《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 Forces,Chemical Bonds,Chemical bond (化学键): A strong attractive force that exists between certain atoms in a substance.,Ionic

2、 bonds (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 Bonds,Covalent bonds (共价键): In a covalent bond, two atoms share valence electrons, which are attracted to the positively charged cor

3、es of both atoms.,diamond,Chemical Bonds,Metallic 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.,sodium,Theory of Chemical Bonding,1. In 1916 Gilbert N. Lewis proposed that the strong attract

4、ive 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 explained by the newly discovered theory of quantum

5、 mechanics.,Theory of Chemical Bonding,3. 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).,4. Based on valence bond theory and wave properties of electrons, L. Pauling and J. C. Slater proposed

6、hybrid orbital theory (杂化轨道理论) in 1931.,Theory of Chemical Bonding,5. In 1932 R. S. Mulliken and F. Hund proposed molecular orbital theory (分子轨道理论).,6. To 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

7、theory was later modified by R. J. Gillespie.,1. Valence Bond Theory,I. Formation of H2 molecules,Heitler and London solved the Schrdinger equation for H2 molecules and found that,1. Valence Bond Theory, When two H atoms are some distance apart, the potential energy of the atoms are nearly zero., Wh

8、en two H atoms (with unpaired electrons of same spins) approach one 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 t

9、he bonding of the atoms. Their 1s orbitals begin to overlap (重叠), each 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

10、bond length (键长) of H2.,The nature of covalent bond: The orbital of one 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

11、considered as arising from the attraction of oppositely charged particles: nuclei and electrons.,II. Valence bond theory(价键理论),1. Valence Bond Theory,Valence bond theory: A bond is formed by overlap of orbitals from two atoms., An orbital (with unpaired electron) on one atom overlaps (comes to occup

12、y a portion of the same region of space) with an orbital (with unpaired electron 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 ();

13、 H3O (); HCl ().,1. Valence Bond Theory, Strength of bonding depends on the amount 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).,HCl,III. Types of covalent bond,1. V

14、alence Bond Theory, -bond: The bond formed when atomic orbitals that contain unpaired 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 i

15、nternuclear axis, the electron distribution resembles that of an s-orbital.,1. Valence Bond Theory,Much 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 .,III. Types of c

16、ovalent bond,1. Valence Bond Theory, -bond: The bond formed when atomic orbitals that 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

17、internuclear axis, a -bond resembles a pair of electrons in a p-orbital.,1. Valence Bond Theory,A 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 The

18、ory,A single bond is a -bond. A double bond is a -bond plus one -bond. A triple bond is a -bond plus two -bonds.,Multiple bonds,NN,1. Valence Bond Theory,Coordinate 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

19、form between atoms that both donate an electron,It is possible for both electrons to come from the same atom,1. Valence Bond Theory,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 paramete

20、rs (键参数),1. Valence Bond Theory,Bonding pair (成键电子对) 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:,1. Valence Bond Theory,1. Bond length (键长): The distance

21、 between the nuclei in a bond.,C-C in diamond (154.2 pm), in ethane (153.3 pm), in propane (154 pm), in cyclohexane (153 pm). The average value is 154 pm.,1. Valence Bond Theory,2. Bond order (键级): The number of pairs of electrons in a bond.,NN,As the bond order increases, the bond strength increase

22、s and the nuclei are pulled inward, decreasing the bond length.,Single bond (C-C); Double bond (C=C); Triple bond (CC).,154 pm 134 pm 120 pm,1. Valence Bond Theory,Example 10-1: Consider the molecules, N2H4, N2, N2F2. Which molecule has the shortest nitrogen-nitrogen bond? Which has the longest nitr

23、ogen-nitrogen bond?,Solution:,First write the structural 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).,N2H4,N2,N2F2,1. Valence Bond Theory,3. Bond energy (键能):,The

24、 energy that must be added to separate the atoms in H2 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 Theory,Bond energy: A-B bond energy is the average enthalpy change for the breakin

25、g of an A-B bond in a molecule in the gas phase.,1. Valence Bond Theory,Bond length and bond energies,Protective gas,1. Valence Bond Theory,Bond 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: t

26、he larger the bond energy, the stronger the chemical bond.,Although 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

27、 results from the simultaneous formation of a number of strong, single bonds.,1. Valence Bond Theory,4. Bond angle (键角): The angle between two bonds from the same atom.,1. Valence Bond Theory,5. Bond polarity (键的极性),A covalent bond involves the sharing of at least one pair of electrons between two a

28、toms.,When the atoms are alike (the electronegativity 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 Theory,When the tw

29、o atoms are of different elements (different 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 covalent,Polar covalent,Ionic,1. Valence Bond Theory

30、,The absolute values of the difference in electronegativity 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.

31、18), O(3.44), C(2.55), and Cl(3.16), respectively.,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 Theory,Differences in electronegativity explains (1) why ionic bonds usually form between a metal atom and a

32、 nonmetal atom, and (2) why covalent bonds from primarily between two nonmetals.,I. Hybrid orbitals (杂化轨道),2. Hybrid Orbitals,The number of bonds formed by a given atom equals the number of unpaired electrons in its valence shell?,HCl ,H2O ,CH2 ,CH4 ,2. Hybrid Orbitals,Explanation: Four unpaired ele

33、ctrons are formed when an electron 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.,C atom (excited st

34、ate),2. Hybrid Orbitals,The four orbitals of the carbon atom combine during the bonding process to form four new, but equivalent (等价的), hybrid orbitals.,hybridized,Nuclear magnetic resonance (NMR) and infrared spectroscopy both show that CH4 has four equivalent CH bonds.,2. Hybrid Orbitals,The shape

35、 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 Orbitals,Outline of hybrid orbitals, Only a

36、tomic 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 reorientation of the hybrid orbita

37、ls 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 orbitals,2. Hybrid Orbitals,1. sp hybridization (sp 杂化): Be

38、Cl2,ClBeCl,Diagram of sp hybrid orbitals: Linear arrangement,2. Hybrid Orbitals,Only one of the three p orbitals are used to form sp hybrid orbitals. The two unhybridized p orbitals is perpendicular (垂直的) to the axis of the sp hybrid orbitals and perpendicular to each other.,2. Hybrid Orbitals,2. sp

39、2 hybridization: BF3,BF3,Diagram of sp2 hybrid orbitals: Trigonal planar arrangement.,2. Hybrid Orbitals,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 Orbitals,3. sp3 hybridi

40、zation: CH4,CH4,Diagram of sp3 hybrid orbitals: Tetrahedral arrangement.,2. Hybrid Orbitals,NH3 ?,4. Nonequivalent hybridization (不等性杂化): H2O,H2O: V shaped.,Diagram of sp3 hybrid orbitals: Tetrahedral arrangement.,2. Hybrid Orbitals,NH3: trigonal pyramidal.,Diagram of sp3 hybrid orbitals: Tetrahedra

41、l arrangement.,5. Multiple bonds,2. Hybrid Orbitals,ethylene,One hybrid orbital is needed for each bond (whether a single bond or a multiple bond) and for each lone pair.,2. Hybrid Orbitals,A single 2p orbital still remains on each C atom. These orbitals are perpendicular to the plane of the hybrid

42、orbitals; that is, perpendicular to the CH2 plane.,2. Hybrid Orbitals,The carbon-carbon double bond can be described as 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 orbital

43、s overlap to give a bond.,The two CH2 plane can rotate about the C-C axis without affecting the overlap of the hybrid orbitals. As these planes rotate, the 2p orbitals also rotate.,2. Hybrid Orbitals,C-H bonds,C=C bond,2. Hybrid Orbitals,acetylene,3. Valence Shell Electron Pair Repulsion Theory,trig

44、onal planar,trigonal pyramidal,A simple model, valence shell electron pair repulsion (价层电子对互斥理论), can allow us to predict molecular geometries, or shapes.,Valence bond theory provides an insight into why bonds form and, at the same time, reveals that bonds have definite directions in space. However,

45、 this theory can only be used to explain the experimental data.,I. Valence shell electron pair repulsion (VSEPR) theory,3. Valence Shell Electron Pair Repulsion Theory,VSEPR model (pronounce as “vesper”) predicts the shapes of molecules and ions by assuming that the valence-shell electron pairs are

46、arranged about each atom so that electron pairs are kept as far away from one another as possible, thus minimizing electron-pair repulsions.,3. Valence Shell Electron Pair Repulsion Theory,II. Steps to follow in order to predict the geometry of an ABn molecule by the VSEPR method,Molecular geometry

47、(分子几何学): The general shape of a molecule, as determined by 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 Pa

48、ir Repulsion Theory,A: central atom; B: ligand (配体). A,B: atoms 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 Theory,1. Determine how many (v

49、alence) electron pairs are around the central atom.,II. Steps to follow 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 a

50、tom contributes 7 electrons, and oxygen group atom contributes 6 electrons; H2O.,3. Valence Shell Electron Pair Repulsion Theory,2. 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

51、dividing the number of electrons by 2., Count a multiple bond as one pair.,III. Molecular geometry,3. Valence Shell Electron Pair Repulsion Theory,CO2 ?,O=C=O,1. Two electron pairs (linear arrangement) Three electron pairs (trigonal planar arrangement),2. Four electron pairs (tetrahedral arrangement

52、),3. Valence Shell Electron Pair Repulsion Theory,3. Valence Shell Electron Pair Repulsion Theory,Example 10-3: Predict the geometry of the following molecules or ions, using the VSEPR method: a. BeCl2; b. NO2-; c. SiCl4.,Solution:,a. BeCl2,The central Be atom has 2 valence electrons, and each Cl li

53、gand contributes one electron. The number of the valence electron pairs on Be is (2+2)/2=2. The two pairs on Be have a linear arrangement, indicating a linear molecular geometry for BeCl2.,3. Valence Shell Electron Pair Repulsion Theory,b. NO2-,The central N atom has 5 valence electrons, O atoms do

54、not have contribution. The charge (-1) on the ion having considered, the number of valence electron 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. SiCl4,The central Si atom has 4 valence

55、electrons, and each chlorine contributes one electron. The number of valence electron pairs on 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 Theory,3. Five electron pairs (trigonal bipyramidal arrange

56、ment),3. Valence Shell Electron Pair Repulsion Theory,4. Six electron pairs (octahedral arrangement),3. Valence Shell Electron Pair Repulsion Theory,Example 10-4: According to the VSEPR model, what molecular geometry would you expect for iodine trichloride ICl3?,Solution:,The central I atom has 7 va

57、lence electrons, and each chlorine contributes one electron. The number of valence electron pairs on I 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 Theory,1. A lone pair tends to require more space

58、 than a corresponding bonding pair.,2. Multiple bonds require more space than single bonds because of the greater number of electrons.,Notes:,A limitation of the VSEPR model is that it cannot explain why both CH2 groups lie in the same plane.,I. Dipole moment (偶极矩),4. Intermolecular Forces,The polar

59、ity of a bond is characterized by a separation of electric charge (电荷分离). We can represent this in HCl by indicating partial charges, q+ and q-, on the atoms.,nonpolar bond,polar bond,Red: negative charge; Blue: positive charge.,4. Intermolecular Forces,Any molecule that has a net separation of charge has dipole moment (偶极矩). A molecule in which the distribution of charge is equivalent to charges q+ and q- separated by a distance d has a dipole moment equal to:,The SI unit of dipole moment is 1Cm (1 coulomb-meter). It is the dipole moment of a charge of 1C separated fro