Lecture+12+-+Chemical+Bond

Lecture+12+-+Chemical+Bond - Problem Set 2: due at end of...

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Problem Set 2: due at end of class today. Practice Final: on website Final Exam: Sep 8 th on Chapters 11, 12 and 14.
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The Chemical Bond (Chapter 12) Valence Bond Theory Molecular Orbital Theory Hw: 2, 4, 6, 8, 9, 10, 12, 14, 16, 18, 20, 22, 24, 26, 28, 36, 38, 40, 42, 46, 47, 48, 50
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H H .. - Covalent bond formed by equal sharing of electrons between atoms. - Each atom (except H) surrounded by 8 valence electrons: octet rule . - Octet rule applies to 2 nd row elements C, N, O, and F (but NOT B). - Formal Charge = valence electrons vs bonding/unpaired electrons. - Lewis structures do not explain geometry or shape of molecule. - Lewis structures fail to give quantitative bond strength and length. Sulfate Ion Lewis Electron Dot Structures O H H C H H H H C O - O S + O - O O - O S B Cl Cl Cl .. .. .. .. .. .. N H .. .. H H -1 -1 H · + · H
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Bond is formed by overlapping valence electrons. s-orbitals: Valence Bond Theory Valence Bond Theory determines bond lengths (R e ) and bond energies. R e Δ E A B × τψ ψ d B s A s 1 1 A B ψψ ) ( ˆ r E H = + +
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) 2 ( ) 1 ( B A ψ × = ) 1 ( ) 2 ( ) 2 ( ) 1 ( B A B A × + × = Bond Length and Bond Energy ψψ E H = ˆ ionic covalent (r = 1 Å, see a) (r = 0.9 Å, see b) (r = 0.74 Å, c) A B
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Valence Bonds (Sigma vs. π ) p z + p z overlap p z + s “good overlap” p y + s “poor overlap” π -Bond between p x + p x or p y + p y Electron density above and below nuclei N N O C O F F H F Sigma ( σ ) bonds with p-orbitals (e- density between nuclei) : Low e - density between nuclei cancels
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Valence Bond theory gives number of bonds formed, bond lengths, bond energies etc., but fails to explain molecular geometry. Why is CO 2 linear and water bent?
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Lecture+12+-+Chemical+Bond - Problem Set 2: due at end of...

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