The VSEPR (Valence Shell Electron Pair Repulsion) theory is a model in chemistry that is used to predict the three-dimensional geometry of molecules.
It is based on the idea that electron pairs, whether they are in the form of bonding pairs or lone pairs, repel each other, and the molecular geometry is determined by the arrangement of these electron pairs to minimize their repulsions.
The VSEPR (Valence Shell Electron Pair Repulsion) theory is based on a set of postulates that provide the foundation for predicting the three-dimensional geometry of molecules. The main postulates of VSEPR theory are as follows:
Steps to apply the VSEPR (Valence Shell Electron Pair Repulsion) procedure:
1. Start by sketching the Lewis electron structure for the molecule or ion.
2. Choose the central atom, opting for the least electronegative one to facilitate effective electron sharing.
3. Count the total bond pairs associated with the central atom, considering the atoms connected by single bonds. Also, tally the electrons in the outermost shell of the central atom.
4. Identify other atoms and their bonds, calculating the total electrons associated with them, including those involved in bonds with the central atom.
5. Determine the optimal arrangement of electron groups around the central atom to minimize repulsions.
6. Recognize interactions between lone pairs (LP–LP, LP–BP, or BP–BP).
7. Calculate the number of lone pairs by subtracting shared pairs from the total electrons.
8. Adjust the valence shell electron pair number (VSEP) for ions by adding or subtracting electrons based on their charges.
9. Anticipate deviations from ideal bond angles.
10. Conclude by describing the final molecular geometry.
The VSEP number describes the shape of the molecule, as described in the table provided below:
VSEP Number and Shapes
Note: The VSEPR theory cannot be used to obtain the exact bond angles between the atoms in a molecule.
1. Linear Shape of Molecule
2. Trigonal Planar Shape of Molecule
3. Tetrahedral Shape of Molecule
4. Trigonal Bipyramid Shape of Molecule
5. Octahedral Shape of Molecule
In Octahedral shape, the central atom attaches 6 different atoms i.e. in total there are 6 bond pairs. These 6 bond pairs and the central atom arrange Octahedral to minimize electron repulsion.
Example of an Octahedral Shape is SF6
Octahedral Shape of SF65. Pentagonal Bipyramidal Shape of Molecule
In a Pentagonal Bipyramidal Shape, the central atom is attached to seven atoms at the corner to minimize the repulsion between the electron pairs.
Example of a Pentagonal Bipyramidal Shape is IF7
Pentagonal Bipyramidal Shape of IF7
The electron pairs around the central atom repel each other and move so far apart from each other that there are no greater repulsions between them. This results in the molecule having minimum energy and maximum stability.
Some significant limitations of the VSEPR theory include:
Q.1. How to explain the shape of molecules using VSEPR theory?
Ans:
Q.2. Determine the geometry of the following molecules using the VSEPR model.
Solution:
Q.3. The shape of the methane molecule is
A. Tetrahedral
B. Pyramidal
C. Octahedral
D. Square planar
Answer: Methane molecule has 4 electron pairs with zero lone pairs so the shape along with geometry is tetrahedral.
Q 4: Which of the following has trigonal planar geometry?
As we know that the electronic geometry of a molecule depends on the number of bond pairs and lone pairs, while the shape depends only on the number of bond pairs. So, for type AB3 molecule, the shape of the molecule is the same as its geometry, if there is no lone pairs. The repulsions are minimal when electron pairs (bond pairs) are 120° apart from each other (the three bond pairs are farthest from each other when the bond angle B‒A‒B is equal to 120°). The geometry and shape of the molecule are trigonal planar. BF3 molecule has trigonal planar geometry. The central atom B has 3 bond pairs and zero lone pairs of electrons. It undergoes sp2 hybridization. IF3 has trigonal pyramidal geometry, and PCl3 and NH3 have tetrahedral geometry.
Q.5. Trigonal bipyramidal is not considered a symmetrical geometry. Why?
Answer: Trigonal bipyramidal is not a perfect symmetrical geometry. In this geometry, all the positions are not equivalent. There are two types of positions axial and equatorial. The three bonds in the plane form a trigonal planar-like geometry. These are known as equatorial bonds. There are bonds above and below the plane. These bonds are known as axial bonds. The axial bonds are longer and weaker, while the equatorial bonds are shorter and stronger.
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1. What is VSEPR Theory and why is it important in chemistry? |
2. What are the main postulates of VSEPR Theory? |
3. How can you predict the shape of a molecule using VSEPR Theory? |
4. What are some limitations of VSEPR Theory? |
5. Can you provide an example of a practice problem involving VSEPR Theory? |
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