CHEMISTRY 161
HW #9B
9B
16, 18, 20, 22, 24, 26, 28, 30, 32, 68.
9-16
B
B
A
B
B
Trigonal bipyramidal geometry
0 lp—lp, 0 lp—bp, 6 bp—bp
B
B
B
B
Square pyramidal geometry
A
0 lp—lp, 0 lp—bp, 8 bp—bp
B
B
The trigonal bipyramid has the fewest bp—bp interactions, so it has a lower in energy than the square
pyramid.
9-18
In order of increasing bond angle, (a) seesaw = (c) square pyramidal < (b) tetrahedral
9-20
From the table we can see that replacing one atom in all the structures with a lone pair does not
change any of the bond angles (ideally). When we remove two atoms from a trigonal bipyramidal
structure we lose the 180˚ bond angle, and when we replace three atoms on the trigonal bipyramidal
structure with lone pairs, we lose the 90˚ and 120˚ bond angles between atoms. Thus, the molecular
geometries that have the same bond angles when lone pairs replace one or more atoms are linear,
trigonal planar, tetrahedral, trigonal pyramidal, trigonal bipyramidal (replacing one atom only),
octahedral, and square pyramidal.
9-22
(c)
9-24
Removing two adjacent edge atoms gives a
pyramidal shape
Removing two atoms across the diagonal of a
face gives a very distorted octahedron
Removing two atoms at opposite corners
gives an approximate octahedral geometry
9-26
O
(a)
SN = 3
Electron-pair geometry = trigonal
planar
No lone pairs
Molecular geometry = trigonal
planar
N
O
O
O
(b)
SN = 4
Electron-pair geometry =
tetrahedral
No lone pairs
Molecular geometry = tetrahedral
(c)
S
S
O
N
O
O
O
SN = 3
Electron-pair geometry = trigonal
planar
One lone pair
Molecular geometry = bent
(d)
SN = 4
Electron-pair geometry =
tetrahedral
One lone pair
Molecular geometry = trigonal
pyramidal
N
F
F
F
9-28
(a)
SN = 2
Electron-pair geometry = linear
No lone pairs
Molecular geometry = linear
(b)
S
C
N
+
H
H
Cl
C
P
H
Cl
Cl
SN = 4 (around P atom)
Electron-pair geometry =
tetrahedral
No lone pairs
Molecular geometry = tetrahedral
Cl
(c)
SN = 5
Electron-pair geometry = trigonal
bipyramidal
Three lone pairs
Molecular geometry = linear
(d)
SN = 4
Electron-pair geometry =
tetrahedral
One lone pair
Molecular geometry = trigonal
pyramidal
I
Cl
P
O
O
9-30
(a)
SN = 4
Electron-pair geometry = tetrahedral
One lone pair + one unpaired
electron
Molecular geometry = bent
(b)
SN = 4
Electron-pair geometry = tetrahedral
One unpaired electron
Molecular geometry = trigonal
pyramidal
(c)
F
F
I
F
F
SN = 5
Electron-pair geometry = trigonal
bipyramidal
Two lone pairs
Molecular geometry = T-shaped
F
O
I
F
(d)
F
F
F
S
SN = 5
Electron-pair geometry = trigonal
bipyramidal
One lone pair
Molecular geometry = seesaw
F
F
S
F
F
F
9-32
Lewis structure
SN
Electron-pair
geometry
Number of lone
pairs
Molecular
geometry
Bond angle
Both N3– and CO2 are linear.
2
Linear
3
Trigonal planar
2
Linear
0
1
0
Linear
Bent
Linear
180˚
<120˚
180˚
9-68
Because the steric number, and thus the geometry, around each central atom in a larger molecule must
be defined separately and may be different for adjacent atoms, the overall molecular geometry is
sometimes hard to name. For example, if a tetrahedral atom is bonded to a trigonal planar atom, just
one term cannot describe the geometry of the molecule
X
Y
A
X
X
B
Y