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Chapter 2 CHEMICAL BONDS AND MOLECULAR STRUCTURE 1 2 3 Electronegativity • Relative attraction of an atom for electrons, • • • its own and those of other atoms. Pauling Scale. Same trends as ionization energy, increases from lower left corner to the upper right corner. fluorine: E.N. = 4.0 4 Electronegativities of the Elements 5 6 Type of Chemical Bonding • Non-polar bonds: diff. EN = 0 diff. EN > 0 • Polar bonds: diff. EN > 1.5 • Ionic bonds: 7 8 9 10 11 12 13 14 15 16 Ionic Compounds Ionic Compounds NaCl; copper(II) sulfate pentahydrate (CuSO4•5H2O, blue); nickel(II) chloride hexahydrate (NiCl2•6H2O, green); potassium dichromate K2Cr2O7, orange); and cobalt(II) chloride hexahydrate (CoCl2•6H2O) 18 19 20 21 22 Covalent Bonding: Sharing of electrons  A covalent bond is formed when two atoms share one or more pairs of electrons.  Covalent bonding occurs when the lectronegativity difference between elements (atoms) is zero or relatively small. 23 24 25 Compare O-O and O=O. Is O=O expected to be stronger, weaker, or the same strength? 26 Shell Model of the Atom • • • • • G.N.Lewis 1875-1946 American Chemist Lewis Symbol valence shell electrons = Group A # 27 28 Lewis Structures • Octet Rule In compound formation an atom gains or loses electrons, or shares pairs of electrons, until its valence shell has eight electrons. 29 30 31 32 33 34 35 36 37 38 39 Lewis Structures CH4 methane C2H6 ethane 40 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 41 Multiple Covalent Bonds 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 Shape: PCl3 Lewis Electron Dot Structure .. .. .. :Cl .. : P .. : Cl: .. :Cl: .. 3 bond pairs 1 lone pair => AB3E trigonal pyramidal shape 69 Shape: IF5 Lewis Electron Dot Structure .. .. :F::F: .. .. .. : I :F: .. .. .. : F : :F : 5 bonds pairs 1 lone pair => AB5E Square pyramidal shaped 70 Shape: IF4-1 Lewis Electron Dot Structure .. .. -1 :F::F: .. .. .. : I :F: .. .. .. :F: 4 bond pairs 2 lone pairs => AB4E2 square planar shape 71 Shape: SO3 Lewis Electron Dot Structure .. :O:S::O: .. .. .. :O: 3 bond pairs 0 lone pairs => AB3 trigonal planar shape 72 What would be expected to be the shape of chloroform, CHCl3? “see-saw” square planar tetrahedral 73 74 75 76 77 78 79 80 81 82 83 84 85 Carbon monoxide, CO and carbon dioxide, CO2, are linear molecules. Which one is the polar molecule: CO CO2 86 87 88 89 90 91 92 93 94 95 96 97 98 99 100 101 102 103 Sigma Bond σ Bond with the greatest electron density on a line connecting the atomic nuclei s-s type p-p type s-p type s-sp3 type s-sp2 type s-sp type p-sp3 type p-sp2 type p-sp type sp3 - sp3 type sp2-sp2 type sp-sp type etc., including all combinations of s, p, d, and hybrid orbitals 104 Pi Bond π Bond with the greatest electron density above and below a line connecting the atomic nuclei p-p type 105 106 Bonding in Ethylene 107 Bonding in Acetylene 108 109 110 111 112 113 Dipole-Dipole Forces 114 115 London (Dispersion) Forces 116 117 118 Hydrogen Bonds 119 Hydrogen Bonds 120 Water, H2O, and methane, CH4, have very similar molar masses yet the boiling point for water is approximately 200oC higher than that for methane. The higher boiling point is due to: hydrogen bonding in water polarity of water molecules both of the above none of the above 121