2020高中化学路易斯共价键理论

1、2020高中化学路易斯共价键理论2020高中化学路易斯共价键理论1、Lewis 符号H He: K :Cl:Cl:2、Lewis 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。八隅体规则1、Lewis 符号H He: K :ClLewis SymbolsRepresent the number of valence electrons as dotsValence number is the same as the Periodic Table Group NumberFor example, Groups12 3 4 5 6 7

2、8 Lewis SymbolsRepresent the numElements 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 electronsOctet RuleElements want to achieve the sNobel Gas Has a Stable Electron ConfigurationElectronic confi

3、guration of Neon achieved in both casesExample of Ionic Bonding10119Nobel Gas Has a Stable ElectroIonic Bonding refers to electrostatic forces between ions, usually a metal cation and a non-metal anion Covalent Bonding results from the sharing of two electrons between two atoms (usually non-metals)

4、resulting in moleculesThere are two types of bonding;Octet Rule appliesEach Covalent Bond contains two electronsTriple bondIonic Bonding refers to electr Covalent Bonding Atoms Share Electrons Covalent Bonding Atoms Shar2020高中化学路易斯共价键理论2020高中化学路易斯共价键理论2020高中化学路易斯共价键理论2020高中化学路易斯共价键理论2020高中化学路易斯共价键理论

5、2020高中化学路易斯共价键理论2020高中化学路易斯共价键理论2020高中化学路易斯共价键理论 Covalent Bonding Atoms Share Electrons Covalent Bonding Atoms SharHydrogen molecule, H2Concentration 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 octe

6、t6A 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 bonding, so to achieve octet4A elements (e.g. C) use four valence electrons for covalent bonding, so to achieve octetHydrogen molecule, H2ConcentraRu

7、les 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 bonded to the central atom (remember that H can only have two electrons)Place any left over electrons on the central atom, even

8、 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) = 26Rules for Drawing Lewis StructCHBr3Total Number of valence electrons = 4 + 1 + (3 x 7) = 26共价键八隅体规则的特例Molecule

9、s with an odd number of electronsOther Natural Radicals, which do not obey Lewis Structures (e.g. O2) Molecules in which an atom has less than an octet3.Molecules in which an atom has more than an octetCHBr3Total Number of valence e1. Odd Number of ElectronsNONumber of valence electrons = 11NO2Numbe

10、r 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 which are neutral (contain no formal charge) and with an unpaired electron are called Radicals1. Odd Number of ElectronsNONuR

11、adicals 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 importance for the chemical reactions that take place in the upper atmosphere, where they contribute to

12、 the formation and decomposition of ozone.A biradical is a molecule with two unpaired electrons.Radicals and BiradicalsSpeciesOne of the most important examples is the oxygen atom itself. Its electron configuration is He2s22px 22py 12pz1 and its Lewis symbol is . The O atom has two unpaired electron

13、s, and so it can be regarded as a special type of biradical.One of the most important exam2. 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 electrons readily from Lewis bases

14、 to establish a stable octet2. Less than an OctetIncludes AlX3Aluminium chloride is an ionic solid in which Al3+ is surrounded by six Cl-. However, it sublimes at 192 C to vapour Al2Cl6 moleculesB2H6A Lewis structure cannot be written for diborane. This is explained by a three-centre bond single ele

15、ctron is delocalized over a B-H-B AlX3Aluminium chloride is an i3. 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) = 4010 electrons around the phosphorusSF4

16、S : (Ne) 3s2 3p4 3d0Number of valence electrons = 6 + (4 x 7) = 34The 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.3. More than an OctetPCl5ElemePCl3(l) + Cl2(g) PCl5(s)Phosphorus trichl

17、oride 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).PCl3(l) + Cl2(g) Phosphorus pentachloride is an ionic solid consisting of PCl4 cations and PCl6 anions; but at

18、 160C, it vaporizes to a gas of PCl5 molecules.Phosphorus pentachloride, PCl5(s)Phosphorus pentachloride is an2020高中化学路易斯共价键理论2020高中化学路易斯共价键理论2020高中化学路易斯共价键理论Electronegativity is defined as the ability of an atom in a molecule to attract electrons to itselfProf. Linus PaulingNobel Prize for Chemistr

19、y 1954Nobel Prize for Peace 1962Electronegativity is a function of two properties of isolated atoms;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 (ne

20、gative) electron affinityA high ionization (always endothermic, or positive for neutral atoms)Will: Attract electrons from other atoms and Resist having electrons attracted awaySuch atoms will be highly electronegativeElectronegativity is defined aPauling scale of electronegativity;Fluorine is the m

21、ost electronegative element followed by O and N, Cl are equal third. Cs is least.Electronegativity increases from left to right along the Periodic Table.For the representative elements (s & p block), the electronegativity decreases as you go down a group.No trend in the transition metals.Pauling sca

22、le of electronegati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 quantized energy level. You are only filling sub-shells, so electronegativity increases from left to rightThe distance from the nucleusdown

23、 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 from the nucleus. The amount of screening by the inner electrons level of screening upon bonding electrons increases down groups, and

24、 adds to the reduction in electronegativity. Screening is caused by repulsion of electrons for each other.电负性的决定因素：分子内的三种主要化学键:非极性共价键：原子间的电负性差别较小 (less than 0.5).极性共价键：原子间的电负性差别(between 0.5 and 2.0).离子键：原子间的电负性差别(2.0 or greater), 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

25、) is polar covalentLiF (4.0 1.0 = 3.0) is ionic +-分子内的三种主要化学键:E.g. F-F (4.0 42020高中化学路易斯共价键理论2020高中化学路易斯共价键理论2020高中化学路易斯共价键理论Dipole Moment occurs in any polar covalent bond, because of an unequal sharing of the electron pair between two atomsE.g. Which of the following bonds is most polar: S-Cl, S-B

26、r, 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 to 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 th