Week 10/Tu: Units ‘23,24’ Valence Bonds

© DJMorrissey, 2o12

Unit 22: VESPR -- Electron Arrangements -- 3D shapes Unit 23,24: Structure & Orbitals -- Molecular polarity -- Molecular Shapes and Orbitals -- Valence bond theory -- sigma and pi bonds Unit 25: Gases -- properties of gases -- gas laws Issues: CONNECT Lewis Drawing … Homework resumes this week, due Saturday 8AM

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Week 10/Tu: Bond Polarity

© DJMorrissey, 2oo9

In general the energy levels of elements are different which leads to the unequal sharing of electrons in chemical bonds. Electronegativity is the quantitative measure of the unequal sharing that we have already discussed. Thus, most chemical bonds are polarized to a greater or lesser extent. The molecular electron distribution can be polarized (or unequally distributed) depending on the atoms in the molecule and its shape …

O O

EN = 3.5 3.5 Δ EN = 0

Cl H

EN = 2.1 3.0 Δ EN = 0.9

O N

EN = 3.0 3.5 Δ EN = 0.5

Week 10/Tu: Molecular Polarity -1-

© DJMorrissey, 2o12

1)  Lewis Diagrams for Main Group elements (8 atomic diagrams) 2)  VSEPR Structures for molecules (5 arrangements, 11 shapes) à symmetric shapes: non-polar molecules à asymmetric shapes: polar molecules The polarization of water molecules is widely known and gives water unique properties.

H O H

EN = 2.1 3.5 2.1 Δ EN = 1.4 each bond

4 pairs on Oxygen – tetrahedral 2 lone pairs on Oxygen – bent

O C O

2 pairs on Carbon – linear (ignore multiple bonds for arrangement) 0 lone pairs on Carbon – linear

EN = 3.5 2.5 3.5 Δ EN = 1.0 each bond

Week 10/Tu: Molecular Polarity -2-

© DJMorrissey, 2o12

1)  Lewis Diagrams for Main Group elements (8 atomic diagrams) 2)  VSEPR Structures for molecules (5 arrangements, 11 shapes) à symmetric shapes: non-polar molecules à asymmetric shapes: polar molecules

3 pairs on Nitrogen – trigonal planar (ignore multiple bonds for arrangement) 0 lone pairs on Nitrogen – trigonal planar Resonance structure …

2 pairs on central Nitrogen – linear (ignore multiple bonds for arrangement) 0 lone pairs on central Nitrogen – linear

EN = 3.5 3.0 3.0 Δ EN = 0.5 N-O bond

N N O

O N O

O

O N O

O

Week 10/Tu: Valence Bond Theory

© DJMorrissey, 2o12

1)  Lewis Diagrams for Main Group elements (8 atomic diagrams) 2)  VSEPR Structures for molecules (5 arrangements, 11 shapes)

3)  Valence Bond Theory: a) Use the VSEPR & Lewis Diagrams to determine structure b) Combine the available valence orbitals to describe structure à Hybridize (mix) atomic orbitals as necessary to correspond to the electron distribution in the molecule. à the number of orbitals is conserved; 2 AO’s give 2 VBO’s, etc. à Each mixed (hybrid) orbital can hold one pair of electrons, that are either a bonding pair or a or a non-bonding (lone) pair c) Use (remaining) atomic orbitals to describe multiple bonds if present. Note that there are only five “arrangements” of electrons in

the VSEPR theory. Thus, only five hybrids (mixtures) of atomic orbitals needed to describe these arrangements.

Week 10/Tu: Valence Bond Hybrids -1a-

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Top Row Fluorides : Beryllium Fluoride BeF2 Be -- 1s2 2s2 2p0

Linear .. (symmetric)

H 1s & Be sp Be sp & H 1s Single Bonds … called “sigma” bonds

Week 10/Tu: Valence Bond Hybrids -1b-

© DJMorrissey, 2o12

Top Row Fluorides : Beryllium Fluoride BeF2 à sp Boron trifluoride BF3 B -- 1s2 2s2 2p1

Trigonal Planar

Week 10/Tu: Valence Bond Hybrids -1c-

© DJMorrissey, 2o12

Top Row Fluorides : Beryllium Fluoride BeF2 à sp Boron trifluoride BF3 à sp2

Carbon tetrafluoride CF4

C – 1s2 2s2 2p2 Tetrahedral Oxygen difluoride OF2 Tetrahedral / Bent

Week 10/Tu: Valence Bond Hybrids -2-

© DJMorrissey, 2o12

Unusual Fluorides : Arsenic pentafluoride PF5 P – [He] 3s23p3 4s0 4d0

Trigonal Bipyramidal

Tellurium tetrafluoride TeF4 Te – [Kr]5s24d105p4 6s0 5d0

Trigonal Bipyramidal / See-saw Selenium hexafluoride IF5 I – [Kr]5s24d105p5 6s0 5d0

Octahedral / Square Pyramidal

Week 10/Tu: Valence Bond Hybrids – Summary

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Atkins & Jones Fig. 3-17, 18, 19

Week 10/Tu: Valence Bond, Multiple Bonds

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Multiple bonds – build the underlying structure, use remaining atomic orbitals to build multiple bonds.

H H C C H H

C -- 1s2 2s2 2p2 Trigonal Planar – sigma bonds Double bond – pi bond

Week 10/Tu: Valence Bond Examples in Notes

© DJMorrissey, 2o12

Molecule VSEPR Valence Bond Arrangement Structure Hybridization BF3 trigonal planar trigonal planar sp2 ✓

PF5 trigonal bipyramidal trigonal bipyramidal sp3d ✓

XeOF4 octahedral square pyramidal sp3d2

H3O+ tetrahedral trigonal pyramidal sp3

CH4 tetrahedral tetrahedral sp3 ✓

ClO2- tetrahedral bent sp3

CO2 linear linear sp SF6 octahedral octahedral sp3d2 ✓ BeCl2 linear linear sp ✓