Theories of Bonding and Structure CHAPTER 10

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: Execption 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