7_lesson7.pdf - Molecular Structure Lesson 7 Bond Polarity...

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Molecular Structure Mary J. Bojan Chem 110 1 Lesson 7 Bond Polarity Electronegativity Bond Dipoles Lewis Structures drawing Lewis structures deciding between alternate Lewis structures: formal charge resonance structures exceptions to the octet rule Electrons are not always shared equally. Mary J. Bojan Chem 110 2 Nonpolar covalent: electrons are shared equally (H 2 , Br 2 , Cl 2 ) Polar covalent: unequal sharing of electrons (HF, ICl) Ionic: no sharing of electrons: (NaCl, LiF) Lesson 7 Unit 2 How to read Maps of Electrostatic Potential (MEPs)
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Electron sharing in covalent bonds depends on the electronegativity of atoms. Mary J. Bojan Chem 110 3 ELECTRONEGATIVITY: not the same as electron affinity not directly measurable: Most electronegative: Least electronegative: The Periodic Table can be used to remember trends in electronegativity. Mary J. Bojan Chem 110 4 Periodic Trends: nonmetals are most electronegative metals are least electronegative (especially active metals, Groups I and II)
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Mary J. Bojan Chem 110 One measure of real charges in molecules is dipole moment (μ) For a diatomic molecule : μ = Q r | r | unit = debye (D) = 3.33 x10 30 C-m ! ———— ! +Q Q For two charges (+1 and 1) separated by 1 Å (= 100 pm) μ = 4.79 D Units of Q = 1.6 x10 19 C = charge of an electron *** see Lesson 7.2 5 Mary J. Bojan Chem 110 μ = Q r We expect that μ will increase as r increases, but that is not what happens. HX bond length Å μ(ionic) D μ(exp) D % ionic H F 0.93 4.45 1.83 41% H Cl 1.28 6.13 1.11 18% H Br 1.43 6.85 0.83 12% H I 1.62 7.76 0.45 6% 6 μ(ionic) = dipole moment the molecule would have IF complete charge transfer occurs. μ = Q r As expected: as r μ(ionic) μ(exp) = experimental (real) dipole moment. As r μ(exp) % ionic = μ(exp)/ μ(ionic) x 100% = 1.11D / 6.13D x 100% = 18% Table T07-2-1
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Mary J. Bojan Chem 110 These molecules (HX) are NOT ionic: only partial charge transfer occurs.
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