Week 9/Th: Lecture Unit ‘22’ VSEPR
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
© DJMorrissey, 2o12
Week 9/Th: Exam Results
CEM 141 Fall 2o12 Exam 1
350
CEM 141 Fall 2o12 Exam 2
250
200
150
100
50
0
10
30
50
70
90
110 130 150 170
Score
Count
Mean
Median
Mode
SD
Range
Minimum
Maximum
Exam 1
2344
109.0
110
110
31.7
170
10
180
© DJMorrissey, 2o12
Number of Students
Number of Students
300
Exam 2
2226
105.5
107.5
120
34.8
170
10
180
300
250
200
150
100
50
0
10
30
50
70
90 110 130 150 170
Score
Grade Point Estimator
Week 9/Th: VSEPR Rules, part 1
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 …
© DJMorrissey, 2o12
Week 9/Th: VSEPR – 1st Three “Arrangements”
Top Row Hydrides :
Lithium Hydride
Li – H
Beryllium Hydride
H – Be – H
Boron trihydride
H
BH3
Methane
CH4
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B
H
H
H
H C H
H
Week 9/Th: VSEPR – All Five “Arrangements”
Plus:
F
F P
F
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F
F
PF5
SF6
F F
F S
F
F
F
Week 9/Th: VSEPR & Lone Pairs
Top Row Hydrides :
Methane
H
H C H
CH4
H
Ammonia
H
N H
NH3
H
Water
H2O
Tetrahedron, 109.5o
Trigonal Pyramid
O H
H
Hydrogen Fluoride
H–F
© DJMorrissey, 2o12
Bent 104.5o
Week 9/Th: VSEPR Rules, part 2
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
© DJMorrissey, 2o12
Week 9/Th: VSEPR – Multiple Bonds
O N
O
N2O4
O
Bent
O
NO2-
O N N
O N
NO2
O N N
Linear
O N
N2O
Two parts
Trigonal
Planar
O N
NO3-
O
Bent
O
O N
O
© DJMorrissey, 2o12
Trigonal
Planar
Week 9/Th: VSEPR All Rules
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
© DJMorrissey, 2o12
Week 9/Th: VSEPR – Large Coordination
F F
SF6
F S
F
F F
IF5
XeF4
XeF2 ?
© DJMorrissey, 2o12
F
I
F
F
F
F
F
Xe
F
F Xe F
F
F
Week 9/Th: VSEPR – All eleven Structures
Total # of Electron Pairs à
ß Total # of Lone Pairs
2
0
3
Linear Trigonal
planar
4
5
6
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)
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Week 9/Th: VSEPR – Examples in Notes
1)  Thionyl chloride: SOCl2
2) Boron trichloride: BCl3
3) Xenon tetrafluoride: XeF4
[see analog BH3]
[done]
4) Bromine trifluoride: BrF3
5) Nitrate anion: NO3-
© DJMorrissey, 2o12
[done]
Week 9/Th: Nitrogen Triiodide Demo
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
Covalent Radii
H 25 pm
N 65 pm
I 108 pm
N
H
H
H
I
N
I
I
Nitrogen triiodide is highly strained and tends to decompose …
© DJMorrissey, 2o12