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Chapter 9

Chapter 9. Bonding and Molecular Structure: Fundamental Concepts. Chemical Bonding. Problems and questions — How is a molecule or polyatomic ion held together? Why are atoms distributed at strange angles? Why are molecules not flat? Can we predict the structure?

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Chapter 9

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  1. Chapter 9 Bonding and Molecular Structure: Fundamental Concepts Dr. S. M. Condren

  2. Chemical Bonding Problems and questions — How is a molecule or polyatomic ion held together? Why are atoms distributed at strange angles? Why are molecules not flat? Can we predict the structure? How is structure related to chemical and physical properties? Dr. S. M. Condren

  3. Structure & Bonding NN triple bond. Molecule is unreactive White phosphorus is a tetrahedron of P atoms. Very reactive! Red phosphorus, a polymer. Used in matches. Less reactive! Dr. S. M. Condren

  4. Forms of Chemical Bonds There are 2 extreme forms of connecting or bonding atoms: Ionic—complete transfer of 1 or more electrons from one atom to another Covalent—some valence electrons shared between atoms Most bonds are somewhere in between. Dr. S. M. Condren

  5. Ionic Compounds Metal of low IE Nonmetal of high EA 2 Na(s) + Cl2(g) ---> 2 Na+ + 2 Cl- Dr. S. M. Condren

  6. Covalent Bonding The bond arises from the mutual attraction of 2 nuclei for the same electrons.Electron sharingresults. Bond is a balance of attractive and repulsive forces. Dr. S. M. Condren

  7. •• •• Cl H H Cl • • + • • •• •• Bond Formation A bond can result from a “head-to-head”overlapof atomic orbitals on neighboring atoms. Note that each atom has a single, unpaired electron. Overlap of H (1s) and Cl (3p) Dr. S. M. Condren

  8. Chemical Bonding: Objectives Objectivesare to understand: 1. valence e- distribution in molecules and ions. 2. molecular structures 3. bond properties and their effect on molecular properties. Dr. S. M. Condren

  9. G. N. Lewis 1875 - 1946 Electron Distribution in Molecules • Electron distribution is depicted withLewis electron dot structures • Valence electrons are distributed as shared orBOND PAIRS and unshared orLONE PAIRS. Dr. S. M. Condren

  10. Bond and Lone Pairs •• H Cl • • •• shared or bond pair • Valence electrons are distributed as shared orBOND PAIRS and unshared orLONE PAIRS. lone pairs This is called a LEWIS ELECTRON DOTstructure. Dr. S. M. Condren

  11. Valence Electrons Electrons are divided between core and valence electrons B 1s2 2s2 2p1 Core = [He] , valence = 2s2 2p1 Br [Ar] 3d10 4s2 4p5 Core = [Ar] 3d10 , valence = 4s2 4p5 Dr. S. M. Condren

  12. Rules of the Game No. of valence electrons of a main group atom = Group number •For Groups 1A-4A, no. of bond pairs = group number. • For Groups 5A -7A, BP’s = 8 - Grp. No. Group 5A Group 3A Dr. S. M. Condren

  13. Rules of the Game No. of valence electrons of an atom = Group number For Groups 1A-4A, no. of bond pairs = group number For Groups 5A -7A, BP’s = 8 - Grp. No. •Except for H (and sometimes atoms of 3rd and higher periods), BP’s + LP’s = 4 This observation is called the OCTET RULE Dr. S. M. Condren

  14. Hydrophobic vs. Hydrophilic • Hydrophobic - translation of Greek – water fear • Hydrophilic – translation of Greek – water friendship Dr. S. M. Condren

  15. Building a Dot Structure Ammonia, NH3 1. Decide on the central atom; never H. Central atom is atom of lowest affinity for electrons. Therefore, N is central 2. Count valence electrons H = 1 and N = 5 Total = (3 x 1) + 5 = 8 electrons / 4 pairs Dr. S. M. Condren

  16. H H N H •• H H N H Building a Dot Structure 3. Form a single bond between the central atom and each surrounding atom 4. Remaining electrons form LONE PAIRS to complete octet as needed. 3 BOND PAIRS and 1 LONE PAIR. Note that N has a share in 4 pairs (8 electrons), while H shares 1 pair. Dr. S. M. Condren

  17. Lewis Structures Step 1. Central atom = C CH4 methane Step 2. Count valence electrons C = 4 4 x H = 4 x 1 = 4 TOTAL = 8 e- or 4 pairs Step 3. Form bonds Dr. S. M. Condren

  18. Lewis Structures C2H6 ethane Dr. S. M. Condren

  19. Multiple Covalent Bonds double bond => 2 pairs shared triple bond => 3 pairs shared normally occurs between: C atoms; N atoms; O atoms; a C atom and a N, O or S atom a N atom and a O or S atom a S atom and an O atom Dr. S. M. Condren

  20. Double and even triple bonds are commonly observed for C, N, P, O, and S H2CO SO3 C2F4 Dr. S. M. Condren

  21. Carbon Dioxide, CO2 C 1. Central atom = _______ 4 2 2. Valence electrons = __ or __ pairs 3. Form bonds. This leaves 6 pairs. Dr. S. M. Condren

  22. Carbon Dioxide, CO2 4. Place lone pairs on outer atoms. 5. So that C has an octet, we shall form DOUBLE BONDS between C and O. The second bonding pair forms api (π)bond. Dr. S. M. Condren

  23. Steps to form Lewis Electron Dot Structure 1. Central atom = _______ 2. Valence electrons = __ or __ pairs • Form bonds. • Place lone pairs on outer atoms. • Form multiple bonds as necessary to obey Lewis’ “Octet Rule”. • Remember that there are MANY compounds that do not obey the Octet Rule. Dr. S. M. Condren

  24. Lewis Structures CO carbon monoxide Dr. S. M. Condren

  25. Exceptions to Octet Rule NO nitric oxide Dr. S. M. Condren

  26. Exceptions to Octet Rule NO2 nitrogen dioxide resonance Dr. S. M. Condren

  27. Exceptions to Octet Rule PF5 expanded octet Dr. S. M. Condren

  28. Exceptions to Octet Rule expanded octet SF4 Dr. S. M. Condren

  29. Exceptions to Octet Rule SF6 expanded octet Dr. S. M. Condren

  30. Formal Atom Charges • Atoms in molecules often bear a charge (+ or -). • The predominant resonance structure of a molecule is the one with charges as close to 0 as possible. • Formal charge = Group number – 1/2 (no. of bonding electrons) - (no. of LP electrons) Dr. S. M. Condren

  31. +6 - ( 1 / 2 ) ( 4 ) - 4 = 0 • • • • O C O • • • • +4 - ( 1 / 2 ) ( 8 ) - 0 = 0 Carbon Dioxide, CO2 Dr. S. M. Condren

  32. • • S C N • • • • • • Thiocyanate Ion, SCN- 6 - (1/2)(2) - 6 = -1 5 - (1/2)(6) - 2 = 0 4 - (1/2)(8) - 0 = 0 Dr. S. M. Condren

  33. • • • • • • S C N S C N • • • • • • • • • • • • S C N • • • • • • Thiocyanate Ion, SCN- Which is the most important resonance form? Dr. S. M. Condren

  34. MOLECULAR GEOMETRY Dr. S. M. Condren

  35. MOLECULAR GEOMETRY VSEPR Valence Shell Electron Pair Repulsion theory. Most important factor in determining geometry is relative repulsion between electron pairs. Molecule adopts the shape that minimizes the electron pair repulsions. Dr. S. M. Condren

  36. Electron Pair Geometries Dr. S. M. Condren

  37. OR bring in bring in right pair left pair •• •• •• O S O • • • • •• •• Sulfur Dioxide, SO2 1. Central atom = S 2. Valence electrons = 18 or 9 pairs 3. Form double bond so that S has an octet — but note that there are two ways of doing this. Dr. S. M. Condren

  38. Sulfur Dioxide, SO2 This leads to the following structures. These equivalent structures are called RESONANCE STRUCTURES. The true electronic structure is aHYBRIDof the two. Dr. S. M. Condren

  39. Geometries for Four Electron Pairs Dr. S. M. Condren

  40. •• H H N H Structure Determination by VSEPR Ammonia, NH3 1. Draw electron dot structure 2. Count BP’s and LP’s = 4 3. The 4 electron pairs are at the corners of a tetrahedron. Dr. S. M. Condren

  41. Structure Determination by VSEPR Ammonia, NH3 The electron pair geometry is tetrahedral. The MOLECULAR GEOMETRY, the positions of the atoms, is PYRAMIDAL. Dr. S. M. Condren

  42. Structure Determination by VSEPR Water, H2O 1. Draw electron dot structure 2. Count BP’s and LP’s = 4 3. The 4 electron pairs are at the corners of a tetrahedron. The electron pair geometry is TETRAHEDRAL. The molecular geometry is BENT. Dr. S. M. Condren

  43. Consequences of H2O Polarity Dr. S. M. Condren

  44. Structures with Central Atoms with More Than or less Than 4 Electron Pairs Often occurs with Group 3A elements and with those of 3rd period and higher. Dr. S. M. Condren

  45. All based on trigonal bipyramid Molecular Geometries for Five Electron Pairs Dr. S. M. Condren

  46. All are based on the 8-sided octahedron Molecular Geometries for Six Electron Pairs Dr. S. M. Condren

  47. Bond Properties • What is the effect of bonding and structure on molecular properties? Free rotation around C–C single bond No rotation around C=C double bond Dr. S. M. Condren

  48. Double bond Single bond Triple bond Bond Order # of bonds between a pair of atoms Acrylonitrile Dr. S. M. Condren

  49. Bond Order 1 1 Fractional bond ordersoccur in molecules with resonance structures. Consider NO2- The N—O bond order = 1.5 Dr. S. M. Condren

  50. Bond Order 414 kJ 123 pm 745 kJ 110 pm Bond order is proportional to two important bond properties: (a) bond strength (b) bond length Dr. S. M. Condren

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