Transcript 4 - VSEPR & POLARITY

MOLECULAR SHAPES
In 1940 Sidgwick proposed an explanation of the
shapes of molecules based on Lewis's model of
molecular shape.
He assumed Lewis's assumption that electrons tend to
group in pairs.
Sidgwick further assumed that electron pairs will
arrange themselves so as to be as far apart as possible.
The basis for this second assumption is the repulsion
between like charges.
This theory has since been named VSEPR (Valence
Shell Electron Pair Repulsion).
BASIC SHAPES
Using Lewis Stuctures 1st assess the
number of electron pairs surrounding the
central atom.
Place these electron pairs as far away as
possible.
If there are 2 pairs this is the shape:
This shape is:
linear
A simple system using a 3 number code
can be used where the 1st number
represents the number of electron pairs
around the central atom,
For this Lewis Structure the 1st
number would be
2
the 2 nd number represents the number
of atoms around the central atom,
For this Lewis Structure the 2nd
number would be
2
the 3rd number represents the number
of unshared electron pairs around the
central atom.
For this Lewis Structure the 3rd
number would be
0
What is the 3 number code for each of the
following:
431
541
If there are 3 pairs the shape is:
The 3
number code
is: 3 3 0
This shape is:
Trigonal
planar
If there are 4 pairs the shape is:
This shape is:
tetrahedral
The 3
number code
is: 4 4 0
If there are 5 pairs the shape is:
This shape is:
Trigonal
bipyramidal
If there are 6 pairs the shape is:
This shape is:
octahedral
If there are 7 pairs the shape is:
This shape is:
Pentagonal
bipyramidal
DERIVED SHAPES - From 4 4 0
Some molecules have unshared pairs of
electrons which influence the shape. Here
are some examples, NH3 , the Lewis
Structure is:
This shape is:
pyramidal
DERIVED SHAPES - From 4 4 0
Another example, H2O, the Lewis
Structure is:
Notice it has 2 unshared pairs of electrons.
This shape is:
angular
DERIVED SHAPES - From 5 5 0
Another example, SF4, the Lewis Structure
is:
Notice it has 1 unshared pair of electrons.
This shape is:
See saw
The 3 number code is:
541
When determining which sphere to
remove from the 5 5 0, remember,
remove the sphere where there is the
greatest repulsion:
DERIVED SHAPES - From 5 5 0
Another example, FCl3, the Lewis
Structure is:
Notice it has 2 unshared pairs of electrons.
This shape is:
T Shaped
The 3 number code is:
532
DERIVED SHAPES - From 5 5 0
Another example, XeF2, the Lewis
Structure is:
Notice it has 3 unshared pairs of electrons.
This shape is:
linear
The 3 number code is:
523
DERIVED SHAPES - From 6 6 0
Another example, lF5, the Lewis Structure
is:
Notice it has 1 unshared pairs of electrons.
This shape is:
Square-based
pyramid
The 3 number code is:
651
DERIVED SHAPES - From 6 6 0
Another example, XeCl4, the Lewis
Structure is:
Notice it has 2 unshared pairs of electrons.
This shape is:
Square
planar
The 3 number code is:
642
Shapes with Double Bonds
This simple model assumes the
repulsive force of 2 pairs of
electrons is almost the same as one
pair of electrons so treat a double
bond like a single bond for
repulsive purposes.
Examples: CO2
What is the Lewis Structure?
This shape is:
The 3 number code is:
420
linear
Examples: CF2
What is the Lewis Structure?
This shape is:
angular
The 3 number code is:
421
Examples: SO2
What is the Lewis Structure?
This shape is:
angular
The 3 number code is:
421
Examples: SO3
What is the Lewis Structure?
This shape is:
Trigonal
planar
The 3 number code is:
430
Sample Problems:
Draw Lewis Structures, state the 3 #
code, name the shape and provide an
alternative formula which has the
same shape, for each of the
following:
a) SeS3
b) FBr3
c) SeCl6
d) AsH3
e) PbF2
f) OTe2
g) XeCl2
h) AsBr3
i) H2S
j) FBr5
k) BrF7
l) SeBr6
m)NeF2
n) PoBr4
o) TeS3
p)AlF3
q) PO4
3-
r) H2CO3
Electric Dipoles and Molecular
Shape
To determine whether a molecule
containing more than two atoms is
polar or non-polar, we must also
know the shape of the molecule.
Consider the two molecules BeF2
and H2O.
From electronegativity values it can be
seen that in BeF2 the electrons will drift
toward the fluorine atoms.
This molecule is linear so both ends of
the molecule become negative and the
opposing polar character of the bonds
cancel each other.
-ve <--- +ve ---> -ve
F ------- Be------- F
Be
+
The negative charges at
each end can be resolved
to a single charge on top
of the positive charge
resulting in no net charge
NON POLAR
Although the bonds are polar,
the molecule itself does not
have positive and negative
ends. Hence BeF2 is not
polar.
In contrast to BeF2, the water
molecule is angular.
The polar O -- H bonds do
not cancel one another, and
the water molecule is left
with a positive end and a
negative end, thus making
water a polar molecule.
H
H
O
+
+
In this situation the positive charges
resolve themselves to a single positive
charge which is not on top of the negative
charge.
+
This molecule is polar due to charge
separation
Now compare the polarity of OF2 with that of water.
+
-
Since electrons drift away from oxygen in the OF2
molecule, the molecular dipole of OF2 is in the
opposite direction to that of water.
Sample Problems:
Draw Lewis Structures and determine the geometry of
each molecule. Which ones are polar?
a) SiO2
b) C2H4 c) PCl3
e) CH2Br2 f) NH3
g) CS2
d) H2S
h) SiF4
+
The negative charges at each end
can be resolved to a single charge
on top of the positive charge
resulting in no net charge
NON POLAR