MOLECULAR SHAPESIn 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 SHAPESUsing 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:
4 3 1
5 4 1
If there are 3 pairs the shape is:
This shape is:
Trigonal planar
The 3 number code is: 3 3 0
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:
Trigonalbipyramidal
If there are 6 pairs the shape is:
This shape is:
octahedral
If there are 7 pairs the shape is:
This shape is:
Pentagonalbipyramidal
DERIVED SHAPES - From 4 4 0Some 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 0Another example, H2O, the Lewis Structure is:Notice it has 2 unshared pairs of electrons.
This shape is:angular
DERIVED SHAPES - From 5 5 0Another example, SF4, the Lewis Structure is:Notice it has 1 unshared pair of electrons.
This shape is:See saw
The 3 number code is: 5 4 1
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 0Another example, FCl3, the Lewis Structure is:Notice it has 2 unshared pairs of electrons.
This shape is:T Shaped
The 3 number code is: 5 3 2
DERIVED SHAPES - From 5 5 0Another example, XeF2, the Lewis Structure is:Notice it has 3 unshared pairs of electrons.
This shape is:linear
The 3 number code is: 5 2 3
DERIVED SHAPES - From 6 6 0Another example, lF5, the Lewis Structure is:Notice it has 1 unshared pairs of electrons.
This shape is:Square-basedpyramid
The 3 number code is:6 5 1
DERIVED SHAPES - From 6 6 0Another example, XeCl4, the Lewis Structure is:Notice it has 2 unshared pairs of electrons.
This shape is:Squareplanar
The 3 number code is:6 4 2
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?
The 3 number code is:4 2 0
This shape is:
linear
Examples: CF2
What is the Lewis Structure?
The 3 number code is:
4 2 1
This shape is:
angular
Examples: SO2
What is the Lewis Structure?
The 3 number code is:
4 2 1
This shape is:
angular
Examples: SO3
What is the Lewis Structure?
The 3 number code is:4 3 0
This shape is:Trigonalplanar
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) PO43- 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 ---> -veF ------- 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
Since electrons drift away from oxygen in the OF2 molecule, the molecular dipole of OF2 is in the opposite direction to that of water.
- -+
Now compare the polarity of OF2 with 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 d) H2S
e) CH2Br2 f) NH3 g) CS2 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 chargeNON POLAR
