Limitation of VSEPOR theory
Answers
Answer:
limitations of VSEPER theory is it doesn't explain the shapes of electron deficient compounds
According to the VSEPR theory, the repulsion between two electrons is caused by the Pauli exclusion principle that has greater importance than electrostatic repulsion in the determination of molecular geometry.
Postulates of VSEPR Theory:
The postulates of the VSEPR theory are listed below
In polyatomic molecules (i.e. molecules made up of three or more atoms), one of the constituent atoms is identified as the central atom to which all other atoms belonging to the molecule are linked.
The total number of valence shell electron pairs decides the shape of the molecule.
The electron pairs have a tendency to orient themselves in a way that minimizes the electron-electron repulsion between them and maximizes the distance between them.
The valence shell can be thought of as a sphere wherein the electron pairs are localized on the surface in such a way that the distance between them is maximized.
Should the central atom of the molecule be surrounded by bond pairs of electrons, then, the asymmetrically shaped molecule can be expected.
Should the central atom be surrounded by both lone pairs and bond pairs of electrons, the molecule would tend to have a distorted shape.
The VSEPR theory can be applied to each resonance structure of a molecule.
The strength of the repulsion is strongest in two lone pairs and weakest in two bond pairs.
If electron pairs around the central atom are closer to each other, they will repel each other. This results in an increase in the energy of the molecules.
If the electron pairs lie far from each other, the repulsions between them will be less and eventually, the energy of the molecule will be low.
Limitations of VSEPR Theory:
Some significant limitations of the VSEPR theory include:
This theory fails to explain isoelectronic species (i.e. elements having the same number of electrons). The species may vary in shapes despite having the same number of electrons.
The VSEPR theory does not shed any light on the compounds of transition metals. The structure of several such compounds cannot be correctly described by this theory. This is because the VSEPR theory does not take into account the associated sizes of the substituent groups and the lone pairs that are inactive.
Another limitation of VSEPR theory is that it predicts that halides of group 2 elements will have a linear structure, whereas their actual structure is a bent one.
Predicting the Shapes of Molecules:
The following steps must be followed in order to decide the shape of a molecule.
The least electronegative atom must be selected as the central atom (since this atom has the highest ability to share its electrons with the other atoms belonging to the molecule).
The total number of electrons belonging to the outermost shell of the central atom must be counted.
The total number of electrons belonging to other atoms and used in bonds with the central atom must be counted.
These two values must be added in order to obtain the valence shell electron pair number or the VSEP number.
What is VSEP Number?
The VSEP number describes the shape of the molecule, as described in the table provided below.
VSEP Number Shape of the Molecule
2 Linear
3 Trigonal Planar
4 Tetrahedral
5 Trigonal Bipyramidal
6 Octahedral
7 Pentagonal Bipyramidal
Each of these corresponding shapes can also be found in the illustration provided earlier. However, the VSEPR theory cannot be used to obtain the exact bond angles between the atoms in a molecule.
Now, we will discuss each shape in detail:
Linear Shape of Molecule:
In this type of molecule, we find two places in the valence shell of the central atom.
They should be arranged in such a manner such that repulsion can be minimized (pointing in the opposite direction).
Example: BeF2
Trigonal Planar Shape of Molecule:
In this type of molecule, we find three molecules attached to a central atom.
They are arranged in such a manner such that repulsion between the electrons can be minimized (toward the corners of an equilateral tr