Chem 102 lec 1c W’11
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Last time: orientation
a) discussed syllabus
b) electron configurations
c) Lewis structures: F2 , NO2Today,
We review: Chapt 8 material
1) rules for Lewis structures.
2) hydrocarbon line structures
3) multiple covalent bonds
4) resonance
5) bond polarity
In addition: Chapt 9 material
6) VSEPR model and e domains
7) molecular polarity
8) hybridization
Many atoms are most stable when full octet is formed.
Lecture: 1c:
(1) How to draw Lewis structure:
1) count # valence electrons
2) draw skeleton: central atom, peripheral atoms
3) draw single bond ; fill octet
4) recount the e’s
5) usually # bonds: C =4, O=2., H=1
(2) Do: NH3 , NO2+
(3) Do C4H10 , C6H6
(4) Drawing hydrocarbon skeletons using line structures.
(5) Electronegativity and bond polarity
(6) What is shape of the molecule?
Use VSEPR theory
Likened to balloons repelling each other.
Now learn about the various electron domain geometries
of these molecular arrangements.
VSEPR Model.
2 e domains = linear; 180°
3 = trigonal planar; 120°
4 = tetrahedral; 109.5°
5 = trigonal bipyramidal; 120° and 90°
6 = octahedral; 90°
(7) NOTE: electron domain geometry is not the same as
molecular geometry
molec geometry is what remains after you remove the
lone pairs.
Review the terminology for the various types of molecular
geometries
(refer to tables 9.5 and 9.6)
Linear,
triangular (or “trigonal”) planar,
tetrahedral,
triangular pyramidal,
angular (or “bent”),
triangular bipyramidal,
seesaw, T-shaped,
octahedral,
square pyramidal,
square planar.
(8) Note that lone e pairs repel more than lone pairs. So
the actual angles differ from the ideal.
Also, it is impt to note where the e pairs will go for say, 5
or 6 e domains
(9) Do examples: Draw the Lewis structures. From that
determine the electron domain shapes, the molecular
shapes, the angles between the bonds, the polarity of
bonds and the polarity of the molecules themselves.
CH4
BF3
CO2
NH3
H2 O
NO2+
CH3CO2H
PF5
SCl6
(10) Hybridization:
a) relate e pair geometry to hybridization
b) understand hybridization; draw the shapes
VBT based on calculations on atomic orbitals.
Hybrid orbitals.
Valence bond theory (VBT)
1) draw H-H molec. 1s & 1s combine: sigma (s)
bond. Electron density is along internuclear axis
2) H-F bond: 2p and 1s overlap: also sigma bond
3) draw F-F molec: 2p & 2p combine. : sigma bond
also 2p-2p sigma
4) draw O=O molec: double bond. 2p and 2p but
now have: sigma bond and : pi bond (describe
properties of a pi bond: more easily broken. Not
really 2 equally strong bonds. Sigma bonds can be
rotated. But not pi bonds. Pi bonds are only found
in double and triple bonds. Sigma bonds always.)
5) draw CH4: problem! need to invoke a new thing:
hybrid orbitals.
Hybrid orbitals: formed from s and p (and d).
# hybrid orbitals = # orbitals hybridized.
P| p| p
P | P|
=>
2s ||
2sp| 2 sp|
2s
if we consider a C atom: 1s2, 2s2 2p2
can have: two sp suborbitals and two 2 p suborbitals: sp
suborbitals are linearly placed.
can have three sp2 suborbitals and 1 2p orbital: sp2
suborbitals are trig planar placed
can have four sp3 suborbitals. Tetrahedral.
Know:
sp orbital
sp2
sp3
sp3d
sp3d2