Unit 10: Molecular Geometries and Bonding Theories

1. CHM 1045: General Chemistry and Qualitative Analysis Unit 10:Molecular Geometriesand Bonding Theories Dr. Jorge L. Alonso Miami-Dade College – Kendall Campus Miami, FL • Textbook Reference: • Chapter # 8 (skip 9) • Module #12 (skip 13)

2. Molecular Shape and Reactivity: Lock & Key Theory The shape of a molecule plays an important role in its reactivity Receptor, active site {MolecularShape&Reactivity} Molecular Shape determines function! t-RNA

3. Molecular Shape: Effect of Cocaine on CNS Dopamine binding to receptors and uptake pumps. Cocaine concentrates in areas of the brain that are rich in dopamine synapses. Presynaptic Neuron synapse Postsynaptic Neuron Cocaine binding to uptake pumps; inhibition of dopamine uptakewhen cocaine is present in the synapse.Cocaine (turquoise) binds to the uptake pumps and prevents them from removing dopamine from the synapse. This results in more dopamine in the synapse, and more dopamine receptors are activated.

4. Molecular Shape determines function! Dopamine Cocaine

5. Molecular Shapes • By noting the number of bonding and nonbondingelectron pairs we can easily predict the shape of the molecule. 2. electron pairs, whether they be bonding or nonbonding, repel each other. 3. By assuming the electron pairs are placed as far as possible from each other.

6. Bonding Theories (1)Valence Shell Electron Pair Repulsion (VSEPR) Theory: considers only the valence electrons and their geometry when they repel each other when they form bonds. (2)Valence Bond (VB) Theory: considers the valence electrons and the quantum mechanical orbitals and how they hybridize to form bonds. (3) Molecular Orbital (MO) Theory: uses bonding and antibonding molecular orbitals which best explain the energy characteristics of molecules.

7. Electron Domains • We can refer to the electron pairs as electrondomains. • In a double or triple bond, all electrons shared between those two atoms are on the same side of the central atom; therefore, they count as one electron domain. • This molecule has four electron domains.

8. Valence Shell Electron Pair Repulsion Theory (VSEPR) “The best arrangement of a given number of electron domains is the one that minimizes the repulsions among them.”

9. Electron-Domain Geometries These are the electron-domain geometries for two through six electron domains around a central atom. {VSEPRTheoryElectrDomain}

10. Electron-Domain Geometries How would the structure of CH4 and NH3 differ? • Count the number of electron domains in the Lewis structure. • The geometry will be that which corresponds to that number of electron domains.

11. Molecular Geometries • The electron-domain geometry is often not the shape of the molecule, however. • The molecular geometry is that defined by the positions of only the atoms in the molecules, not the nonbonding pairs.

12. Linear Electron Domain • In this domain, there is only one molecular geometry: linear. • NOTE: If there are only two atoms in the molecule, the molecule will be linear no matter what the electron domain is.

13. Trigonal Planar Electron Domain • There are two molecular geometries: • Trigonal planar, if all the electron domains are bonding • Bent, if one of the domains is a nonbonding pair.

14. Multiple Bonds and Bond Angles • Double and triple bonds place greater electron density on one side of the central atom than do single bonds. • Therefore, they also affect bond angles.

15. Tetrahedral Electron Domain • There are three molecular geometries: • Tetrahedral, if all are bonding pairs • Trigonal pyramidal if one is a nonbonding pair • Bent if there are two nonbonding pairs

17. Nonbonding Pairs and Bond Angle • Nonbonding pairs are physically larger than bonding pairs. • Therefore, their repulsions are greater; this tends to decrease bond angles in a molecule.

18. Trigonal Bipyramidal Electron Domain • There are two distinct positions in this geometry: • Axial • Equatorial Where will the nonbonding electrons be for SF4? SF4

19. Trigonal Bipyramidal Electron Domain Lower-energy conformations result from having nonbonding electron pairs in equatorial (where there is more space), rather than axial, positions in this geometry.

20. Trigonal Bipyramidal Electron- Domain

21. Octahedral Electron Domain • All positions are equivalent in the octahedral domain. • There are three molecular geometries: • Octahedral • Square pyramidal • Square planar

22. Electron Domains determined from Molecular Geometries What is the electron domain geometries that led to the formation of these molecular structures? CO2: two double bonds SO2: trigonal planar SO3: trigonal planar (1 double bond) NF3: tetrahedral ClF3: trigonal bipyramidal (5 e- pairs on Cl)

23. Larger Molecules: HC2H3O2 3 In larger molecules, it makes more sense to talk about the geometry about a particular atom rather than the geometry of the molecule as a whole. 1 2 4

24. Larger Molecules This approach makes sense, especially because larger molecules tend to react at a particular site in the molecule. {OtherLargeMolecules}

25. Molecular Structure

26. Polarity Just because a molecule possesses polar bonds does not mean the molecule as a whole will be polar.

27. Polarity By adding the individual bond dipoles, one can determine the overall dipole moment for the molecule.

28. Polarity

29. Valence Bond Theory:Orbital (s,p,d,f) Overlap and Bonding • Covalent bonds are formed by the sharing of electrons from adjacent atoms whose orbitals overlap. • Which orbitals are involved in the bonding of the molecules shown below? H2 HCl Cl2

30. Atomic Orbitals & Molecular Geometries? Molecular Geometries It is hard to imagine that from the atomic orbitals we know (s, px, py & pz) sp Atomic Orbitals sp2 ……..could arise the molecular geometries we know to exist: tetrahedral, trigonal planar, etc. p orbitals have Angles that are 900 from each other sp3

31. Hybrids: Mixing Orbitals ¨ ¨ BeF2 = : F : Be : F : (Be & B, accept less than octet) ¨ ¨ • Consider beryllium (Be): In its ground electronic state, it would not be able to form bonds because it has no singly-occupied orbitals. • But if it absorbs the small amount of energy needed to promote an electron from the 2s to the 2p orbital, it can form two bonds. sp hybrid • With hybrid orbitals the orbital diagram for beryllium would look like this. • The sp orbitals are higher in energy than the 1s orbital but lower than the 2p.

32. Hybrid Orbitals • Mixing the s and p orbitals yields two degenerate orbitals that are hybrids of the two orbitals. • These sp hybrid orbitals have two lobes like a p orbital. • One of the lobes is larger and more rounded as is the s orbital. sp hybrid orbitals for Beryllium

33. Hybrid Orbitals • These two degenerate orbitals would align themselves 180 from each other. • This is consistent with the observed geometry of beryllium compounds: linear. ¨ ¨ BeF2 = : F : Be : F : (Be & B, accept less than octet) ¨ ¨ ¨ ¨ : : ¨ ¨

34. Hybrid Orbitals Using a similar model for boron (B) leads to… BF3

35. Hybrid Orbitals …three degenerate sp2 orbitals. BF3

36. Hybrid Orbitals With carbon we get… CH4

37. Hybrid Orbitals …four degenerate sp3 orbitals. CH4

38. Hybrid Orbitals For geometries involving expanded octets on the central atom, we must use d orbitals in our hybrids. PF5

39. Hybrid Orbitals PF5 This leads to five degenerate sp3d orbitals… …or six degenerate sp3d2 orbitals. SF6

40. Hybrid Orbitals (1) (2) Once you know the electron-domain geometry, you know the hybridization state of the atom. (3) (4) (5) {HybridOrbitals}

41. Valence Bond Theory: Bond Types • Hybridization is a major player in this approach to bonding. • There are two ways orbitals can overlap to form bonds between atoms.

42. Sigma () Bonds HCl s-px bond Cl2 px-px bond H2 s-s bond • Sigma bonds are characterized by • Head-to-head overlap. • Cylindrical symmetry of electron density about the internuclear axis.

43. Pi () Bonds • Pi bonds are characterized by • Side-to-side overlap. • Electron density above and below the internuclear axis.

44. Single Bonds Single bonds are always  bonds, because  overlap is greater, resulting in a stronger bond and more energy lowering.

45. Multiple Bonds In a multiple bond one of the bonds is a  bond and the rest are  bonds.

46. Multiple Bonds • In a molecule like formaldehyde (shown at left) an sp2 orbital on carbon overlaps in  fashion with the corresponding orbital on the oxygen. • The unhybridized p orbitals overlap in  fashion.

47. Multiple Bonds In triple bonds, as in acetylene, two sp orbitals form a  bond between the carbons, and two pairs of p orbitals overlap in  fashion to form the two  bonds. H C C H

48. Delocalized Electrons: Resonance When writing Lewis structures for species like the nitrate ion, we draw resonance structures to more accurately reflect the structure of the molecule or ion.

49. Delocalized Electrons: Resonance • In reality, each of the four atoms in the nitrate ion has a p orbital. • The p orbitals on all three oxygens overlap with the p orbital on the central nitrogen.

50. Delocalized Electrons: Resonance This means theelectrons are not localized between the nitrogen and one of the oxygens, but rather aredelocalized throughout the ion.