Molecular Geometry and Bonding Theories

1. Molecular Geometry and Bonding Theories 7.1 Molecular Geometry The VSEPR Model Electron-Domain Geometry and Molecular Geometry Deviation from Ideal Bond Angles Geometry of Molecules with More Than One Central Atom 7.2 Molecular Geometry and Polarity 7.3 Valence Bond Theory 7.4 Hybridization of Atomic Orbitals Hybridization of s and p Orbitals Hybridization of s, p, and d Orbitals 7.5 Hybridization of Molecules Containing Multiple Bonds 7.6 Molecular Orbital Theory Bonding and Antibonding Molecular Orbitals σMolecular Orbitals Bond Order π Molecular Orbitals Molecular Orbital Diagrams 7.7 Bonding Theories and Descriptions of Molecules with Delocalized Bonding 7

2. VSEPR Valence Shell Electron Pair Repulsion Electrons tend to repel each other due to their like charges. The electrons around the central atom typically exist in groups. These associated groups are referred to as …. a) lone pairs unshared pair of electrons on atom b) bonding domains: shared pairs of electrons between atoms single, double, or triple bonds (all count as 1 BD) c) radical single electron on atom (rare and reactive) The lone pairs and bonding domains will be arranged around the central atom in a way that maximizes their separation distance. A molecular model treats lone pairs as if they are invisible, and yet they influence the positioning of the bonds in a molecule. Electron distribution vs. Bond distribution electron geometry vs. molecular geometry

3. VSEPR Valence Shell Electron Pair Repulsion • Draw the Lewis dot structure for the compound • Count the number of bonding domains around the central atom • Count the number of lone pairs around the central atom • Add these two numbers – this determines the electron geometry • Describe the molecular geometry based on bonding domains only Bond domains electron + lone pairsgeometry 2 3 4 linear trigonal planar tetrahedral

4. CH4 H | H – C – H | H VSEPR Valence Shell Electron Pair Repulsion electron geometry = tetrahedral Molecular geometry = tetrahedral considers bond domains only  = 109 If there are no lone pairs the electron geometry and the molecular geometry are the same Bond domains = 4, Lone pairs = 0

5. VSEPR Valence Shell Electron Pair Repulsion • Draw the Lewis dot structure for the compound • Count the number of bonding domains around the central atom • Count the number of lone pairs around the central atom • Add these two numbers H C Bond domains electron + lone pairsgeometry H H H 2 3 4 linear trigonal planar tetrahedral

6. ~ 105º Molecular geometry = bent Molecular geometry = pyramid • < 109 Lone pairs have greater repulsion than bond domains H – O – H ~ 107º bds= 2, lp= 2 e- geometry = tetrahedral H – N – H | H Electron geometry vs. Molecular geometry bd + lone pairs bd only lp= 1, bd= 3 e- geometry = tetrahedral

7. Deviation from Ideal Bond Angles Some electron domains are better than others at repelling neighboring domains. Lone pairs take up more space than bonded pairs of electrons. Multiple bonds repel more strongly than single bonds.

8. H C .. :O .. N Tetrahedral pyramid bent 3-sided H H H H H All have tetrahedral electron geometries H H H Determine the molecular geometry for ….Ozone, O3 a) linear b) pyramid c) trigonal planar d) bent What is the approximate bond angle in ozone? a) 90˚ b) 109˚ c) 120˚ d) 180˚

9. Orbital shapes: s spherical p double lobe along each axis d four lobes between each axis pair or along xy axes double lobe along z-axis with doughnut shaped loop in xy plane. What is the bond angle between any pair of p orbitals from an atom? a) 90˚ b) 109˚ c) 120˚ d) 180˚ How does bonding theory explain that tetrahedral molecules have 109˚ bond angles and not 90˚? a) The s orbitals on the central atom are not used in bonding. b) The p orbitals on the central atom change their orientation. c) The s and p orbitals on the central atom mix to form 4 equal energy orbitals.

10. The electron geometry of the central atom in a molecule is determined by the sum of the bonding domains and lone pairs present on the central atom. One s orbital and the smallest number of p orbitals required to equal bd + lp are “hybridized” to form ….. linear bd + lp = 2 sp hybrids trigonal planar bd + lp = 3 sp2 hybrids tetrahedral bd + lp = 4 sp3 hybrids

11. Draw Lewis dot - # bd - #lp – sum – electron geometry – molecular geometry Predict the molecular geometry for ……. 1) BH3 2) SO2 3) CO2

12. VSEPR Valence Shell Electron Pair Repulsion Draw Lewis dot - # bd - #lp – sum – electron geometry – molecular geometry Molecules with expanded octets have electron geometries that are either trigonalbipyramid or octahedral. Bond domains electron + lone pairsgeometry PF5 2 3 4 5 6 linear trigonal planar tetrahedral trigonalbipyramid octahedral

13. Bond domainshybridElectronmolecular + lone pairstypeGeometrygeometry 2sp 3 sp2 4 sp3 5dsp3 6 d2sp3 Linear trigonal/bent tetrahedral; trigonal pyramid bent Trigonalbipyramid seesaw; T-shaped; linear Octahedral; square pyramid; square planar Linear Trigonal Tetrahedral Trigonal bipyramid Octahedral

14. Electron-Domain Geometry and Molecular Geometry The electron domain geometryis the arrangement of electron domains around the central atom. The molecular geometryis the arrangement of bonded atoms. 180° 120° Linear Trigonal planar 90° 109.5° 120° 90° Tetrahedral Trigonal bipyramidal Octahedral

15. Electron-Domain Geometry and Molecular Geometry AB5 molecules contain two different bond angles between adjacent bonds. Axial positions; perpendicular to the trigonal plane 90° Equatorial positions; three bonds arranged in a trigonal plane. 120° Trigonal bipyramidal

17. Deviation from Ideal Bond Angles Some electron domains are better than others at repelling neighboring domains. Lone pairs take up more space than bonded pairs of electrons. Multiple bonds repel more strongly than single bonds.

18. Geometry of Molecules with More Than One Central Atom The geometry of more complex molecules can be determined by treating them as though they have multiple central atoms. Central O atom No. of electron domains: 4 Electron-domain geometry: tetrahedral Molecular geometry: bent Central C atom No. of electron domains: 4 Electron-domain geometry: tetrahedral Molecular geometry: tetrahedral

19. H F Bond Type DEN examples Ionic ≥ 2.0 Na-Cl, Mg-O Polar covalent 0.5 – 1.9 N←H, C→O, O ← H, Nonpolar ≤ 0.4 C — H, Si — H, X - X Bond Polarity vs. Molecule Polarity Both the HF bond and the HF molecule are polar. For any diatomic molecule the bond polarity determines the molecular polarity. d+ d-

20. .. .. .. .. O H H polar bonds O H H Shape = bent dipole moment d- d+ Dipole Moments & Molecular Shape H2O is a dipole

21. H H C = C H H polar bonds cancel A molecule with polar bonds will be nonpolar if it is symmetrical. nonpolar bonds nonpolar molecule O = C = O polar bonds nonpolar molecule H C ClCl Cl Cl C ClCl Cl No dipole dipole

22. ClF3 Best shape? Cl Cl F A : B : F .. .. .. .. direction? a. ↑ b. ↓ c. → d. ← :F — Cl — F: | :F: .. .. F F : F .. F : Cl— F bond? a. ionic b. Nonpolar c. Polar covalent Dipole moment? a. Yes a. no

23. Valence Bond Theory The H−H bond in H2 forms when the singly occupied 1s orbitals of the two H atoms overlap: The probability of finding the electrons between the 2 H atomsincreases. This produces a bonding molecular orbital (MO) which would be fatter in the middle and shrink in size on the outside of the two H atoms.

24. Valence Bond Theory According to valence bond theory, atoms share electrons when atomic orbitals overlap. A bond forms when single occupied atomic orbitals on two atoms overlap. Formation of a bond results in a lower potential energy for the system. The overlapping atomic orbitals now form a molecular orbital. The Pauli Exclusion Principle applies to MOs as well as AOs. The pair of electrons must have opposite spins and the orbital occupancy is limited to 2. 7.3

25. F F H H Valence Bond Theory … Molecular Orbital Theory A bond is formed when two atomic orbitals (AO) from different atoms overlap. H 1s orbital (with 1e-) overlaps with F 2p orbital (with only 1e-) The bonding electrons now occupy a Molecular Orbital (MO). The probability of finding e- between nuclei 

26. Valence Bond Theory The H−F bond in HF forms when the singly occupied 1s orbital on the H atom overlaps with the single occupied 2p orbital of the F atom:

27. Valence Bond Theory The F−F bond in F2 forms when the singly occupied 2p orbitals of the two F atoms overlap:

28.   sp p Hybrid Atomic Orbitals- BeH2 Be 1s2 2s2 H – Be – H 2p  2s Why does Be share 2 electrons at even spacing when its valence e- are already paired in the 2s orbital?  1s Hybridized AOs # = # bonding domains + # lone pairs and only occur on central atom(s) = 2 for BeH2use s first then as many p orbitals as needed

29. Hybridization of s and p Orbitals BeH2 Experimentally the bond in BeH2bonds are identical in length and strength. Mixing of one s orbital and one p orbital to yield two sp orbitals.

30.     sp3 Hybrid Orbitals C 1s2 2s2 2p2   2p  2s Why does C share 4 electrons at even spacing when 2 of its valence e- are already paired in the 2s orbital?  1s Hybridized AOs = # bonding domains + # lone pairs

31.    sp3 Hybrid Orbitals N 1s2 2s22p3     2p  2s  1s Hybridized AOs = # bonding domains + # lone pairs

32. : : H C .. N O H H H H H H Tetrahedral pyramid bent 3-sided H H All have tetrahedral electron geometries All have sp3 hybridization

33. Hybridization of s, p and d Orbitals

35. Sigma (s) vs. Pi (p) bonds The first bond formed between two atoms is usually a sbond with overlap in bonding axis. Subsequent bonds to make double or triple bonds are p bonds. Overlap above/below bond axis.

36. C2H4 Draw the Lewis dot structure for this molecule Best Frame? a) H b) H C C H H C C H H H H hybrid? a) sp b) sp2 c) sp3 d) sp3d The p bond cold be due to an overlap of which of the following two orbitals? Assuming the bonding axis is the z-axis. a) px-px b) px-py c) pz-pz d) sp2-sp2 p bond H ― C = C ― H | | H H s bond

37. MO Theory AOs from two different atoms will combine to form MO pairs. One is a bonding MO – Energy less than that of AOs The other is an antibonding MO – Energy greater than the AOs Electrons of the molecule are filled into MOs from low to high energy. MO Theory is better than VSEPR theory in explaining resonance/delocalization and magnetism, energy calculations

38. Energy levels of bonding and antibonding molecular orbitals in hydrogen (H2). A bonding molecular orbital has lower energy and greater stability than the atomic orbitals from which it was formed. An antibonding molecular orbital has higher energy and lower stability than the atomic orbitals from which it was formed.

39. s* antibonding MO H H Energy H1s AO H1s AO H H s bonding MO MO = LCAOs

40. s* MO Energy s MO p AO p AO

41. . . p* MO Energy p AO p AO . . p MO

42. Review of Molecular Structure and Bonding Lewis Dot structures – bonding as shared electron pair VSEPR – Molecular Shape and Polarity Valence Bond Theory – bond = overlap of atomic orbitals hybridization of central atom AOs s bond – overlap along bond axis pbond – overlap above/belowbond axis MO Theory – bond = LCAO (linear combination of atomic orbitals) bonding MO – energy lower than AOs antibonding MO – energy higher than Aos Molecular electron configurations - Aufbau

43. Valence Bond Theory – bond = overlap of atomic orbitals s bond – overlap along bond axis The probability of finding the electrons between the 2 H atomsincreases. This produces a bonding molecular orbital (MO) which would be fatter in the middle and shrink in size on the outside of the two H atoms.

44. Hybridization of s, p and d Orbitals Valence Bond Theory – bond = overlap of atomic orbitals hybridization of central atom AOs s bond – overlap along bond axis pbond – overlap above/belowbond axis

45. VSEPR – Molecular Shape and Polarity

46. C2H4 Valence Bond Theory – bond = overlap of atomic orbitals hybridization of central atom AOs s bond – overlap along bond axis pbond – overlap above/belowbond axis H ― C = C ― H | | H H p bond H H \ ⁄ C = C ⁄ \ H H s bond

47. The molecule acetylene C2H2 has the Lewis dot structure H – C ≡ C - H What is the hybridization of AOs on each C atom? a) sp b) sp2 c) sp3 d) sp3d How many p bonds are in this molecule? a) 0 b) 1 c) 2 d) 3 One p bond results from which of the following C(AO) overlaps? Assume the bonding axis is the z axis. a) sp - sp b) pz - pz c) px-px d) px-py

48. Valence Bond Theory – bond = overlap of atomic orbitals • The new orbital produced by this overlap is called …. • A molecular orbital (MO) • A hybridized AO MO Theory AOs from two different atoms will combine to form MO pairs. The # of MOs formed = #AOs combined to form them. One is a bonding MO – Energy less than that of AOs The other is an antibonding MO – Energy greater than the AOs Electrons of the molecule are filled into MOs from low to high energy. MO Theory is better than VSEPR theory in explaining resonance/delocalization and magnetism, energy calculations

49. s* antibonding MO H H Energy H1s AO H1s AO H H s bonding MO MO = LCAOs

50. s* MO Energy s MO p AO p AO