what is vsepr theory ?
describe it's rule and limitations.( explain 300 words)
Answers
VSEPR is a theory that chemists apply in order to determine the shape of a molecule. This principle says that in order to minimize repulsion interactions by electrons, molecules will adopt a shape in such a way that the bonds and lone pairs of electrons are as far away from each other as possible.
Chemical bonds are made up of electrons which are negatively charged. Since like-charges repel one another, the bonds and electrons want to have as much space in between them as possible so they aren't repelling one another. It's kind of like someone getting really close to you and 'getting in your bubble,' your natural reaction is to back up right?
Let's use methane as an example. Methane contains four hydrogen atoms bonded to a central carbon atom. In order to maximize the distance between the four bonds in methane, it adopts a geometrical shape called a tetrahedron.
Methane adopts a tetrahedral shape according to VSEPR theory
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VSEPR theory can be used to predict the shape of virtually any molecule provided we know how many bonds and lone pairs of electrons the central atoms contains. The term lone pairs is used to describe a pair or pairs of electrons that reside on the central atom but aren't involved in chemical bonding.
Limitations of VSEPR
Although VSEPR is a great tool for determining molecular shape, there are two big limitations to this theory.
Bond Angle Deviations
One of the main limitations of VSEPR theory is that it cannot predict the degree to which bond angles may deviate from idealized cases. For instance let's look at methane and water side-by-side. VSEPR theory predicts that both molecules should have a tetrahedral shape since they both contain four groups of electrons around their central atoms (carbon for methane and oxygen for water).
Water actually adopts a bent shape instead of a tetrahedral shape. The fact that the bond angle for water is so different from methane is not something that VSEPR theory is able to predict or explain, especially from a numerical standpoint. OR
Steps to Using VSEPR
Draw a Lewis structure for the ion or molecule in question.
Determine the number of electron groups around the central atom. Each lone pair of electrons counts as a single group. Each bond counts as a single group, even if it is a double or triple bond. Find the corresponding electron geometry from the table.
Determine the number of lone pairs and the number of bonding pairs around the central atom, and use that to find the molecular geometry.
VSEPR Notation
VSEPR notation gives a general formula for classifying chemical species based on the number of electron pairs around a central atom. Note, however, that not all species have the same molecular geometry. For example, carbon dioxide and sulfur dioxide are both species, but one is linear and the other is bent.
Sometimes, the notation is expanded to include lone pair electrons. This can get confusing because water can be referred to as a species depending on the conventions the author or text chooses. In general,
A is used to represent the central atom.
B or X is used to represent the number of atoms bonded to the central atom.
E represents the number of lone pairs on the central atom (ignore lone pairs on bonded atoms).
Again, this theory is also not void of any limitations. We will now discuss the common limitations of the VSEPR theory.
Limitations of the VSEPR theory
The VSEPR model is not a theory. It does not explain or attempt to explain any observations or predictions. Rather, it is an algorithm that accurately predicts the structures of a large number of compounds.
VSEPR is simple and useful but does not work for all chemical species.
First, the idealized bond angles do not always match the measured values. For example, VSEPR predicts that and will have the same bond angles, but structural studies have shown the bonds in the two molecules are different by 12 degrees.
VSEPR also predicts that group-2 halides such as will be linear when they are actually bent. Quantum mechanics and atomic orbitals can give more sophisticated predictions when VSEPR is inadequate.
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Importance of VSEPR Models
Lewis structures only tell the number and types of bonds between atoms, as they are limited to two dimensions. The VSEPR model predicts the 3-D shape of molecules and ions but is ineffective in providing any specific information regarding the bond length or the bond itself.
VSEPR models are based on the concept that electrons around a central atom will configure themselves to minimize repulsion, and that dictates the geometry of the molecule.
It can predict the shape of nearly all compounds that have a central atom, as long as the central atom is not a metal. Each shape has a name and an idealized bond angle associated with it.
The following terms are commonly used in discussing the shapes of molecules.
Bond Angle: This is the angle between a bonded atom, the central atom, and another bonded atom.
Lone Pair: This refers to a pair of valence electrons that are not shared with another atom.
Molecular Geometry: This is the 3-D arrangement of bonded atoms in a polyatomic ion or molecule.
Electron Pair Geometry: This is the 3-D arrangement of electron pairs around the central atom of a polyatomic ion or molecule.
The main difference between molecular geometry and electron pair geometry is that molecular geometry does not include unpaired electrons, whereas electron pair geometry includes both bonded atoms and unpaired electrons. If there are no unpaired electrons in the compound being assessed, the molecular and electron pair geometries will be the same.
Molecular Geometry

Steps to Using VSEPR
Draw a Lewis structure for the ionor molecule in question.
Determine the number of electron groups around the central atom. Each lone pair of electrons counts as a single group. Each bond counts as a single group, even if it is a double or triple bond. Find the corresponding electron geometry from the table.
Determine the number of lone pairs and the number of bonding pairs around the central atom, and use that to find the molecular geometry.
VSEPR Notation
VSEPR notation gives a general formula for classifying chemical species based on the number of electron pairs around a central atom. Note, however, that not all species have the same molecular geometry. For example, carbon dioxide and sulfur dioxide are both species, but one is linear and the other is bent.
Sometimes, the notation is expanded to include lone pair electrons. This can get confusing because water can be referred to as a species depending on the conventions the author or text chooses. In general,
A is used to represent the central atom.
B or X is used to represent the number of atoms bonded to the central atom.
E represents the number of lone pairs on the central atom (ignore lone pairs on bonded atoms).
Again, this theory is also not void of any limitations. We will now discuss the common limitations of the VSEPR theory.
Limitations of the VSEPR theory
The VSEPR model is not a theory. It does not explain or attempt to explain any observations or predictions. Rather, it is an algorithm that accurately predicts the structures of a large number of compounds.
VSEPR is simple and useful but does not work for all chemical species.
First, the idealized bond angles do not always match the measured values. For example, VSEPR predicts that and will have the same bond angles, but structural studies have shown the bonds in the two molecules are different by 12 degrees.
VSEPR also predicts that group-2 halides such as will be linear when they are actually bent. Quantum mechanics and atomic orbitals can give more sophisticated predictions when VSEPR is inadequate.