atom packing in an ionic material attempts to arrange as many positive ions around each negative one as possible, and vice versa. 22inverse r242When voltage is applied to an ionic material, entire ions must move to cause a current to flow. Ion m

41、ovement is slow and the electrical conductivity is poor.43The ionic bond has the following characteristics:Electrons transferred between atoms, producing ions No directionality - each ion surrounded by maximum number of opposite sign High strength (150-370 kcal/mol) Forms between atoms of different

42、electronegativity values (one high, one low) 44Arises from interaction between dipoles Permanent dipoles-molecule induced Fluctuating dipoles-general case:-ex: liquid HCl-ex: polymerAdapted from Fig. 2.13, Callister 6e. Secondary Bonding45The Van der Waals bond: polarization due to bond structure ca

43、uses attractive and repulsive force between molecules. The Van der Waals bond is a weak bond that forms by electrostatic attraction between molecules. A molecule with covalent bonds between the atoms localizes the electrons in the region of the bond. The fractional amount of the charge is much less

44、than produced by ionization that removes or adds a whole electron. 46The Hydrogen bond is a common example of a Van der Waals bond. The H2O molecules in water have polarization charge that is positive on the exposed tips of the hydrogen atoms and negative where the valence electrons are localized. T

45、his produces a Van der Waals force between the molecules. -+-+OxygenOxygenHHHH47Since the bond angle is 109.5 , close to 120 , this results in a roughly hexagonal arrangement of molecules. Indeed, when water freezes to make ice, it has a hexagonal structure due to these same bond forces. The hexagon

46、al geometry of the ice has a lower density than the water which is why ice floats.Van der Waals bonds are secondary bonds, the atoms within the molecule or group of atoms are joined by strong covalent or ionic bonds.48The Van der Waals bond has the following characteristics:Polarization produces sli

47、ght electrostatic charge between molecules Not directional but affects regions of molecules Weak bond (1/100th of strong bonds; 10 kcal/mol) Hydrogen bond is an example 49Metallic bondIonic bondCovalent bondVan der Waals bondElectrons shared among all atomsElectrons transferred between atoms, produc

48、ing ionsElectrons shared among two adjacent atomsPolarization produces slight electrostatic charge between moleculesNo directionality No directionality directionality Not directional but affects regions of moleculesHigh strength (25-200 kcal/mol)High strength (150-370 kcal/mol)High strength (125-300

49、 kcal/mol)Weak bond (1/100th of strong bonds; 10 kcal/mol)Forms between atoms with low electronegativityForms between atoms of different electronegativity values (one high, one low)Forms between atoms with high electronegativityHydrogen bond is an example50TypeIonicCovalentMetallicSecondaryBond Ener

50、gyLarge!Variablelarge-Diamondsmall-BismuthVariablelarge-Tungstensmall-MercurysmallestCommentsNondirectional (ceramics)Directionalsemiconductors, ceramicspolymer chains)Nondirectional (metals)Directionalinter-chain (polymer) inter-molecularSummary: Bonding51Mixed BondsIn most materials, bonding betwe

51、en atoms is a mixture of two of more types. The Ionic and covalent bond types represent the extremes of transfer and sharing of electrons. Real bonds are a mixture of these extremes, depending on the electronegativities of the elements involved. Compounds formed from two or more metals may be bonded by a mixture of metallic and ionic bonds, particularly when there is a large difference in electronegativity between the elements.52It is the electronegativities of atoms that determine what bond type forms. If the atoms have high electronegativity, they share atoms in a covalen