Lewis Structures Valence Bond Thery路易斯结构、价键理论

1、 chemistry a molecule is composed of atoms that are bound together by sharing electron pairs. a bonding pair is of which two electrons are localized between two adjacent nuclei. a lone pair is that the electron pair is localized in an atom.localized electron bonding modeloctet rule1 electron dot (ba

2、r) formula 2 valence eletrons & net charge3 multiple bond4 dative bond (arrow to the acceptor) 5 formal charge what is the difference between f. c. & ox. no6 generally good thru the 3rd periodwhat is the difference between f. c. & ox. noformal charge is for the comparison of the elctron

3、change between the bonded and the non-bonded atomic molds.oxidation number is for the comparison of the electron change between the oxidized and the reduced molds.formal charge and oxidation numbers are the same for the ionic compounds.disadvantage of lewis structure1 octet rule is no good for d orb

4、ital or d electrons involved species use 18 electron rule2 no good for electron delocalization use resonance3 octet rule is no good for odd electron species4 no information for geometry- use vsepr 5 it does not explain the bonding character use vbt or mot valence bond theory (vbt)atomic orbitals tha

5、t involve in chemical bonding are the major concern.ij 0, wherein i and j represent the orbital wave functions of different atoms. vbt for h2consideration of simple orbital coulomb overlap:i = 1sa(1)1sb (2) curve ai = 1sa(1) 1sb (2)experimentalconsideration of indistinguishability of the electrons:h

6、eitler london functions:attractive ii+ = 1sa(1)1sb (2) + 1sa(2)1sb (1) curve brepulsive ii = 1sa(1)1sb (2) 1sa(2)1sb (1) curve g i = 1sa(1) 1sb (2)experimentalii+ = 1sa(1) 1sb(2) + 1sa(2) 1sb(1)ii = 1sa(1) 1sb(2) 1sa(2) 1sb(1)consideration of effective nulear charge: 1s = n(z/a0)3/2exp(-zr/a0), z =

7、1.17 curve c the z-modification gives good bond length, but energy is still substantially off.i = 1sa(1) 1sb (2)experimentalii+ = 1sa(1) 1sb(2) + 1sa(2) 1sb(1)ii = 1sa(1) 1sb(2) 1sa(2) 1sb(1)z = 1.17consideration of polarizability or hybridization:a = n (1sa + g g 2pza)b = n (1sb + g g 2pzb), assumi

8、ng z is the nuclear axis.normalization condition gives n = 1/(1 + g g2)1/2contribution of 1sa to a (or 1sb to b ) is 1/(1 + g g2)contribution of 2pza to a (or 2pzb to b ) is g g2/(1 + g g2)if g g is set as 0.1, which gives 99% (1/1.01) of 1s and 1% (0.01/1.01) of 2pz. note that 1% of contribution of

9、 pz accounts for about 5% energy stabilization, mainly due to the enhancement of orbital overlap efficiency. i =1sa(1) 1sb (2)experimentalii+ = 1sa(1) 1sb(2) + 1sa(2) 1sb(1)ii =1sa(1) 1sb(2) 1sa(2) 1sb(1)z = 1.17a = n (1sa + g g 2pza)b = n (1sb + g g 2pzb) g g = 0.1combine indistinguishability and h

10、ybridization:attractive heitler london functions:iii = a(1)b (2) + a(2)b (1)consideration of other overlaps 1sa2pb/2pa1sb and 2pa2pb hh hahb+ ha+hbiv = a(1)b (2) + a(2)b (1) + l la(1)a (2) + l lb(1)b (2)l= 0.25 would minimize the total energy. the value of l l2 0.06, indicating that about 6% ionic c

11、ontribution will only improve the energy to 23%.consideration of ionization:i = 1sa(1) 1sb (2)experimentalii+ = 1sa(1) 1sb(2) + 1sa(2) 1sb(1)ii = 1sa(1) 1sb(2) 1sa(2) 1sb(1)z = 1.17a = n (1sa + g g 2pza)b = n (1sb + g g 2pzb) g g = 0.1iv = a(1) b(2) + a(2) b(1) + l la(1) a(2) + l lb(1) b(2)valence b

12、ond of hfconsideration of indistinguishable orbital overlapi = 1s(h)(1)2pz(f)(2) + 1s(h)(2)2pz(f)(1)consideration of hybridization:hybridization for a.o. of f would give the configuration of 2s12p6 which allows the use of 2s for bonding.f = n (2pz(f) + g g 2s(f), g g 0.5 and g g2 0.25, the contribution of 2s (i.e. g g2 /(1+g g2) in f is less than 20%.consideration of ionic model: = 1s(h)(1)f (2) + 1s(h)(2)f (1)+ l lf(1)f(2)pauling estimated that l l2 50%. 1 it helps to have the electron density concentrat