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Lecture 19 - Topics for the day Administrative stuff...

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Topics for the day Administrative stuff Polarity one more time Valence bond theory Sigma bonds Multiple bonds Ethylene Carbon dioxide Nitrogen The oxygen mystery Molecular orbital theory Administrative stuff Syllabus clarification “Your ALEKS contribution to the HW grade will be the higher of two things, either the average of your ALEKS % of topics learned for each objective, or your final % of topics learned over the entire semester.” (Recall that the worst two homeworks of the semester are dropped.) Each ALEKS objective % is set on each due date. At the end of the semester to study for the final you’ll have access to the entire pie and could potentially get your pie to 100%. Syllabus correction You get to drop the worst exam of the four. That fact is totally missing from the syllabus. Sorry. “Best three of four class exams ......... 50%” What I intended 1 2 3
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Note that the two “Final Options” do count all four exams: Get out of Jail early Betting the farm Polarity revisited again Nonpolar covalent bond Δ EN = 0 X—Y Polar covalent bond Δ EN 0 – 1.3 X—Y δ + δ Ionic bond Δ EN 1.3 X + Y Large EN difference More ionic Small EN difference More covalent Δ EN 1.3 The values of Δ EN that mark the boundaries of nonpolar/polar and polar/ ionic have some variability. .5 1.3, 1.7, 2.0 ..... For this course, a nonpolar bond has Δ EN=0. In particular, C–H is polar , relative to homonuclear bonded atoms like H 2 . A. NaCl B. CaO C. KCl D. LiF iClicker Time Which would you predict to have the highest melting point? Charge has a bigger effect than bond length differences CaO has charges of ±2, c.f. the others are ±1. Larger charge Larger lattice energy higher m.p. Lattice energy = k Q 1 Q 2 r 4 5 6
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A. NaCl B. CaO C. KCl D. LiF iClicker Time Which would you predict to have the highest melting point? Charge is the same for the remaining three. Consider ion separation (= sum of ionic radii) Lattice energy = k Q 1 Q 2 r And second highest melting point?
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