Lecture+13+-+Chemical+Bond

Lecture+13+-+Chemical+Bond - Bonds formed by different...

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Unformatted text preview: Bonds formed by different atoms have electrons shared unevenly. The tendency of an atom to attract electrons is electronegativity (X) . A 2 + B 2 → 2AB δ + δ- H 2 + F 2 → 2HF ( ) 2 1 2 2 B A AB B A D D D X X − = − Electronegativity and Dipole Moment D m C m C r Q ionic 8 . 4 10 6 . 1 10 10 6 . 1 29 10 19 = × = × × = × = − − − μ μ Positive and negative charges (Q) in a molecule separated by a distance (r) generates a permanent electric dipole moment ( μ ): H F δ- δ + m C D ⋅ × = − 30 10 3356 . 3 1 A(g) + e- → A- (g) X A ∝ K eq = ionic μ μ % ionic character D AB = bond dissociation E of AB H H gas liq ⇒ CO 2 is linear Electronegativity (X) A(g) + e- → A- (g) ( ) 2 1 2 2 B A AB B A D D D X X − = − B(g) + e- → B- (g) Bond Polarity Dipole Moment % 100 character ionic % exp × = ionic μ μ 4.8 D Example: Calculate the % ionic character of H – F (bond length = 92 pm). From Table 12.1, the experimental dipole moment of HF is 1.91 D m C D m C D ⋅ × = ⎟ ⎟ ⎠ ⎞ ⎜ ⎜ ⎝ ⎛ ⋅ × × − − 30 30 10 37 . 6 1 10 33 . 3 91 . 1 % ionic character % 2 . 43 % 100 ) 10 92 )( 10 6 . 1 ( 10 37 . 6 % 100 12 19 30 exp = × × × ⋅ × = × = − − − m C m C ionic μ μ Molecular Orbital Theory (LCAO-MO) Molecular orbitals arise from linear combination of atomic orbitals (LCAO): ( ) B B A A * Ψ c Ψ c σ Ψ − = A + B → A -- B ( ) B B A A Ψ c Ψ c σ Ψ + = Anti-bonding orbital ( σ *) Bonding orbital ( σ ) No electron density repulsion attraction Ψ A Ψ B Heteronuclear Diatomic Bonding A + B → A -- B Ψ A Ψ B Ψ σ Ψ σ * B A c c ψ ψ σ ψ × + × = 2 1 ) ( B A c c ψ ψ σ ψ × − × = 4 3 *) ( E ( ) 2 2 1 B E E c − σ α ( ) 2 1 1 A E E c − σ α ( ) 2 * 3 1 A E E c − σ α ( ) 2 * 4 1 B E E c − σ α c 3 > c 4 c 2 > c 1 ) ( ) ( ˆ r E r H ψ ψ = Heteronuclear Diatomic MOs H-F (bond order =1) C O (bond order=3) 2p z z p s p c c 2 4 1 3 * 2 ) ( ψ ψ σ ψ − = z p s p c c 2 2 1 1 2 ) ( ψ ψ σ ψ + = HF (8 e- ): (2s) 2 ( σ 2p ) 2 (2p x ) 2 (2p y ) 2 ( σ * 2p ) 2 CO (10 e- ): ( σ 2s ) 2 ( σ * 2s ) 2 ( π x ) 2 ( π y ) 2 ( σ 2p ) 2 c 2 > c 1 c 3 > c 4 E( σ * 2p ) ≅ E(H 1s ) H F c 1 < c 2 non-bonding...
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This note was uploaded on 09/16/2011 for the course CHE 107B CHE 107B taught by Professor Ames during the Summer '11 term at UC Davis.

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Lecture+13+-+Chemical+Bond - Bonds formed by different...

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