《基础化学》英文教学课件：chapter-10

1、Chapter 10. Covalent Bonding and Intermolecular Forces 10-1. Valence Bond Theory 10-2. Hybrid Orbitals 10-3. Valence Shell Electron Pair Repulsion Theory 10-4. Intermolecular ForcesChemical BondsChemical bond (化学键): A strong attractive force that exists between certain atoms in a substance. Ionic bo

2、nds (离子键): A chemical bond formed by the electrostatic (静电) attraction between positive and negative ions. Ionic bonds, covalent bonds, and metallic bonds.Chemical BondsCovalent bonds (共价键): In a covalent bond, two atoms share valence electrons, which are attracted to the positively charged cores of

3、 both atoms.H:H-+diamondChemical BondsMetallic bonds (金属键): The valence electrons of a crystal of Na move throughout the crystal (delocalized electrons), attracted to the positive cores of all Na+ ions.sodiumSodium ionsValence electrons move throughout metalTheory of Chemical Bonding1. In 1916 Gilbe

4、rt N. Lewis proposed that the strong attractive force between two atoms in a molecule results from covalent bond, a chemical bond formed by the sharing of a pair of electrons between atoms.2. In 1926 Walter Heitler and Fritz London showed that the covalent bond in H2 could be quantitatively explaine

5、d by the newly discovered theory of quantum mechanics.F. London（1900-1954）GermanyW. Heilter（left）（1904-1981）SwitzerlandTheory of Chemical Bonding3. The explanation of the covalent bond in H2 by the theory of quantum mechanics can be extended to other molecules, resulting in valence bond theory (价键理论

6、).4. Based on valence bond theory and wave properties of electrons, L. Pauling and J. C. Slater proposed hybrid orbital theory (杂化轨道理论) in 1931.L. Pauling(1901-1994)J. C. Slater(1900-1976)Theory of Chemical Bonding5. In 1932 R. S. Mulliken and F. Hund proposed molecular orbital theory (分子轨道理论).6. To

7、 predict molecular geometries, N. V. Sidgwick proposed valence-shell electron-pair repulsion theory (VSEPR, “Vesper”) (价层电子对互斥理论) in 1940 and the theory was later modified by R. J. Gillespie.F. Hund(1896-1997)R. S. Mulliken (1896-1986)1. Valence Bond TheoryI. Formation of H2 moleculesHeitler and Lon

8、don solved the Schrdinger equation for H2 molecules and found thatPotential-energy curve for H2Overlap of orbitals1. Valence Bond Theory When two H atoms are some distance apart, the potential energy of the atoms are nearly zero. When two H atoms (with unpaired electrons of same spins) approach one

9、another, the potential energy gets higher and higher, and thus no covalent bonds are formed. When two H atoms (with unpaired electrons of opposite spins) approach one another, the potential energy gets lower and lower, reflecting the bonding of the atoms. Their 1s orbitals begin to overlap (重叠), eac

10、h electron can then occupy the space around both atoms.1. Valence Bond Theory The potential energy gets lower, reaches a minimum value, and then increases. The distance (74 pm) between nuclei at this minimum energy is called the bond length (键长) of H2.The nature of covalent bond: The orbital of one

11、atom overlaps the orbital of another, and the bonded electron pair were shared by both nuclei. The electrons are attracted simultaneously by the positive charges of the two nuclei. The forces that hold the atoms together can be considered as arising from the attraction of oppositely charged particle

12、s: nuclei and electrons.II. Valence bond theory(价键理论)1. Valence Bond TheoryValence bond theory: A bond is formed by overlap of orbitals from two atoms. An orbital (with unpaired electron) on one atom overlaps (comes to occupy a portion of the same region of space) with an orbital (with unpaired elec

13、tron with opposite spin) on the other atom.1. Valence Bond Theory The number of covalent bonds formed for an atom is limited by the number of unpaired electrons in the atom (saturation, 饱和性). H2 (); He2 (); Cl2 (); H2O (); H3O (); HCl ().1. Valence Bond Theory Strength of bonding depends on the amou

14、nt of overlap; the greater the overlap, the greater the bond strength. Orbitals bond in the direction in which they protrude or point, to obtain maximum overlap (directionality, 方向性).HClIII. Types of covalent bond1. Valence Bond Theory -bond: The bond formed when atomic orbitals that contain unpaire

15、d electrons overlap end-to-end (along the internuclear axis), it has a cylindrical shape (sausage) about the bond axis.The Greek letter sigma, , is the equivalent of letter s. It reminds us that, looking along the internuclear axis, the electron distribution resembles that of an s-orbital.H21. Valen

16、ce Bond TheoryMuch the same kind of -bond formation occurs in the hydrogen halides (e.g., HF).For N2 molecules, (only) one of the three orbitals on each atom can overlap end-to-end to form a -bond .HFN2III. Types of covalent bond1. Valence Bond Theory -bond: The bond formed when atomic orbitals that

17、 contain unpaired electrons overlap side-by-side, it has an anti-symmetrical (反对称) distribution above and below the bond axis.The Greek letter pi, , is the equivalent of letter p. When we imagine looking along the internuclear axis, a -bond resembles a pair of electrons in a p-orbital.1. Valence Bon

18、d TheoryA side-by-side overlap will not give so strong a bond as an end-to-end overlap of orbitals. A bond occurs when two parallel orbitals are still available after strong bonds have formed.1. Valence Bond TheoryA single bond is a -bond.A double bond is a -bond plus one -bond.A triple bond is a -b

19、ond plus two -bonds.Multiple bonds1s2s2pN2NN1. Valence Bond TheoryCoordinate covalent bond(配位共价键)A covalent bond in which both electrons come from one of the atoms is called coordinate covalent bond.When bonds form between atoms that both donate an electron,It is possible for both electrons to come

20、from the same atom,1. Valence Bond TheoryH+:NH3+:N:HHHH+C:.O:.:+C:O:NH4+:CO:孔子曰：“益者三友，损者三友。友直，友谅，友多闻，益矣。友便辟(bin p )，友善柔，友便佞(pin nng)，损矣。” 正直诚信 恕人大度 知识广博便辟(bin p ):谄媚(chn mi )逢迎之人善柔：表面奉承而背后诽谤人之人 便佞(pin nng):善于花言巧语之人 IV. Bond parameters (键参数)1. Valence Bond Theorybonding pairlone pairBonding pair (成键电

21、子对) is an electron pair shared between two atoms.Lone pair (孤电子对, nonbonding pair) is an electron pair that remains on one atom and is not shared.Bonding pairs are often represented by dashes:H Cl1. Valence Bond Theory1. Bond length (键长): The distance between the nuclei in a bond.C-C in diamond (154

22、.2 pm), in ethane (153.3 pm), in propane (154 pm), in cyclohexane (153 pm).The average value is 154 pm.bond lengthl1. Valence Bond Theory2. Bond order (键级): The number of pairs of electrons in a bond. NN As the bond order increases, the bond strength increases and the nuclei are pulled inward, decre

23、asing the bond length.Single bond (C-C); Double bond (C=C); Triple bond (CC). 154 pm134 pm120 pm1. Valence Bond TheoryExample 10-1: Consider the molecules, N2H4, N2, N2F2. Which molecule has the shortest nitrogen-nitrogen bond? Which has the longest nitrogen-nitrogen bond?Solution:First write the st

24、ructural formulas:The nitrogen-nitrogen bond should be shortest in N2, where it is a triple bond, and longest in N2H4, where it is a single bond. Experimental data: N2(109 pm); N2H4(147 pm).N2H4N2N2F21. Valence Bond Theory3. Bond energy (键能):The energy that must be added to separate the atoms in H2

25、molecules is called the bond dissociation energy (解离能). It is essentially the enthalpy change (H) for a gas-phase reaction in which a bond breaks.1. Valence Bond TheoryBond energy: A-B bond energy is the average enthalpy change for the breaking of an A-B bond in a molecule in the gas phase.1. Valenc

26、e Bond TheoryBond length and bond energiesProtective gas1. Valence Bond TheoryBond energy is perhaps of greatest value when you try to understand the relative stabilities of compounds.Bond energy is a measure of the strength of a bond: the larger the bond energy, the stronger the chemical bond.Altho

27、ugh a double bond is stronger than a single bond, it is not necessarily less reactive. Ethylene, CH2=CH2, for example, is more reactive than ethane, CH3CH3, where carbon atoms are linked through a single bond. The reactivity of ethylene results from the simultaneous formation of a number of strong,

28、single bonds.1. Valence Bond Theory4. Bond angle (键角): The angle between two bonds from the same atom.直线形V形平面三角形三角锥形四面体形平面四边形三角双锥形八面体形1. Valence Bond Theory5. Bond polarity (键的极性)A covalent bond involves the sharing of at least one pair of electrons between two atoms.When the atoms are alike (the el

29、ectronegativity of the atoms are the same), the bonding electrons are shared equally. That is, the electrons spend the same amount of time in the vicinity of each atom. Such a bond is called a nonpolar (covalent) bond (非极性共价键).1. Valence Bond TheoryWhen the two atoms are of different elements (diffe

30、rent electronegativity), the bonding electrons are not shared equally. A polar (covalent) bond (极性共价键) is a covalent bond in which the bonding electrons spend more time near one atom than the other.Nonpolar covalentPolar covalentIonic1. Valence Bond TheoryThe absolute values of the difference in ele

31、ctronegativity of two atoms gives a rough measure of the relative bond polarities.Example 10-2: Use electronegativity values to arrange the following bonds in order of increasing polarity: PH, HO, CCl.Solution:Electronegativity values are P(2.19), H(2.18), O(3.44), C(2.55), and Cl(3.16), respectivel

32、y.The absolute values of the electronegativity differences are PH, 0.01; HO, 1.26; CCl, 0.61.Hence, the order is PH, CCl, HO. 1. Valence Bond TheoryDifferences in electronegativity explains (1) why ionic bonds usually form between a metal atom and a nonmetal atom, and (2) why covalent bonds from pri

33、marily between two nonmetals.I. Hybrid orbitals (杂化轨道)2. Hybrid OrbitalsThe number of bonds formed by a given atom equals the number of unpaired electrons in its valence shell?HCl H2O CH2 1s2s2p3s3p1s2s2p1s2s2pCH4 2. Hybrid Orbitals1s2s2pExplanation: Four unpaired electrons are formed when an electr

34、on from the 2s orbital of the carbon atom is promoted (excited) to the vacant 2p orbital.It would require energy to promote the carbon atom this way, but more than enough energy would be obtained from the formation of two additional covalent bonds.1s2s2pC atom (ground state)C atom (excited state)2.

35、Hybrid OrbitalsThe four orbitals of the carbon atom combine during the bonding process to form four new, but equivalent (等价的), hybrid orbitals.1ssp31s2s2pground state1s2s2pexcited statehybridizedNuclear magnetic resonance (NMR) and infrared spectroscopy both show that CH4 has four equivalent CH bond

36、s.2. Hybrid OrbitalsThe shape of a single sp3 hybrid orbital (left). The four hybrid orbitals are arranged tetrahedrally in space (right).Hybrid orbitals are orbitals used to describe bonding that are obtained by taking combinations of atomic orbitals of the isolated atoms.2. Hybrid OrbitalsOutline

37、of hybrid orbitals Only atomic orbitals with approximate energy from the isolated atom can be combined to give hybrid orbitals, in which the energy and orientation of the orbitals are redistributed. The number of hybrid orbitals formed always equals the number of atomic orbitals used. The reorientat

38、ion of the hybrid orbitals is favorable for the maximum overlap during covalent bond formation. The reorientation of the hybrid orbitals minimizes the repulsion between bonding electron pairs, thus the resulting covalent bonds are more stable.II. Types of hybrid orbitals2. Hybrid Orbitals1. sp hybri

39、dization (sp 杂化): BeCl2ClBeClDiagram of sp hybrid orbitals:Linear arrangement1s2s2pBe (ground state)1sspBe (in BeCl2)1sspBe (hybridized)2. Hybrid Orbitals1s2s2p1ssp1sspBe (ground state)Be (in BeCl2)Be (hybridized)Only one of the three p orbitals are used to form sp hybrid orbitals. The two unhybridi

40、zed p orbitals is perpendicular (垂直的) to the axis of the sp hybrid orbitals and perpendicular to each other.2. Hybrid Orbitals2. sp2 hybridization: BF3BF3Diagram of sp2 hybrid orbitals:Trigonal planar arrangement.1s2s2pB (ground state)1ssp2B (in BeF3)1ssp2B (hybridized)2. Hybrid Orbitals1s2s2p1ssp21

41、ssp2B (ground state)B (in BeF3)B (hybridized)Only two of the three p orbitals are used to form sp2 hybrid orbitals. The unhybridized p orbital is perpendicular (垂直的) to the plane of the sp2 hybrid orbitals.2. Hybrid Orbitals3. sp3 hybridization: CH4CH4Diagram of sp3 hybrid orbitals:Tetrahedral arran

42、gement.1s2s2pC (ground state)1ssp3C (in CH4)1ssp3C (hybridized)2. Hybrid OrbitalsNH3 ?4. Nonequivalent hybridization (不等性杂化): H2O1s2s2pO (ground state)1ssp3O-H bondslone pairsO (in H2O)1ssp3O (hybridized)H2O: V shaped.Diagram of sp3 hybrid orbitals:Tetrahedral arrangement.2. Hybrid OrbitalsNH3: trig

43、onal pyramidal.Diagram of sp3 hybrid orbitals:Tetrahedral arrangement.1s2s2pN (ground state)1ssp3N (hybridized)N (in NH3)1ssp3N-H bondslone pair5. Multiple bonds2. Hybrid OrbitalsethyleneOne hybrid orbital is needed for each bond (whether a single bond or a multiple bond) and for each lone pair.1s2s

44、2pC (ground state)1ssp2C (in CH2CH2)1ssp2C (hybridized)2. Hybrid OrbitalsA single 2p orbital still remains on each C atom. These orbitals are perpendicular to the plane of the hybrid orbitals; that is, perpendicular to the CH2 plane.2. Hybrid OrbitalsThe carbon-carbon double bond can be described as

45、 one bond and one bond. The formation of a bond “locks” the two CH2 ends into a flat, rigid molecule.When the CH2 planes rotate so that the 2p orbitals become parallel, the orbitals overlap to give a bond.The two CH2 plane can rotate about the C-C axis without affecting the overlap of the hybrid orb

46、itals. As these planes rotate, the 2p orbitals also rotate. 2. Hybrid Orbitals1s2s2p1ssp21ssp2?acetyleneC-H bondsC=C bondC=C bond2. Hybrid Orbitalsacetylene1s2s2p1ssp1sspC-H bondCC bondCC bonds3. Valence Shell Electron Pair Repulsion Theorytrigonal planartrigonal pyramidalA simple model, valence she

47、ll electron pair repulsion (价层电子对互斥理论), can allow us to predict molecular geometries, or shapes.BF3PF3Valence bond theory provides an insight into why bonds form and, at the same time, reveals that bonds have definite directions in space. However, this theory can only be used to explain the experime

48、ntal data. I. Valence shell electron pair repulsion (VSEPR) theory 3. Valence Shell Electron Pair Repulsion TheoryVSEPR model (pronounce as “vesper”) predicts the shapes of molecules and ions by assuming that the valence-shell electron pairs are arranged about each atom so that electron pairs are ke

49、pt as far away from one another as possible, thus minimizing electron-pair repulsions.3. Valence Shell Electron Pair Repulsion TheoryII. Steps to follow in order to predict the geometry of an ABn molecule by the VSEPR methodMolecular geometry (分子几何学): The general shape of a molecule, as determined b

50、y the relative positions of the atomic nuclei. Note the difference between the arrangement of electron pairs and the molecular geometry.The direction in space of the bonding pairs gives you the molecular geometry.3. Valence Shell Electron Pair Repulsion TheoryA: central atom;B: ligand (配体).A,B: atom

51、s of the main-group elements.ABn molecule:Valence shell electron pair:Electron pairs of -bonds and lone electron pairs.Electron pairs of -bonds are not included.3. Valence Shell Electron Pair Repulsion Theory1. Determine how many (valence) electron pairs are around the central atom. II. Steps to fol

52、low in order to predict the geometry of an ABn molecule by the VSEPR method Ligand H and halogens contribute one electron, O has no contribution; BF3, CO2. The charge on an ion should be considered, SO42-, NH4+; Central halogen atom contributes 7 electrons, and oxygen group atom contributes 6 electr

53、ons; H2O.3. Valence Shell Electron Pair Repulsion Theory2. Arrange the electron pairs as shown in last slides.3. Obtain the molecular geometry from the directions of bonding pairs. The number of electron pairs is obtained by dividing the number of electrons by 2. Count a multiple bond as one pair.II

54、I. Molecular geometry3. Valence Shell Electron Pair Repulsion TheoryCO2 ?O=C=O1. Two electron pairs (linear arrangement)Three electron pairs (trigonal planar arrangement)2. Four electron pairs (tetrahedral arrangement)3. Valence Shell Electron Pair Repulsion Theory3. Valence Shell Electron Pair Repu

55、lsion TheoryExample 10-3: Predict the geometry of the following molecules or ions, using the VSEPR method: a. BeCl2; b. NO2-; c. SiCl4.Solution:a. BeCl2The central Be atom has 2 valence electrons, and each Cl ligand contributes one electron. The number of the valence electron pairs on Be is (2+2)/2=

56、2. The two pairs on Be have a linear arrangement, indicating a linear molecular geometry for BeCl2.3. Valence Shell Electron Pair Repulsion Theoryb. NO2-The central N atom has 5 valence electrons, O atoms do not have contribution. The charge (-1) on the ion having considered, the number of valence e

57、lectron pairs on N is (5+1)/2=3. The N atom has three valence electron pairs, two of which are bonding pairs. Therefore, the molecular geometry of the NO2- is bent.c. SiCl4The central Si atom has 4 valence electrons, and each chlorine contributes one electron. The number of valence electron pairs on

58、 Si is (4+4)/2=4, all of which are bonded. Therefore, the molecular geometry of the SiCl4 is tetrahedral.3. Valence Shell Electron Pair Repulsion Theory3. Five electron pairs (trigonal bipyramidal arrangement)3. Valence Shell Electron Pair Repulsion Theory4. Six electron pairs (octahedral arrangemen

59、t)3. Valence Shell Electron Pair Repulsion TheoryExample 10-4: According to the VSEPR model, what molecular geometry would you expect for iodine trichloride ICl3?Solution:The central I atom has 7 valence electrons, and each chlorine contributes one electron. The number of valence electron pairs on I

60、 is (7+3)/2=5, three of which are bonding pairs. Therefore, the molecular geometry of the ICl3 is T-shaped.3. Valence Shell Electron Pair Repulsion Theory1. A lone pair tends to require more space than a corresponding bonding pair.2. Multiple bonds require more space than single bonds because of the