Drawing Good Lewis Structures. Molecular Shape

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1 3//05 Drawing Good Lewis Structures. # valence e in atoms (± charge) must = # e in structure ; always. determine connectivity: least EN usually central; avoid small rings; always terminal ( e ); work out of a central atom (avoid long chains) 3. complete octet for each atom (except ); check against # 4. remove/add required e in pairs from/to central atom 5. if needed, move e pairs from outside atoms to bond with central atom to complete octet again 6. formal charge minimized as much as possible (# and magnitude) N 3 3 x ( ve ) 3 e x N (5 ve ) 5 e N 8 e octet 8 e total? Molecular Shape Ronald J. Gillespie English Ronald S. Nyholm Australian 957 Valence-Shell Electron-Pair Repulsion e regions on central atom repelled as far as possible = = = x x x x x linear trigonal planar tetrahedral trigonal bipyramidal octahedral

2 Energy (ev) 3// o 0 o VSEPR and Deviations 09.5 o 80 o linear 0 o bent trigonal planar o 09.5 o bent trigonal pyramidal tetrahedral 90 o 90 o 90 o linear 0 o T-shaped see-saw trigonal bipyramidal 90 o square planar square pyamidal octahedral deviate from ideal when lone-pair involved bonding-bonding < Lp-bonding < Lp-Lp Valence Bond, A, B ; each e (, ) in s orbital ( ) 0 r (Å) experimental

3 3//05 verlap Symmetry s cross section p cross section s s s s p p p s d S σ > S π > S δ d ½(s + p x + p y + p z ) Valence Bond Treatment: C 4 ½(s + p x p y p z ) ½(s p x + p y p z ) 4 sp 3 orbitals ½(s p x p y + p z ) E sp 3 weighted average E 3

4 3//05 sp ybrid rbitals 3 s + px 3 3 s 6 p x + p y sp 3 s 6 p x p y trigonal planar E sp sp ybrid rbitals (s + p z ) (s p z ) sp linear E sp 4

5 3//05 ther ybrid rbitals equatorial s + p x s + p x + d z s p x d z s p x + p y s + p z d z d x y axial s p x p y p z + d z dsp 3 s + p y d z d x y d sp 3 s p x d z + d x y p z d z s p y d z d x y trigonal bipyramidal octahedral ybrid rbitals and Bond Strength bond strength S s character S sp > sp > sp S S C-C S C % s 33% s 5% s 0.3 sp 3 sp sp % s character 5

6 Energy 3//05 Multiple Bonds Cl 3 sp Cl low E d multiple bonds from π (and δ) overlap: Cl C l sp 3 more bonds, lower E sp sp Si Si non-vsepr N(Si 3 ) 3 D 3h not C 3v Si N sp Si low E d Si Si M Treatment no e density between nuclei antibonding (u) s * s DE s s E = E DE s s e density between nuclei bonding (g) lower energy than by x DE. 6

7 Energy 3//05 Molecular rbitals S E/symmetry: S AB > 0, bonding: E stabilized S σ > S π > S δ S AB < 0, antibonding: E destabilized S AB = 0, nonbonding: no stabilization + + π* g + + π u s* u s g s* s u s s g omonuclear Diatomic M Diagram: A complicated by s-p mixing when s and p close in E changes relative M E s s* p* x y z z y x p s* s s* s s s* B C N F A s A 7

8 Energy Energy 3//05 s-p Energy Separation in First Row Elements complicated by s-p mixing when s and p close in E changes relative M E s B C N F s* s s* s B C N F DE (ev) omonuclear Diatomic M Diagram complicated by s-p mixing when s and p close in E changes relative M E s s* p* x y z z y x p s s* s 8

9 3//05 omonuclear Diatomic M Diagram complicated by s-p mixing when s and p close in E changes relative M E s 6s u g x y z z y x 5s g p u 4s u 3s g s s g, u M Diagram: Li, Be, B, C, N 6s u g x y z z y x 5s Li g p u 4s u Li 3s g Li 9

10 3//05 M Diagram: Li, Be, B, C, N 6s u g x y z z y x 5s g Be p u 4s u Be 3s g Be M Diagram: Li, Be, B, C, N 6s u g x y z z y x 5s g B p u 4s u B 3s g B 0

11 3//05 M Diagram: Li, Be, B, C, N 6s u g x y z z y x 5s g C p u 4s u C 3s g C M Diagram: Li, Be, B, C, N crosses over at to simpler diagram 6s u g x y z z y x 5s g N p u 4s u N 3s g N

12 3//05 M Diagram:, F, Ne s* p* x y z z y x p s s* s M Diagram:, F, Ne s* p* x y z z y x F p s s* F s F

13 3//05 M Diagram:, F, Ne s* p* x y z z y x Ne p s s* Ne s Ne Bond Length /Bond rder s* p* p s s* + order length, pm s 3

14 3//05 Cr : d Bonds Cr s* s s* z p* xz, yz d* x y, xy x y xy yz xz z z xz yz xy x y 3d 3d d 4s x y, xy 4s Cr p xz, yz Cr s z p xz, yz s z s s d x y, xy Carbon Monoxide M Diagram s* p* p C s nb s C C s nb 4

15 3//05 b * a * B A A B B b a A b a C 4 SALC 5

16 Electronegativity 3//05 Diborane, B 6 : 3-Center, e Bond B D h B A g s * B u s nb A g B, B s 0 0 Electronegativity (c) Linus Pauling American F Cl Li Na K Br Rb I Xe Atomic Number Cs At Fr 6

17 Total Energy (MJ/mole) Total Energy 3// Total Energy Ne F Cl 45 0 E = aq + bq where E = S IE or S EA q = ionic charge xidation State Mulliken-Jaffe Electronegativity 4 Cl 0 8 Cl larger c than Na 6 4 Na 0 4 Charge 7

18 Total Energy 3//05 Electronegativity Equalization 4 Cl Na 0 4 Charge 8