Week 9/Th: Lecture Unit ‘22’ VSEPR
© DJMorrissey, 2o12
Unit 21: Lewis Structures, II -- multiple bonds -- “resonance” Unit 22: VSEPR -- Rules -- Electron Arrangements -- 3D shapes Unit 23,24: Structure & Orbitals -- Molecular Shapes and Orbitals Issues: Test Results, Grade Estimation No Homework this week
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10 30 50 70 90 110 130 150 170 Num
ber
of S
tude
nts
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CEM 141 Fall 2o12 Exam 1
Week 9/Th: Exam Results
© DJMorrissey, 2o12
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10 30 50 70 90 110 130 150 170 N
umbe
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CEM 141 Fall 2o12 Exam 2
Exam 1 Exam 2 Count 2344 2226 Mean 109.0 105.5
Median 110 107.5 Mode 110 120
SD 31.7 34.8 Range 170 170
Minimum 10 10 Maximum 180 180
Grade Point Estimator
Week 9/Th: VSEPR Rules, part 1
© DJMorrissey, 2o12
The ValenceShellElectronPairRepusion (VSEPR) Theory is applied to Covalent Bonds: Electron Sharing, Main group elements Ø focus on the ‘central atom’ Ø construct Lewis electron-pair diagram Ø multiple bonds ?? Ø mutual repulsion of all remaining pairs in 3D à “arrangement” à Only five electronic arrangements on central atom Ø Plus one more rule …
Week 9/Th: VSEPR – 1st Three “Arrangements”
© DJMorrissey, 2o12
Top Row Hydrides : Lithium Hydride Li – H Beryllium Hydride H – Be – H Boron trihydride BH3 Methane CH4
H H C H H
B H H
H
Week 9/Th: VSEPR – All Five “Arrangements”
© DJMorrissey, 2o12
Plus: PF5 SF6
S F
F F F
F F P F
F F F
F
104.5o
Tetrahedron, 109.5o
Trigonal Pyramid
Bent
Week 9/Th: VSEPR & Lone Pairs
© DJMorrissey, 2o12
Top Row Hydrides : Methane CH4 Ammonia NH3 Water H2O Hydrogen Fluoride H – F
H H C H H
H N H H
O H H
Week 9/Th: VSEPR Rules, part 2
© DJMorrissey, 2o12
The ValenceShellElectronPairRepusion (VSEPR) Theory is applied to Covalent Bonds: Electron Sharing, Main group elements Ø focus on the ‘central atom’ Ø construct Lewis electron-pair diagram Ø multiple bonds ?? Ø mutual repulsion of all remaining pairs in 3D à “arrangement” à Only five electronic arrangements on central atom Ø LonePair&LP > LP&BondingPair > BP&BP à“molecular shape”
à Only eleven 3D shapes for central atom in molecule
Week 9/Th: VSEPR – Multiple Bonds
© DJMorrissey, 2o12
N2O NO2 N2O4 NO2
-
NO3
-
N N O N N O
O N O
O
N
O O
N
O
O N O
O N O O
Linear Bent Two parts Trigonal Planar Bent
Trigonal Planar
Week 9/Th: VSEPR All Rules
© DJMorrissey, 2o12
The ValenceShellElectronPairRepusion (VSEPR) Theory is applied to Covalent Bonds: Electron Sharing, Main group elements Ø focus on the ‘central atom’ Ø construct Lewis electron-pair diagram Ø treat multiple bonds as a single unit Ø mutual repulsion of all remaining pairs in 3D à “arrangement” à Only five electronic arrangements on central atom Ø LonePair&LP > LP&BondingPair > BP&BP à“molecular shape”
à Only eleven 3D shapes for central atom in molecule
Week 9/Th: VSEPR – Large Coordination
© DJMorrissey, 2o12
XeF4
XeF2 ?
SF6
IF5
S F
F F F
F F
I F
F F
F F
F Xe F F
F
Xe F F
Week 9/Th: VSEPR – All eleven Structures
© DJMorrissey, 2o12
2 3 4 5 6 0 Linear Trigonal
planar Tetrahedral Trigonal
bipyramidal Octahedral
1 X Bent Trigonal pyramidal
See-saw Square pyramidal
2 X X Bent T-shaped Square planar
3 X X X Linear (no examples)
Total # of Electron Pairs à
ß T
otal
# o
f Lon
e Pa
irs
Week 9/Th: VSEPR – Examples in Notes
© DJMorrissey, 2o12
1) Thionyl chloride: SOCl2 2) Boron trichloride: BCl3 [see analog BH3] 3) Xenon tetrafluoride: XeF4 [done] 4) Bromine trifluoride: BrF3 5) Nitrate anion: NO3
- [done]
Week 9/Th: Nitrogen Triiodide Demo
© DJMorrissey, 2o12
We wrote the Lewis Structure for ammonia, NH3, and we used that to identify its VSEPR structure as a trigonal pyramid. We also have the concept that elements lower in the periodic table with have analogous structures to those containing top-row elements. Nitrogen triiodide, NI3, should have the same structure (Lewis, VSEPR) However, there is a substantial size difference between H & I
N H
H H N I
I I
Covalent Radii H 25 pm N 65 pm I 108 pm
Nitrogen triiodide is highly strained and tends to decompose …
