1、第 2 节 共价键理论 例如例如 H +H =H H 通过共用一对电子，每个通过共用一对电子，每个 H 均均成成为为 He 的电子构型，形成一个共价键。的电子构型，形成一个共价键。又如又如ClClHH一一 路易斯理论路易斯理论 1916 年，美国科学家年，美国科学家 Lewis 提出共价键理论。认为分子中的提出共价键理论。认为分子中的原子都有形成稀有气体电子结构的趋势，求得本身的稳定。原子都有形成稀有气体电子结构的趋势，求得本身的稳定。而达而达到这种结构，可以不通过电子转移形成离子和离子键来完成，而到这种结构，可以不通过电子转移形成离子和离子键来完成，而是通过共用电子对来实现。是通过共用电子对
2、来实现。Lewis 的贡献，在于提出了一种不同于离子键的新的键型，的贡献，在于提出了一种不同于离子键的新的键型，解释解释了了 X 比较小的元素之间原子的成键事实。比较小的元素之间原子的成键事实。1、Lewis 符号符号H He:K:Cl:Cl:2、Lewis 符号和离子键符号和离子键+Ca:+:Cl:Cl:-Ca2+:Cl:-:可以通过共用电子对形成分子，共价键，共价分子。可以通过共用电子对形成分子，共价键，共价分子。八隅体规则八隅体规则Lewis SymbolsRepresent the number of valence electrons as dotsValence number is
3、 the same as the Periodic Table Group NumberHLiBeBCNOFNeHeNa;Is2,2s2,2p6,3s1=Ne 3s1Lewis Structure=NaFor example,Groups 12 3 4 5 6 7 8 Elements want to achieve the stable electron configuration of the nearest noble gasAtoms tend to gain,lose or share electrons until they are surrounded by 8 electron
4、sOctet RuleNeNobel Gas Has a Stable Electron ConfigurationArNe;1s2,2s2,2p6Ar;Ne 3s2,3p6 FNa+Na+F _Electronic configuration of Neon achieved in both casesExample of Ionic Bonding10119Ionic Bonding refers to electrostatic forces between ions,usually a metal cation and a non-metal anion Covalent Bondin
5、g results from the sharing of two electrons between two atoms(usually non-metals)resulting in moleculesThere are two types of bonding;Octet Rule appliesHHHHClClClClNNNN+number of electrons around each atom=He+number of electrons around each atom=Ar+number of electrons around each atom=NeEach Covalen
6、t Bond contains two electronsTriple bondHCHHHmethaneCarbon has 4 valence electronsCHHHHHCNe NeonStable Octet required Covalent Bonding Atoms Share ElectronsHCHHHmethaneCarbon has 4 valence electronsCHHHHHCNe NeonStable Octet required Covalent Bonding Atoms Share ElectronsHydrogen molecule,H2Concentr
7、ation of negative charge between two nuclei occurs in a covalent bond7A elements(e.g.F)have one valence electron for covalent bonding,so to achieve octet6A elements(e.g.O)use two valence electrons for covalent bonding,so to achieve octet5A elements(e.g.N)use three valence electrons for covalent bond
8、ing,so to achieve octet4A elements(e.g.C)use four valence electrons for covalent bonding,so to achieve octetPClClClPClClClRules for Drawing Lewis StructuresFirst sum the number of valence electrons from each atomThe central atom is usually written first in the formulaComplete the octets of atoms bon
9、ded to the central atom(remember that H can only have two electrons)Place any left over electrons on the central atom,even if doing so it results in more than an octetIf there are not enough electrons to give the central atom an octet,try multiple bonds PCl3Total Number of valence electrons=5+(3 x 7
10、)=26PClClClCHBr3Total Number of valence electrons=4+1+(3 x 7)=26BrCBrHBr共价键八隅体规则的特例共价键八隅体规则的特例1.Molecules with an odd number of electrons2.Other Natural Radicals,which do not obey Lewis Structures(e.g.O2)2.Molecules in which an atom has less than an octet3.Molecules in which an atom has more than an
11、 octet1.Odd Number of ElectronsNONumber of valence electrons=11N ON OResonace ArrowsO OO OOxygen is a ground statediradicalNO2Number of valence electrons=17O2Resonance occurs when more than one valid Lewis structure can be written for a particular molecule(i.e.rearrange electrons)Molecules and atoms
12、 which are neutral(contain no formal charge)and with an unpaired electron are called RadicalsN OON OON OORadicals and BiradicalsSpecies having electrons with unpaired spins are called radicals.One example is the methyl radical,CH3,which is so reactive that it cannot be stored.Radicals are of crucial
13、 importance for the chemical reactions that take place in the upper atmosphere,where they contribute to the formation and decomposition of ozone.A biradical is a molecule with two unpaired electrons.One of the most important examples is the oxygen atom itself.Its electron configuration is He2s22px 2
14、2py 12pz1 and its Lewis symbol is.The O atom has two unpaired electrons,and so it can be regarded as a special type of biradical.2.Less than an OctetIncludes Lewis acids such as halides of B,Al and compounds of BeBCl3Group 3A atom only has six electrons around itHowever,Lewis acids“accept”a pair of
15、electrons readily from Lewis bases to establish a stable octetClAlClClNHHHClAlClClNHHH+Lewis acidLewis basesalt+_BClClClAlX3Aluminium chloride is an ionic sol i d i n w h i c h A l3+i s surrounded by six Cl-.However,it sublimes at 192 C to vapour Al2Cl6 moleculesB2H6A Lewis structure cannot be writt
16、en for diborane.This is explained by a three-centre bond single electron is delocalized over a B-H-B BHHHHBHH3.More than an OctetPCl5Elements from the third Period and beyond,have ns,np and unfilled nd orbitals which can be used in bondingP:(Ne)3s2 3p3 3d0Number of valence electrons=5+(5 x 7)=40PClC
17、lClClCl10 electrons around the phosphorusSF4S:(Ne)3s2 3p4 3d0Number of valence electrons=6+(4 x 7)=34SFFFFThe Larger the central atom,the more atoms you can bond to it usually small atoms such as F,Cl and O allow central atoms such as P and S to expand their valency.PCl3(l)+Cl2(g)PCl5(s)Phosphorus t
18、richloride is a colorless liquid.When it reacts with chlorine(the pale yellow-green gas in the flask),it forms the very pale yellow solid phosphorus pentachloride(at the bottom of the flask).Phosphorus pentachloride is an ionic solid consisting of PCl4 cations and PCl6 anions;but at 160C,it vaporize
19、s to a gas of PCl5 molecules.Phosphorus pentachloride,PCl5(s)Electronegativity is defined as the ability of an atom in a molecule to attract electrons to itselfProf.Linus PaulingNobel Prize for Chemistry 1954Nobel Prize for Peace 1962Electronegativity is a function of two properties of isolated atom
20、s;The atoms ionization energy(how strongly an atom holds onto its own electrons)The atoms electron affinity(how strongly the atom attracts other electrons)For example,an element which has:A large(negative)electron affinityA high ionization(always endothermic,or positive for neutral atoms)Will:Attrac
21、t electrons from other atoms and Resist having electrons attracted awaySuch atoms will be highly electronegativePauling scale of electronegativity;Fluorine is the most electronegative element followed by O and N,Cl are equal third.Cs is least.Electronegativity increases from left to right along the
22、Periodic Table.For the representative elements(s&p block),the electronegativity decreases as you go down a group.No trend in the transition metals.电负性的决定因素：电负性的决定因素：The number of protons in the nucleusacross a period you are increasing the number of protons,but filling electrons in the same Bohr qua
23、ntized energy level.You are only filling sub-shells,so electronegativity increases from left to rightThe distance from the nucleusdown groups,you are placing electrons into new quantized energy levels,so moving further away from the attractive power of the nucleus.Outer shell becomes further away fr
24、om the nucleus.The amount of screening by the inner electrons level of screening upon bonding electrons increases down groups,and adds to the reduction in electronegativity.Screening is caused by repulsion of electrons for each other.分子内的三种主要化学键分子内的三种主要化学键:非极性共价键非极性共价键：原子间的电负性差别较小：原子间的电负性差别较小(less t
25、han 0.5).极性共价键极性共价键：原子间的电负性差别原子间的电负性差别(between 0.5 and 2.0).离子键：离子键：原子间的电负性差别原子间的电负性差别(2.0 or greater),电子能发生转移。电子能发生转移。E.g.F-F(4.0 4.0=0)is non-polar covalentH-F(4.0 2.1=1.9)is polar covalentLiF(4.0 1.0=3.0)is ionic HF+-Dipole Moment occurs in any polar covalent bond,because of an unequal sharing of
26、 the electron pair between two atomsE.g.Which of the following bonds is most polar:S-Cl,S-Br,Se-Cl or Se-Br?S-Cl(3.0 2.5)=0.5S-Br(2.8-2.5)=0.3Se-Cl(3.0-2.4)=0.6Se-Br(2.8-2.4)=0.4Therefore,Se-Cl is the most polar!We should be able to reach the same conclusion using the Periodic Table,Cl is furthest t
27、o the right and to the top of the Periodic Table,so is the most electronegative.Se is furthest to the left(metallic like)and towards the bottom.Therefore,difference in electronegativity should be the greatest!CompoundBond Length()Electronegativity DifferenceDipole Moment(D)H-F0.921.91.82H-Cl1.270.91
28、.08H-Br1.410.70.82H-I1.610.40.44Electronegativity difference decreases as bond length increasesDipole Moment:=QrDipole moment is defined as the magnitude of charge(Q)multiplied by the distance between the charges;units are D(Debye)=3.36 x 1030 C.mProf.Peter DebyeNoble Prize 1936When a small,highly charged cation is close to a large anion,the electron cloud of the anion is distorted in the process we call polarization.The green sphere represents the shape of anion in the absence of a cation.The gray shadow shows how the shape of the sphere is distorted by the positive charge of the cation.
