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CHEMISTRY The Molecular Science Chapter eight

CHEMISTRY The Molecular Science Chapter eight. Slides prepared by S. Michael Condren Department of Chemistry Christian Brothers University. to Accompany CHEMISTRY The Molecular Science by John W. Moore, Conrad Stanitski, & Peter C. Jurs. Chapter 8. Covalent Bonding. Covalent Bonding.

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CHEMISTRY The Molecular Science Chapter eight

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  1. CHEMISTRYThe Molecular ScienceChapter eight Slides prepared by S. Michael Condren Department of Chemistry Christian Brothers University to Accompany CHEMISTRY The Molecular Science by John W. Moore, Conrad Stanitski, & Peter C. Jurs

  2. Chapter 8 Covalent Bonding

  3. Covalent Bonding sharing of electrons H:H H–H

  4. Atomic Interactions

  5. H-H Bond Formation

  6. Shell Model of the Atom G.N.Lewis 1875-1946 AmericanChemist Lewis Symbol valence shell electrons = Group A

  7. Lewis Symbol Sulfur  S Group VIA 6 valence electron . : S : .

  8. Lewis Structures • combinations of Lewis symbols which represent possible bonding Octet Rule In compound formation an atom gains or loses electrons, or shares pairs of electrons, until its valence shell has eight electrons.

  9. Lewis Structures H CH4 methane H C H H bonding pair of electrons

  10. Lewis Structures C2H6 ethane H H ×××× H : C : C : H ×××× H H

  11. Selected Lewis Structures Number of electrons Number shared to complete an Group of valence octet (8-group number electrons number) Example 4A 4 4 C in CH4 5A 5 3 N in NF3 6A 6 2 O in H2O 7A 7 1 F in HF

  12. Guidelines for Writing Lewis Structures 1. Add up the number of valence electrons in the molecule or ion, adding electrons for each negative charge on ion and removing electrons for each positive charge. 2. Create a skeleton structure using the chemical symbols for the atoms and a line for bonding pairs of electrons.

  13. Guidelines for Writing Lewis Structures 3. Satisfy the Octet Rule for each atom, starting with the central atom. 4. All remaining electrons should be placed around central atom 5. Add multiple bonds to central atom if less than eight electrons are around central atom.

  14. Single Covalent Bondsin Hydrocarbons Saturated hydrocarbons - C - C - C - C - C - C - C - C - C - C - C - or CH3CH2CH2CH2CH2CH2CH2CH2CH2CH2CH3

  15. Saturated hydrocarbons butane 2-methylpropane

  16. Cycloalkanes

  17. Pentanes H H H H H | | | | | H - C - C - C - C - C - H | | | | | H H H H H pentane H H H H | | | | H - C - C - C - C - H | | | | H CH3 H H 2-methylbutane H CH3 H | | | H - C - C - C - H | | | H CH3 H 2,2-dimethylpropane

  18. 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

  19. Lewis Structures CO2 carbon dioxide ×××× : O :: C :: O : double bonds

  20. Lewis Structures CO carbon monoxide : C ::: O : triple bond

  21. Multiple Covalent Bondsin Hydrocarbons

  22. Double Bondsand Isomerism

  23. Properties of cis- and trans-1,2-dichloroethene Physical Property cis-1,2-dichloroethene trans-1,2-dichloroethene Melting Point -80.5 °C -50.0 °C Boiling Point (at 1 atm) 60.3 °C 47.5 °C Density (at 20 °C) 1.284 g/ml 1.265 g/ml

  24. Non-rotation around C = C

  25. Triglycerides, Fats and Oils triglyceride - 1 mole of glycerol plus three moles of fatty acids fat - solid triglyceride at room temperature oil - liquid triglyceride at room temperature 3 fatty acids + glycerol  triglyceride + 3H2O the 3 fatty acids can be the same or different

  26. Triglyceride

  27. Fats saturated fats – fats containing only C–C single bonds unsaturated fats – fats containing one or more C=C double bonds monounsaturated fats – fats containing one C=C double bond polyunsaturated fats – fats containing two or more C=C double bonds

  28. Fatty Acids animal fat – primarily saturated fatty acids vegetable fat – primarily unsaturated fatty acids more unsaturation, lower melting point, more liquid in character most natural unsaturated fatty acids are cis- configuration, producing lower melting points than corresponding trans- isomers

  29. Fats animal fat • more likely solid, less unsaturation vegetable fat • more likely liquids, more unsaturation • converted to semisolids by hydrogentation (addition of gaseous hydrogen to some of the double bonds)

  30. Bond Length

  31. Bond Lengths

  32. Relative Atom Sizes of Second and Third Period Elements

  33. Bond Energy

  34. EXAMPLE: Using bond energy data, calculate the DHo for: CH4(g) + 2 O2(g)CO2(g) + 2 H2O(g) DHo = ([ 4( 414 ) + 2 ( 498 )]broken - [2 ( 715 ) + 4 ( 464 )]formed)kJ/mol = ([ 1656 + 996 ] - [ 1430 + 1856 ])kJ/mol = ([ 2652 ] - [ 3286 ])kJ/mol = - 634 kJ/mol from DHfo data: DHo = - 802.3 kJ/mol

  35. Linus Pauling 1901–94 American chemist One of the few recipients of two Nobel prizes. His paper “The Nature of the Chemical Bond and the Structure of Molecules and Crystals” won him the 1954 Nobel Prize in chemistry. Won the peace prize in 1962.

  36. Electronegativity Pauling Scale • relative attraction of an atom for electrons, its own and those of other atoms • same trends as ionization energy, increases from lower left corner to the upper right corner • fluorine: E.N. = 4.0

  37. Trends in Electronegativity

  38. Bond Characteristics

  39. Covalent BondProperties electronegativity nonpolar bonds DEN = 0 polar bonds DEN > 0 ionic bonds DEN > 1.5

  40. Formal Charge Formal Charge is the charge on an atom if all of the shared electrons were equally shared • lone pair electrons are assigned to atom about which they are found • half of the bonding electrons are assigned to each atom of the bonded pair • the sume of formal charges on a molecule is zero and the magnitude of the charge on ions

  41. Formal Charge Calculations Formal charge = (# valence electrons on an atom) - [(number of lone pair electrons) + (1/2number of shared electrons)] N C O Valence electrons 5 4 6 Lone pair electrons 2 0 6 1/2 shared electrons 3 4 1 Formal charge 0 0 –1

  42. Resonance Resonance is present when two or more electron dot structures that differ only in the arrangement of the electrons. The resonant hybrid, an average structure, best represents the structure

  43. Ozone O = O - O O - O = O

  44. Resonance in SO3 O S O O O S O O O S O O

  45. Exceptions to Octet Rule NO nitric oxide less than octet ×××× ×N :: O:

  46. Exceptions to Octet Rule expanded octet PF5 F F P F F F

  47. Exceptions to Octet Rule expanded octet SF6 F F F S F F F

  48. Exceptions to Octet Rule

  49. Aromatics

  50. Aromatics

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