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Trigonal planar
electron-group arrangement
with
one lone pair
Two
atoms attached to the central atom +
one
lone
pair (
AX
E 2
)^
Bent shape
The
lone pair is bulkier
and repels the bonding
pairs stronger
bond angle is less than 120
Example: SO
2
Bent shapeBond angle of ~
Tetrahedral
electron-group arrangement with
one lone pair
Three
atoms attached to the central atom +
one
lone
pair (
AX
E 3
)^
Trigonal pyramidal shape
The
lone pair is bulkier
and repels the bonding
pairs stronger
bond angles are less than 109.
Example: NH
3
Trigonal pyramidalshapeBond angle of 107.
Tetrahedral
electron-group arrangement with
two lone pairs
Two
atoms attached to the central atom +
two
lone
pairs (
AX
2
E
Bent shape
The
two lone pairs
have even greater repelling
effect
bond angles are less than 109.
Example: H
2
O
Bent shapeBond angle of 104.
(lower than in NH
3
- Strengths of electron group repulsions
lone pair-lone pair > lone pair-bonding pair >bonding pair-bonding pair^ ⇒
In the electron arrangement, lone pairs occupypositions as far from one another and frombonding pairs as possible
Trigonal bipyramidal
electron-group
arrangement with
,^
or
3 lone pairs
AX
E 4
,^
AX
E 3
2
and
AX
E 2
3
lone pairs occupy
equatorial
positions
(provides more space for the lone pairs andminimizes the repulsion)
Octahedral
electron-group arrangement with
or
2 lone pairs
AX
E 5
, and
AX
E 4
2
lone pairs occupy positions opposite to
each other
(provides the lowest repulsion)
Bent
e-groups = 3# lone pairs = 1
Trigonal pyramidal
e-groups = 4# lone pairs = 1
See-saw
e-groups= 5# lone pairs = 1
Square pyramidal
e-groups = 6# lone pairs = 1
~
o
~
o
~
o (^) ~
o
~
o
~
o
Bent
e-groups = 4# lone pairs = 2
~
o
T-shaped
e-groups = 5# lone pairs = 2
~
o
Linear
e-groups= 5# lone pairs = 3
Square planar# e-groups = 6# lone pairs = 2
90
o
Summary of molecular shapes for central atomswith lone pairs
Steps
in determining molecular shapes using
the VSEPR model:– Write the Lewis structure– Determine the electron-group arrangement, ideal
bond angles and VSEPR class
- Place the surrounding atoms and lone pairs in
appropriate positions around the central atom andpredict any deviations from the ideal bond angles
more than one central
atom
, find the electron-group arrangement
and corresponding shape around each centralatom (one central atom at a time)
Polar
molecules – nonzero dipole moment
- Heteronuclear diatomic molecules (
HF
,^
CO
- Polyatomic molecules where the bond dipoles do
not cancel each other
H
O 2
is a polar
molecule because thebond dipoles of theO-H bonds do notcancel due to the bentmolecular shape
H
O
H
H
O
δ+H
The polarity of molecules depends on both thepolarity of the bonds and the moleculargeometry which must be known
Example:
The isomers 1,2-dichlorobenzene and
1,4-dichlorobenzenehave the sameformula, C
6
H
4
Cl
2
but differentphysical properties
B.p. 180
oC
Very soluble
in EtOH
Cl
Cl
Cl
Cl
B.p. 174
o^ C
Slightly soluble
in EtOH
δ−
δ−
δ−
δ−
Polar
Non-polar
- Highly symmetric molecules are normally
nonpolar–
AX
n
molecules (n=2, 3, 4, 5, 6) where
X
are
atoms of the same element
- Molecules with symmetrically positioned lone
pairs (
AX
E 2
AX
E 4
- Molecules with asymmetrically positioned
lone pairs or different atoms attached to thecentral atom are normally polar–
AX
E 2
,^
AX
E 2
2
,^
AX
3
E
,^
AX
3
E
AX
E 4
,^
AX
E 5
CF
H 3
,^
CF
2
H
2
,^
SO
(bent), ... 2
Example:
Is
PCl
2
F
3
a polar molecule?
The Lewis structure is similar to
PCl
5
(five atoms
bonded to the
P
atom, no lone pairs)
trigonal bipyramidal shape
The
Cl
atoms are larger and take two of the
equatorial positions; the
F
atoms are smaller and
take the two axial and one of the equatorial positions
The
P–F
dipoles are larger than the
P–Cl
dipoles (
EN
is larger for
P
and
F
The molecule is
polar
dipoles don’t cancel (asymmetricarrangement)
Linear (non-polar)
Trigonal planar
(non-polar)
Tetrahedral(non-polar)
Trigonal bipyramidal
(non-polar)
Octahedral(non-polar)
180
o
120
o
o
90
o^120
o^
90
o
90
o
Summary of molecular polarities for moleculeshaving central atoms without lone pairs
Bent(polar)
Trigonal pyramidal
(polar)
See-saw(polar)
Square pyramidal
(polar)
~
o
~
o
~
o (^) ~
o
~
o
~
o
Bent(polar)
~
o
~90 T-shaped(polar)
o
Linear (non-polar)
Square planar (non-polar)
90
o
Summary of molecular polarities for moleculeshaving central atoms with lone pairs
