1、Chapter 10.10-1.Valence Bond Theory 10-2.Hybrid Orbitals 10-3.Valence Shell Electron Pair Repulsion Theory 10-4.Intermolecular ForcesChemical Bonds:A strong attractive force that exists between certain atoms in a substance.A chemical bond formed by the electrostatic()attraction between positive and
2、negative ions.Ionic bonds,covalent bonds,and metallic bonds.(NaCl)ClNaClNaChemical Bonds In a covalent bond,two atoms share valence electrons,which are attracted to the positively charged cores of both atoms.H:HH:H-+Chemical Bonds The valence electrons of a crystal of Na move throughout the crystal(
3、delocalized electrons),attracted to the positive cores of all Na+ions.Sodium ionsValence electrons move throughout metalTheory of Chemical Bonding1.In 1916 Gilbert N.Lewis proposed that the strong attractive force between two atoms in a molecule results from,a chemical bond formed by the sharing of
4、a pair of electrons between atoms.2.In 1926 Walter Heitler and Fritz London showed that the could be quantitatively explained by the newly discovered theory of.F.London（1900-1954）GermanyW.Heilter（left）（1904-1981）SwitzerlandTheory of Chemical Bonding3.The explanation of the covalent bond in H2 by the
5、 theory of quantum mechanics can be extended to other molecules,resulting in.4.Based on valence bond theory and wave properties of electrons,L.Pauling and J.C.Slater proposed in 1931.L.Pauling(1901-1994)J.C.Slater(1900-1976)Theory of Chemical Bonding5.In 1932 R.S.Mulliken and F.Hund proposed.6.To pr
6、edict molecular geometries,N.V.Sidgwick proposed 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 London solved the Schrdinger equation for H2 molecules and found that1.Valence Bond Theory W
7、hen two H atoms are some distance apart,the potential energy of the atoms are nearly zero.When two H atoms(with unpaired electrons of)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)approach one anothe
8、r,the potential energy gets lower and lower,reflecting the bonding of the atoms.Their 1s orbitals begin to,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()between nuclei at this m
9、inimum energy is called the.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 considered as arising from t
10、he.II.Valence bond theory()1.Valence Bond Theory A bond is formed by overlap of orbitals from two atoms.An orbital(with unpaired electron)on one atom (comes to occupy a portion of the same region of space)with an orbital(with unpaired electron with opposite spin)on the other atom.1.Valence Bond Theo
11、ry The number of covalent bonds formed for an atom is limited by the number of unpaired electrons in the atom().A At to omm Z Z 1 1s s 2 2s s 2 2p p Hydrogen 1 Helium 2 1 1s s 2 2s s 2 2p p Oxygen 8 Chlorine 17 1 1s s 3 3s s 3 3p p H2();He2();Cl2();H2O();H3O();HCl().1.Valence Bond Theory Strength of
12、 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().III.Types of covalent bond1.Valence Bond Theory -bond:The bond formed when atomic orbitals that contain unpaired
13、electrons overlap,it has a.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.Valence Bond TheoryMuch the same kind of -bond formation occurs in the hydrogen halides(e.g.,).For molec
14、ules,(only)one of the three orbitals on each atom can overlap end-to-end to form a -bond.1 1s s 2 2s s 2 2p p A At to omm Z Z 1 1s s 2 2s s 2 2p p Hydrogen 1 Nitrogen 7 Fluorine 9 HFN2III.Types of covalent bond1.Valence Bond Theory -bond:The bond formed when atomic orbitals that contain unpaired ele
15、ctrons overlap,it has an anti-symmetrical()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 Bond TheoryA side-by-side overlap will not give s
16、o 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 -bond plus two -bonds.Multiple bonds1s2s2pN2N N1.V
17、alence Bond TheoryCoordinate covalent bond()A covalent bond in which both electrons come from one of the atoms is called.A+BABA+BABWhen bonds form between atoms that both donate an electron,It is possible for both electrons to come from the same atom,1.Valence Bond TheoryH+:NH3+:N:HHHH+C:.O:.:+C:O:N
18、H4+:CO:孔子曰：孔子曰：“益者三友，损者三友。益者三友，损者三友。友直，友友直，友谅，友多闻，益矣。友便辟谅，友多闻，益矣。友便辟(bin p)，友善柔，友善柔，友便佞，友便佞(pin nng)，损矣，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()1.Valence Bond TheoryH:Cl:bonding pairlone pairBonding pair()is an electron pair shared between t
19、wo atoms.Lone pair(,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():The distance between the nuclei in a bond.in diamond(154.2 pm),in ethane(153.3 pm),in propane(154 pm),in cyclohexa
20、ne(153 pm).The average value is.bond length1.Valence Bond Theory2.Bond order():The number of pairs of electrons in a bond.N N As the bond order increases,the bond strength increases and the nuclei are pulled inward,decreasing the bond length.Single bond(C-C);Double bond(C=C);Triple bond(C C).154 pm1
21、34 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 structural formulas:Experimental data:N2(109 pm);N2H4(147 pm).N2H4NNHHHHN2NNN2F2NNFF1.Valence
22、Bond Theory3.Bond energy():The energy that must be added to separate the atoms in H2 molecules is called the.It is essentially the for a gas-phase reaction in which a bond breaks.1.Valence Bond Theory A-B bond energy is the average enthalpy change for the breaking of an A-B bond in a molecule in the
23、 gas phase.CHHHH(g)CHHHH(g)CHHH(g)CHHH(g)+H(g)H=435 kJmol-1CHHHH(g)CHHHH(g)CHHH(g)CHHH(g)+H(g)H=435 kJmol-1CHHHCHHH(g)CHHHCHH(g)+H(g)H=410 kJmol-1CHHHCHHH(g)CHHHCHH(g)+H(g)H=410 kJmol-11.Valence Bond TheoryBond length and bond energiesBond Bond length pm Bond energy kJ mol-1 C-C 154 347 C=C 134 611
24、C C 120 837 N-N 145 159 N N 110 946 F-F 128 158 Cl-Cl 199 244 Br-Br 228 192 I-I 267 150 1.Valence Bond TheoryBond energy is 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 th
25、e 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 results from the simultaneous formation of a numbe
26、r of strong,single bonds.1.Valence Bond Theory4.Bond angle():The angle between two bonds from the same atom.直线形直线形V形形平面三角形平面三角形三角锥形三角锥形四面体形四面体形平面四边形平面四边形三角双锥形三角双锥形八面体形八面体形1.Valence Bond Theory5.Bond polarity()H:HA covalent bond involves the sharing of at least one pair of electrons between two atoms
27、.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.1.Valence Bond TheoryWhen the two atoms are of different elements(different ele
28、ctronegativity),the bonding electrons are not shared equally.A is a covalent bond in which the bonding electrons spend more time near one atom than the other.H:Cl:H:HH:Cl:Na+:Cl:1.Valence Bond TheoryThe absolute values of the difference in electronegativity of two atoms gives a rough measure of the
29、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),respectively.The absolute values of the electronegativity differences are PH,
30、0.01;HO,1.26;CCl,0.61.Hence,the order is.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 primarily between two nonmetals.I.Hybrid orbitals()2.Hybrid OrbitalsThe number of bonds formed
31、 by a given atom equals the number of unpaired electrons in its valence shell?HCl H2O CH2 1s2s2p3s3p1s2s2p1s2s2pCH4 2.Hybrid Orbitals1s2s2pExplanation:are formed when an electron from the 2s orbital of the carbon atom is to the vacant 2p orbital.It would require energy to promote the carbon atom thi
32、s 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.Hybrid OrbitalsThe four orbitals of the carbon atom during the bonding process to form four,but,.1ssp31s2s2pground state1s2s2pexcited statehybridiz
33、edNuclear magnetic resonance(NMR)and infrared spectroscopy both show that CH4 has four equivalent CH bonds.2.Hybrid OrbitalsThe shape of a single sp3 hybrid orbital(left).The four hybrid orbitals are arranged tetrahedrally in space(right).are orbitals used to describe bonding that are obtained by ta
34、king combinations of atomic orbitals of the isolated atoms.2.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 e
35、quals the number of atomic orbitals used.The reorientation 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.T
36、ypes of hybrid orbitals2.Hybrid Orbitals1.sp hybridization(sp):ClBeCl1s2s2pBe(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 unhybridized p orbitals i
37、s to the axis of the sp hybrid orbitals and perpendicular to each other.2.Hybrid Orbitals2.sp2 hybridization:BF31s2s2pB(ground state)1ssp2B(in BeF3)1ssp2B(hybridized)2.Hybrid Orbitals1s2s2p1ssp21ssp2B(ground state)B(in BeF3)B(hybridized)Only two of the three p orbitals are used to form sp2 hybrid or
38、bitals.The unhybridized p orbital is to the plane of the sp2 hybrid orbitals.2.Hybrid Orbitals3.sp3 hybridization:CH41s2s2pC(ground state)1ssp3C(in CH4)1ssp3C(hybridized)2.Hybrid Orbitals4.Nonequivalent hybridization():1s2s2pO(ground state)1ssp3O-H bondslone pairsO(in H2O)1ssp3O(hybridized)H2O:V sha
39、ped.2.Hybrid OrbitalsNH3:trigonal pyramidal.1s2s2pN(ground state)1ssp3N(hybridized)N(in NH3)1ssp3N-H bondslone pair5.Multiple bonds2.Hybrid OrbitalsCCHHHHethyleneOne hybrid orbital is needed for each bond(whether a single bond or a multiple bond)and for each lone pair.1s2s2pC(ground state)1ssp2C(in
40、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.CCHHHH2.Hybrid OrbitalsThe carbon-carbon double bond can be described as one bond and one bond.The fo
41、rmation 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 orbitals.As these planes rotate,the 2
42、p orbitals also rotate.2.Hybrid Orbitals1s2s2p1ssp21ssp2CCHH?acetyleneC-H bondsC=C bondC=C bond2.Hybrid OrbitalsCCHHacetylene1s2s2p1ssp1sspC-H bondC C bondC C bonds3.Valence Shell Electron Pair Repulsion Theorytrigonal planartrigonal pyramidalA simple model,valence shell electron pair repulsion(),ca
43、n allow us to 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,this theory can only be used to the experimental data.I.Valence shell electron pair repulsion(VSEPR)theory 3.Vale
44、nce Shell Electron Pair Repulsion Theory(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 kept as far away from one another as possible,thus minimizing electron-pair repulsions.3.V
45、alence Shell Electron Pair Repulsion TheoryII.Steps to follow in order to predict the geometry of an ABn molecule by the VSEPR method 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 mol
46、ecular geometry.3.Valence Shell Electron Pair Repulsion TheoryA:central atom;B:ligand().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 The
47、ory1.Determine 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 atom contributes 7 el
48、ectrons,and oxygen group atom contributes 6 electrons;H2O.3.Valence Shell Electron Pair Repulsion Theory2.as shown in last slides.3.Obtain the from the directions of.The number of electron pairs is obtained by dividing the number of electrons by 2.Count a multiple bond as one pair.III.Molecular geom
49、etry3.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 Repulsion TheoryExample 10-3:P
50、redict 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=2.The two pairs on Be have a linear ar
