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Theories of Bonding and Structure in Chemistry

This chapter explores VSEPR theory and its application in determining molecular geometry, molecular polarity, and dipole moment. It also covers Valence Bond Theory and Molecular Orbital Theory to understand the stability and bonding in molecules.

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Theories of Bonding and Structure in Chemistry

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  1. Theories of Bonding and Structure CHAPTER 10 Chemistry: The Molecular Nature of Matter, 6th edition By Jesperson, Brady, & Hyslop

  2. CHAPTER 10: Bonding & Structure • Learning Objectives • VESPR theory: • Determine molecular geometry based on molecular formula and/or lewis dot structures. • Effect of bonded atoms & non-bonded electrons on geometry • Molecular polarity & overall dipole moment • Assess overall dipole moment of a molecule • Identify polar and non-polar molecules • Valence Bond Theory • Hybridized orbitals • Multiple bonds • Sigma vs pi orbitals • Molecular Orbital Theory • Draw & label molecular orbital energy diagrams • Bonding & antibonding orbitals • Predict relative stability of molecules based on MO diagrams

  3. Molecular Geometry Basic Molecular Geometries Linear 3 atoms Trigonal Planar or Planar Triangular 4 atoms TrigonalBipyramidal 6 atoms Tetrahedral: 5 atoms Octahedral: 7 atoms Jesperson, Brady, Hyslop. Chemistry: The Molecular Nature of Matter, 6E

  4. VESPR Definition Valence Shell Electron Pair Repulsion Model Electron pairs (or groups of electron pairs) in the valence shell of an atom repel each other and will position themselves so that they are far apart as possible, thereby minimizing the repulsions. Electron pairs can either be lone pairs or bonding pairs. Tetrahedral arrangement of electron pairs Bent geometry Jesperson, Brady, Hyslop. Chemistry: The Molecular Nature of Matter, 6E http://chemistry-desk.blogspot.com/2011/05/prediction-of-shape-of-molecules-by.html

  5. VESPR Definition Valence Shell Electron Pair Repulsion Model Electron pairs (or groups of electron pairs) in the valence shell of an atom repel each other and will position themselves so that they are far apart as possible, thereby minimizing the repulsions. Text uses “electron domain” to describe electron pairs: Bonding domain: contains electrons that are shared between two atoms. So electrons involved in single, double, or triple are part of the same bonding domain. Nonbonding Domain: Valence electrons associated with one atom, such as a lone pair, or a unpaired electron. Jesperson, Brady, Hyslop. Chemistry: The Molecular Nature of Matter, 6E

  6. VESPR Basic Examples 2 bonding domains 3 bonding domains 5 bonding domains 4 bonding domains 6 bonding bonding domains Jesperson, Brady, Hyslop. Chemistry: The Molecular Nature of Matter, 6E

  7. VESPR When Lone Pairs or Multiple Bonds Present Including lone pairs: • Take up more space around central atom • Effect overall geometry • Counted as nonbonded electron domains Including multiple bonds (double and triple) • For purposes of determining geometry focus on the number of atoms bonded together rather then the number of bonds in between them: ie, treat like a single bond. • Treat as single electron bonding domain Jesperson, Brady, Hyslop. Chemistry: The Molecular Nature of Matter, 6E

  8. VESPR Electrons that are Bonding & Not Bonding Bonding Electrons • More oval in shape • Electron density focused between two positive nuclei. Nonbonding Electrons • More bell or balloon shaped • Take up more space • Electron density only has positive nuclei at one end Jesperson, Brady, Hyslop. Chemistry: The Molecular Nature of Matter, 6E

  9. VESPR 3 atoms or lone pairs Number of Bonding Domains 3 2 Number of Nonbonding Domains 0 1 Structure Molecular Shape Planar Triangular (e.g. BCl3) All bond angles 120 Nonlinear Bent or V-shaped (e.g. SnCl2) Bond <120 Jesperson, Brady, Hyslop. Chemistry: The Molecular Nature of Matter, 6E

  10. VESPR 4 atoms or lone pairs Number of Bonding Domains 4 3 2 Number of Nonbonding Domains 0 1 2 Structure Molecular Shape Tetrahedron (e.g. CH4) All bond angles 109.5  Trigonalpyramidal (e.g. NH3) Bond angle less than 109.5 Nonlinear, bent (e.g. H2O) Bond angle less than109.5 Jesperson, Brady, Hyslop. Chemistry: The Molecular Nature of Matter, 6E

  11. VESPR 5 atoms or lone pairs TrigonalBipyramidal • Two atoms in axial position • 90 to atoms in equatorial plane • Three atoms in equatorial position • 120 bond angle to atoms in axial position • More room here • Substitute here first 90 120 Jesperson, Brady, Hyslop. Chemistry: The Molecular Nature of Matter, 6E

  12. VESPR 5 atoms or lone pairs Number of Bonding Domains 5 4 Number of Nonbonding Domains 0 1 Structure Molecular Shape Trigonalbipyramid (e.g. PF5) Ax-eq bond angles 90 Eq-eq 120 Distorted Tetrahedron, or Seesaw (e.g. SF4) Ax-eq bond angles < 90 Jesperson, Brady, Hyslop. Chemistry: The Molecular Nature of Matter, 6E

  13. VESPR 5 atoms or lone pairs • Lone pair takes up more space • Goes in equatorial plane • Pushes bonding pairs out of way • Result: distorted tetrahedron Jesperson, Brady, Hyslop. Chemistry: The Molecular Nature of Matter, 6E

  14. VESPR 5 atoms or lone pairs Number of Bonding Domains 3 2 Number of Nonbonding Domains 2 3 Structure Molecular Shape T-shape (e.g. ClF3) Bond angles 90 Linear (e.g. I3–) Bond angles 180 Jesperson, Brady, Hyslop. Chemistry: The Molecular Nature of Matter, 6E

  15. VESPR 6 atoms or lone pairs Number of Bonding Domains 6 5 Number of Nonbonding Domains 0 1 Structure Molecular Shape Octahedron (e.g. SF6) Square Pyramid (e.g. BrF5) Jesperson, Brady, Hyslop. Chemistry: The Molecular Nature of Matter, 6E

  16. VESPR 6 atoms or lone pairs Number of Bonding Domains 4 Number of Nonbonding Domains 2 Structure Molecular Shape Square planar (e.g. XeF4) Jesperson, Brady, Hyslop. Chemistry: The Molecular Nature of Matter, 6E

  17. VESPR Determining 3-D Structures 1. Draw Lewis Structure of Molecule • Don't need to compute formal charge • If several resonance structures exist, pick only one 2. Count electron pair domains • Lone pairs and bond pairs around central atom • Multiple bonds count as one set (or one effective pair) 3. Arrange electron pair domains to minimize repulsions • Lone pairs • Require more space than bonding pairs • May slightly distort bond angles from those predicted. • In trigonalbipyramid lone pairs are equatorial • In octahedron lone pairs are axial • Name molecular structure by position of atoms—only bonding electrons Jesperson, Brady, Hyslop. Chemistry: The Molecular Nature of Matter, 6E

  18. Molecular Polarity Polar Molecules • Have net dipole moment • Negative end • Positive end • Polar molecules attract each other. • Positive end of polar molecule attracted to negative end of next molecule. • Strength of this attraction depends on molecule's dipole moment • Dipole moment can be determined experimentally • Polarity of molecule can be predicted by taking vector sum of bond dipoles • Bond dipoles are usually shown as crossed arrows, where arrowhead indicates negative end Jesperson, Brady, Hyslop. Chemistry: The Molecular Nature of Matter, 6E

  19. Molecular Polarity Molecular Shape & Polarity • Many physical properties (melting and boiling points) affected by molecular polarity • For molecule to be polar: • Must have polar bonds • Many molecules with polar bonds are nonpolar • Possible because certain arrangements of bond dipoles cancel • For molecules with more than two atoms, must consider the combinedeffects of all polar bonds Jesperson, Brady, Hyslop. Chemistry: The Molecular Nature of Matter, 6E http://wps.prenhall.com/wps/media/objects/3081/3155729/blb0903.html

  20. Molecular Polarity Symmetrical Nonpolar Molecules • Symmetricalmolecules • Nonpolar because bond dipoles cancel • All five shapes are symmetrical when all domains attached to them are composed of identical atoms Jesperson, Brady, Hyslop. Chemistry: The Molecular Nature of Matter, 6E

  21. Molecular Polarity Symmetrical Nonpolar Molecules Cancellation of Bond Dipoles In Symmetrical TrigonalBipyramidal and Octahedral Molecules • All electron pairs around central atom are bonding pairs and • All terminal groups (atoms) are same • The individual bond dipoles cancel Jesperson, Brady, Hyslop. Chemistry: The Molecular Nature of Matter, 6E

  22. Molecular Polarity Polar Molecules Molecule is usually polarif • All atoms attached to central atom are NOT same Or, • There are one or more lone pairs on central atom Jesperson, Brady, Hyslop. Chemistry: The Molecular Nature of Matter, 6E

  23. Molecular Polarity Polar Molecules • Water and ammonia both have non-bonding domains • Bond dipoles do not cancel • Molecules are polar Jesperson, Brady, Hyslop. Chemistry: The Molecular Nature of Matter, 6E

  24. Molecular Polarity Polar Molecules: Exception Exception to these general rules for identifying polar molecules: Nonbonding domains (lone pairs) are symmetrically placed around central atom Jesperson, Brady, Hyslop. Chemistry: The Molecular Nature of Matter, 6E

  25. Problem Set A • For the following molecules: • Draw a lewis dot structure. • Determine the molecular geometry at each central atom. • Identify the bond angles. • Identify all polar bonds: δ+ / δ- • Assess the polarity of the molecule & indicate the overall dipole moment if one exists • AsF5 AsF3 SeO2 GaH3 • ICl2- SiO4-4 TeF6

  26. VB Theory Review: Modern Atomic Theory of Bonding Modern Atomic Theory of Bonding is based on wave mechanics and gave us: • Electrons and shapes of orbitals • Four quantum numbers • Heisenberg uncertainty principle • Electron probabilities • Pauli Exclusion Principle Jesperson, Brady, Hyslop. Chemistry: The Molecular Nature of Matter, 6E

  27. VB Theory Valence Bond Theory & Molecular Orbital Theory Valence Bond Theory • Individual atoms, each have their own orbitals and orbitals overlap to form bonds • Extent of overlap of atomic orbitals is related to bond strength Molecular Orbital Theory • Views molecule as collection of positively charged nuclei having a set of molecular orbitals that are filled with electrons (similar to filling atomic orbitals with electrons) • Doesn't worry about how atoms come together to form molecule Jesperson, Brady, Hyslop. Chemistry: The Molecular Nature of Matter, 6E

  28. VB Theory Valence Bond Theory & Molecular Orbital Theory Both Theories: • Try to explain structure of molecules, strengths of chemical bonds, bond orders, etc. • Can be extended and refined and often give same results Valence Bond Theory Bond between two atoms formed when pair of electronswith paired (opposite) spins is shared by two overlapping atomic orbitals Jesperson, Brady, Hyslop. Chemistry: The Molecular Nature of Matter, 6E

  29. VB Theory H2 H2 bonds form because 1s atomic valence orbital from each H atom overlaps Jesperson, Brady, Hyslop. Chemistry: The Molecular Nature of Matter, 6E

  30. VB Theory F2 • F2 bonds form because atomic valence orbitals overlap • Here 2p overlaps with 2p • Same for all halogens, but different nporbitals Jesperson, Brady, Hyslop. Chemistry: The Molecular Nature of Matter, 6E

  31. VB Theory HF HF involves overlaps between 1s orbital on H and 2p orbital of F 1s 2p Jesperson, Brady, Hyslop. Chemistry: The Molecular Nature of Matter, 6E

  32. VB Theory H2S • Assume that unpaired electronsin S and H are free to form paired bond • We may assume that H—S bond forms between s and p orbital • Predicted 90˚ bond angle is very close to experimental value of 92˚. Jesperson, Brady, Hyslop. Chemistry: The Molecular Nature of Matter, 6E

  33. VB Theory Need to Change Approach to Explain Bonding in CH4 Example: CH4 C 1s 22s 22p 2 and H 1s 1 • In methane, CH4 • All four bonds are the same • Bond angles are all 109.5° • Carbon atoms have • All paired electrons except two unpaired 2p • p orbitals are 90° apart • Atomic orbitals predict CH2 with 90° angles Jesperson, Brady, Hyslop. Chemistry: The Molecular Nature of Matter, 6E

  34. VB Theory Hybridization • Mixing of atomic orbitals to allow formation of bonds that have realistic bond angles. • Realistic description of bonds often requires combining or blending two or more atomic orbitals • Hybridization just rearranging of electron probabilities Why do it? • To get maximum possible overlap • Best (strongest) bond formed Jesperson, Brady, Hyslop. Chemistry: The Molecular Nature of Matter, 6E

  35. VB Theory Hybrid Orbitals • Blended orbitals result from hybridization process • Hybrid orbitals have • New shapes • New directional properties • Each hybrid orbital combines properties of parent atomic orbitals Jesperson, Brady, Hyslop. Chemistry: The Molecular Nature of Matter, 6E

  36. VB Theory Hybrid Orbitals • Symbols for hybrid orbitals combine the symbols of the orbitals used to form them • Use s + p form two sp hybrid orbitals • Use s + p + p form three sp 2 hybrid orbitals • One atomic orbital is used for each hybrid orbital formed • Sum of exponents in hybrid orbital notation must add up to number of atomic orbitals used Jesperson, Brady, Hyslop. Chemistry: The Molecular Nature of Matter, 6E

  37. VB Theory Hybrid Orbitals • Mixing or hybridizing s and p orbital of same atom results in two sp hybrid orbitals • Two sp hybrid orbitals point in opposite directions Jesperson, Brady, Hyslop. Chemistry: The Molecular Nature of Matter, 6E

  38. VB Theory Ex: sp Hybridized Orbitals: BeH2 • Now have two sp hybrid orbitals • Oriented in correct direction for bonding • 180 bond angles • As VSEPR predicts and • Experiment verifies • Bonding = • Overlap of H 1s atomic orbitals with sp hybrid orbitals on Be Jesperson, Brady, Hyslop. Chemistry: The Molecular Nature of Matter, 6E

  39. VB Theory Hybrid Orbitals Jesperson, Brady, Hyslop. Chemistry: The Molecular Nature of Matter, 6E

  40. VB Theory Bonding in BCl3 • Overlap of each half- filled 3p orbital on Cl with each half-filled sp2 hybrid on B • Forms three equivalent bonds • Trigonal planar shape • 120 bond angle Jesperson, Brady, Hyslop. Chemistry: The Molecular Nature of Matter, 6E

  41. VB Theory Bonding in CH4 • Overlap of each half- filled 1s orbital on H with each half-filled sp3 hybrid on carbon • Forms four equivalent bonds • Tetrahedral geometry • 109.5 bond angle Jesperson, Brady, Hyslop. Chemistry: The Molecular Nature of Matter, 6E

  42. VB Theory Hybrid sp Orbitals Two sp hybrids Three sp2hybrids Four sp3 hybrids Linear All angles 120 Planar Triangular All angles 109.5 Tetrahedral Jesperson, Brady, Hyslop. Chemistry: The Molecular Nature of Matter, 6E

  43. VB Theory Expanded Octet Hybridization Hybridization When Central Atom has More Than Octet • If there are more than four equivalent bonds on central atom, then must add d orbitals to make hybrid orbitals Why? • One s and three p orbitals means that four equivalent orbitals is the most you can get using s and p orbitals alone So, only atoms in third row of the periodic table and below can exceed their octet • These are the only atoms that have empty d orbitals of same n level as s and p that can be used to form hybrid orbitals • One d orbital is added for each pair of electrons in excess of standard octet Jesperson, Brady, Hyslop. Chemistry: The Molecular Nature of Matter, 6E

  44. VB Theory Expanded Octet Hybridization Jesperson, Brady, Hyslop. Chemistry: The Molecular Nature of Matter, 6E

  45. VB Theory Hybridization in Molecules with Lone Pairs CH4sp3 tetrahedral geometry 109.5° bond angle NH3 107° bond angle H2O 104.5° bond angle • Angles suggest that NH3 and H2O both use sp3 hybrid orbitals in bonding • Not all hybrid orbitals used for bonding e– • Lone pairs can occupy hybrid orbitals • Lone pairs must always be counted to determine geometry Jesperson, Brady, Hyslop. Chemistry: The Molecular Nature of Matter, 6E

  46. VB Theory Ex: H2O Hybridization Jesperson, Brady, Hyslop. Chemistry: The Molecular Nature of Matter, 6E

  47. VB Theory Multiple Bonds • So where do extra electron pairs in multiple bonds go? • Not in hybrid orbitals • Remember VSEPR, multiple bonds have no effect on geometry • Why don’t they effect geometry? Two types of bond result from orbital overlap • Sigma () bond • Accounts for first bond • Pi () bond • Accounts for second and third bonds Jesperson, Brady, Hyslop. Chemistry: The Molecular Nature of Matter, 6E

  48. VB Theory Sigma () Bonds • Head on overlap of orbitals • Concentrate electron density concentrated most heavily between nuclei of two atoms • Lie along imaginary line joining their nuclei s + s p + p sp + sp Jesperson, Brady, Hyslop. Chemistry: The Molecular Nature of Matter, 6E

  49. VB Theory Pi () Bonds • Sideways overlap of unhybridizedp orbitals • Electron density divided into two regions • Lie on opposite sides of imaginary line connecting two atoms • Electron density above and below  bond. • No electron density along  bond axis •  bond consists of both regions • Both regions = one  bond Jesperson, Brady, Hyslop. Chemistry: The Molecular Nature of Matter, 6E

  50. VB Theory Pi () Bonds • Can never occur alone • Must have  bond • Can form from unhybridized p orbitals on adjacent atoms after forming  bonds •  bonds allow atoms to form double and triple bonds Jesperson, Brady, Hyslop. Chemistry: The Molecular Nature of Matter, 6E

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