We have already covered the Lewis structure and the Valence Bond Theory. Did you get all the answers to your queries? No. We do not know anything about the geometrical arrangement of the various molecules. the VSEPR theory comes to our rescue! In this chapter, we will know more about the arrangement of molecules by the VSEPR theory.
What is the VSEPR Theory?
The Valence Shell Electron Pair Repulsion Model is often abbreviated as VSEPR (pronounced “vesper”). It is basically a model to predict the geometry of molecules. Specifically, VSEPR models look at the bonding and molecular geometry of organic molecules and polyatomic ions. It is useful for nearly all compounds that have a central atom that is not a metal.
Browse more Topics under Chemical Bonding And Molecular Structure
- ond Parameters
- Covalent Compounds
- Fundamentals of Chemical Bonding
- Hydrogen Bonding
- Ionic or Electrovalent Compounds
- Molecular Orbital Theory
- Polarity of Bonds
- Resonance Structures
- Valence Bond Theory
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.
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- 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 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.
- 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.
Solved Example for You
Q: On the basis of VSEPR theory explain the structure of NH3 molecule.
Ans: In ammonia, N is the central atom. Nitrogen is a group 15 element and therefore has 5 electrons in its outmost shell. Three electrons of N are bonded with hydrogen and the rest two which do not take part in bonding form the lone pair. The outer shell then has a share in eight electrons, that is, three pairs bonded and one lone pair.
These four pairs of electrons give rise to a tetrahedral structure where three positions are occupied by H atoms and fourth position by the lone pair. This shape may either be described as tetrahedral or pyramidal. The presence of lone pair causes slight distortion from 109°28’ to 107°48’.