VSEPR theory predicts molecular geometry based on electron pair repulsion. It was developed in 1957 by Gillespie and Nyholm. The theory states that electron pairs around an atom will adopt a geometry that minimizes repulsions between electron pairs. Molecular shapes are determined by counting bonding and non-bonding electron pairs. Common molecular geometries include linear, trigonal planar, tetrahedral, trigonal bipyramidal, and octahedral. The theory successfully predicts geometry for many molecules but has limitations for transition metal complexes and compounds where exact bond angles are sought.
When drawing lewis structure-'s how do we know whether the shape shoul.docxSUKHI5
When drawing lewis structure\'s how do we know whether the shape should be a tetrahedral or a square planar? For example in questions where we have to draw the lewis structure for XEBr4 or XeF4?
Solution
Lewis structures, along with VB theory, can predict the structures of molecules based on the number of electron pairs around the central atom in the molecule. The following shapes are possible based on the number of electron pairs.
Number of electron pairs around the central atom
Shape of the molecule
2
Linear
3
Planer Trigonal
4
Tetrahedral
5
Trigonal bipyramidal
6
Octahedral
7
Pentagonal bipyramid
Now count the number of electron pairs in XeBr 4 or XeF 4 .
Number of electron pairs = ½*(8 + 4) = 6
The electronic geometry of the molecule is octahedral; however, the actual shape of the molecule is square planer. This is due to the fact that there are 4 bonds between the Xe atom and the halogen. The Xe atom has two non-bonded electron pairs in the molecules. Non-bonded electron pairs are subject to maximum repulsions and hence, occupy diagonally opposite positions. Due to repulsions, the Xe-X (X = Br, F) bonds are squeezed and the molecules assume square planer geometry with the two lone pairs far away from each other.
.
When drawing lewis structure-'s how do we know whether the shape shoul.docxSUKHI5
When drawing lewis structure\'s how do we know whether the shape should be a tetrahedral or a square planar? For example in questions where we have to draw the lewis structure for XEBr4 or XeF4?
Solution
Lewis structures, along with VB theory, can predict the structures of molecules based on the number of electron pairs around the central atom in the molecule. The following shapes are possible based on the number of electron pairs.
Number of electron pairs around the central atom
Shape of the molecule
2
Linear
3
Planer Trigonal
4
Tetrahedral
5
Trigonal bipyramidal
6
Octahedral
7
Pentagonal bipyramid
Now count the number of electron pairs in XeBr 4 or XeF 4 .
Number of electron pairs = ½*(8 + 4) = 6
The electronic geometry of the molecule is octahedral; however, the actual shape of the molecule is square planer. This is due to the fact that there are 4 bonds between the Xe atom and the halogen. The Xe atom has two non-bonded electron pairs in the molecules. Non-bonded electron pairs are subject to maximum repulsions and hence, occupy diagonally opposite positions. Due to repulsions, the Xe-X (X = Br, F) bonds are squeezed and the molecules assume square planer geometry with the two lone pairs far away from each other.
.
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2. INTRODUCTION
VSEPR [Valence shell electron pair repulsion]
This theory is developed by RONALD GILLESPIE and RONALD
NYHOLM in the year 1957.
This theory is also called GILLESPIE-NYHOLM theory.
The main idea of VSEPR that is “pairs of electrons in bond pair
and in lone pair repel each other”.
Thus far, we have used two dimentional lewis structure to
represent molecule. However molecular structure is actually
three-dimensional.
3. POSTULATES OF VSEPR THEORY
The central metal atom is surrounded by only bond pair not lone pair,
hence the molecule have REGULER GEOMETRY.
When both the bond pair and lone pair present on central metal
atom, the molecule will have IRREGULER OR DISTORTED GEOMETRY.
One pair of electron in valence shell repel another pair of electron
because they are electro negative.
Triple bond have more repulsion than double bond similarly double
bond have more repulsion than single bond.
The lone pair takes more space around a central atom than a bond
pair.
This theory is used to predict the molecular shapes of various
molecular from the electron pairs that surrounded the central atom
of the molecule
6. GENERIC FORMULA – AB
A – The Central atom
B – An atom bonded to A
Ex: H2
It contains one bond pair and zero lone pair.
Dihydrogen is an element molecule consisting of the two
hydrogen joined by a single bond.
Two hydrogen atoms are bonded together.
It is axial overlap.
It forms linear structure.
The bond angle is 1800.
H H
7. GENERIC FORMULA - AB3E
A-The central atom
B-An atom bonded to A
E-A lone pair on A
Ex:NH3
It contains three bond pair and one lone pair.
Outermost shell contains 8 electron i.e 4 electron pair in that 3
bond pair and 1 lone pair.
Four electron pairs raise to tetrahedral structure and in this case
three positions is occupied by H atoms and fourth position is
occupied by lone pair of electron.
8.
Presence of lone pair causes
slight distortion from bond angle
of 109.50 to 107.50.
The electron geometry of NH3 is
tetrahedral where as molecular
geometry is trigonal pyramidal.
9. GENERIC FORMULA – AB4E2
Ex:- XeF4
Here the central metal atom is Xe,
and it is surrounded by four F
atoms and two pairs of lone pair
electron.
XeF4 contains 6 electron pairs,
means 12 electrons in that 4
electron pairs are bond pair and
the other 4 electron i.e 2 lone pair.
The electron geometry of XeF4 is
octahedral but due to presence of
two lone pair electron the
molecular geometry is square
planar.
The bond angle is 900 or 1800
10. LIMITATIONS
This theory can’t explain the structure of the
transition metals compounds and ions.
This theory can’t be used to obtain the exact
bond angles between the atoms in a molecule.
Halides of group two elements should have a
linear structure according to this theory, but
their actual structure is a bent one.
11. CONCLUSION:-
VSEPR Theory can be used to predict the
shapes of molecule.
The molecular shape gives the arrangement
of atoms in a molecule.
The electron pair geometry gives the
arrangement of bonding and non-bonding
pairs around a central atom.