Chem 105
Wednesday, 18 Nov 2009
Lewis formula practice
Sub-octet and expanded-octet molecules
Molecular geometry
Electron pair geometry vs. molecular geometry
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1
The preferred Lewis formula for “OCNH” contains
how many single bonds?
1.
2.
3.
4.
5.
0
1
2
3
4
36
19
6
4
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4
3
2
1
0
0
2
O
C
N
H
O
C
N
H
O
C
N
H
O
C
N
H
O
C
N
H
O
C
N
H
Bad: + charge on O, and charge
separation that is not necessary.
Isocyanic acid. liquid bp 24 °C First discovered in 1830 by Frederick Wohler.
One of 129 interstellar molecules identified by rotational spectroscopy
(microwave region).
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The 129 reported interstellar and circumstellar molecules
http://www.cv.nrao.edu/~awootten/allmols.html
3
Sub-octet atoms in molecules (Be, B only)
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4
Sub-octet (electron-deficient) atoms
F
Be
F
?
F
Be
F
(“No”)
Do F atoms donate electrons to give Be an octet?
(No: the Be-F bonds are single due to the high
electronegativity of F.)
Compare bond lengths:
O=C=O has = bonds (short).
F―Be―F has ― bonds. (longer)
-2
+1
1.18 Å
1.40 Å
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5
Expanded octet atoms (3rd, 4th, 5th period) can
accommodate more than 8 valence electrons due to
greater size.
10 valence electrons (5 bonds/non-bonding pairs)
12 valence electrons (6 bonds/non-bonding pairs)
F
F
F Cl
F
F
Chlorine pentafluoride ClF5
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F
F
F
P
F
F
Phosphorus pentafluoride PF5
6
Molecular geometry is controlled by valence electron status
Why is the DNA
phosphate P
tetrahedral??
Why are the
bases
(A,G,C,T)
flat?
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7
Molecular geometry is controlled by valence electron status
VSEPR – Valence Shell Electron Pair Repulsion
1. Valence electron pairs occupy orbitals – look like “balloons.”
2. When attached to an atom, they repel each other due to negative charges.
Very approximately, SINGLE, DOUBLE, TRIPLE, & NON-BONDING PAIRS
all look like “a balloon”.
3. These lead to various Electron Pair geometries
X
X
X
2 balloons: “Linear”
H-C≡N:
4 balloons:
“Tetrahedral”
3 balloons:
“Trigonal planar”
S=C=NCl-Be-Cl
O
H
O
F
H
180°
H
H
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120°
B
F
F
H
H
109°
2-
O S
O
O
8
Actual balloons can be used to represent electron orbitals.
Linear
Trigonal
planar
Tetrahedral
One “balloon” can represent a
single, double, or triple bond, or
non-bonding pair.
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9
Non-bonding electron pairs occupy one “balloon.”
Therefore, a molecule may have DIFFERENT “Electron Pair” and
“Molecular” (or “ionic”) geometries.
X
X
ClO3- anion
O
H2S molecule
-
O Cl
H S
Cl
O
O
O
S
H
H
H
O
“Tetrahedral” electron pair geom;
“Trigonal pyramidal” molecular geometry
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“Tetrahedral” electron pair geom;
“Bent” molecular geometry
10
“Methylamine”
H
CH3NH2
H
N
H
H
H
Central N has a
tetrahedral electron
pair geometry
(4 electron pairs)...
Bond angle
~ 109o
...and trigonal pyramidal
molecular geometry
(3 bonds and 1 nb pair)
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11
AX2
Linear
180°
Trigonal
120°
AX3 planar
Electron pairs
can occur in 6
different
amounts,
each with a
characteristic
orientation:
AX4 Tetrahedral 109°
AX5 Trigonal
90°
bipyramidal 120°
AX6 Octahedral 90°
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12
Expanded octet electron pair geometries
Trigonal
bipyramidal
Octahedral
(all positions are equivalent in
the octahedral geometry)
One “balloon” can represent a single, double, or
triple bond, or non-bonding pair.
(In 5-coordinate atoms, a non-bonding pair
occupies the less-crowded equatorial position.)
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13
Trigonal
bipyramidal
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14
Do we need to know the standard
electron pair and molecular
geometries?
Yes.
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15
Expanded Octets: The electron
pair geometry of atoms with
FIVE electron pairs is
“triangular bipyramid”
F
F
F
P
F
F
Phosphorus
pentafluoride PF5
F
F
S
F
F
Sulfur tetrafluoride has
one lone pair
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“See-saw”
molecular shape
16
The non-bonded pair does not occupy an
“apical” position.
The apical positions are more crowded than the
equatorial positions.
Lone pair orbitals are more diffuse than single bond
orbitals. They prefer the roomier equatorial position.
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17
(~99.999% see-saw)
The “binding energy” = energy relative to free atoms, therefore it is very negative.
See-saw SF4 shape is ~7 kcal/mole more stable than the trigonal pyramidal
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18
Expanded Octet: 6 e- pairs)
•
Octahedral e- pair geometry
SF6
•
1 non-bonding pair: square pyramidal
F
F
F Cl
F
F
Chlorine pentafluoride
has one lone pair
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ClF5
Octahedral e- pr
Square pyramidal molecular
19
In ClF5 the minimum F-Cl-F bond angle is about..
90°
109 °
120
180
16
5
18
0
12
0
10
9
90
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°
7
°
1.
2.
3.
4.
41
20
90°
ClF5
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90°
21
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22
JK’s “ctrl-c file”
OWL
linear
trigonal planar
tetrahedral
trigonal bipyramidal
octahedral
bent
trigonal pyramidal
seesaw
t-shaped
square pyramidal
square planar
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23
“First draw the Lewis structure.”
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