VSEPR theory-Postulates, Prediction of geometrical shapes, Limitations, Practice problems, FAQs

In daily life every one wants to get into their comfort zone. Isn’t it so! Everyone wants a stable life in which problems are less. Same thing happens in chemistry also.

Every system strives to become more stable over time, and bonding is nature's approach to lowering the system's energy in order to achieve stability. VSEPR theory has closely been related to the developments in the understanding of the structure of atoms. Let’s begin!

Table of contents

• What is VSEPR theory?
• Main postulates of VSEPR theory
• Prediction of geometrical shapes using VSEPR theory
• Limitation of VSEPR theory
• Practice problems
• Frequently asked questions-FAQs

What is VSEPR theory?

In 1940, Sidgwick and Powell suggested a basic theory based on the repellent interactions of electron pairs in atom’s valence shells. This theory lays out a straightforward method for predicting the shapes of covalent compounds. The repulsive interactions of electron pairs in the valence shell of the atoms are the basis for this theory. VSEPR is abbreviated as valence shell electron pair repulsion.

Main postulates of VSEPR theory

• The shape of a molecule depends upon the number of valence shell electron pairs (bonded or non-bonded) around the central atom.
• Pairs of electrons in the valence shell repel one another since their electron clouds are negatively charged.
• These pairs of electrons tend to occupy such positions in space that minimise repulsion and thus maximise the distance between them.
• The valence shell is taken as a sphere with the electron pairs localising on the spherical surface at maximum distance from one another. 
• A multiple bond is treated as if it is a single electron pair and the two or three electron pairs of a multiple bond are treated as a single super pair. 
• Where two or more resonance structures can represent a molecule, the VSEPR model is applicable to any such structure.
  
  Electron pair’s repulsive interactions decrease in the following order: Lone pair (lp) – Lone pair (lp) > Lone pair (lp) – Bond pair (bp) > Bond pair (bp) – Bond pair (bp)

Prediction of geometrical shapes using VSEPR theory

It is convenient to divide molecules into two groups when using VSEPR theory to predict the geometrical shapes of molecules:
 I. Molecules with no lone pairs in the central atom
 II. Molecules with one or more lone pairs in the central atom

The shape and geometry of a molecule with two electron pairs

The shape and geometry of a molecule with three electron pairs

The shape and geometry of the molecules with four electron pairs

The shape and geometry of the molecules with five electron pairs

The shape and geometry of the molecules with six electron pairs

Limitation of VSEPR theory

The VSEPR theory has a number of severe limitations, including 

• Isoelectronic species that are not explained by this idea (i.e. elements having the same number of electrons). Despite having the same number of electrons, the shapes of the species can differ.
• The VSEPR theory provides little insight into transition metal compounds. This theory cannot accurately describe the structure of several such molecules. This is due to the fact that the VSEPR theory hypothesis ignores the substituent groups' associated sizes as well as the inactive lone pairs.

Practice problems

Q1: The shape of the methane molecule is 

A. Tetrahedral
B. Pyramidal
C. Octahedral
D. Square planer

Answer: Methane molecule has 4 electron pairs with zero lone pairs so the shape along with geometry is tetrahedral.

Q 2: Which of the following has trigonal planer geometry?

A.
B.
C.
D.

Answer: 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 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. molecule has trigonal planer geometry. The central atom B has 3 bond pairs and zero lone pairs of electrons. It undergoes hybridization. has trigonal pyramidal geometry, and ha tetrahedral geometry.

Q 3: Using VSEPR theory, determine the shape of the molecule .
Answer: In order to determine the shape of molecule, follow the following steps:

(1) Lewis structure can be drawn for as follows:

(2) In this Lewis structure, the number of sigma bonds = 3 and the number of lone pairs on the central atom = 1
(3) Using VSEPR theory, the shape of will be of type , i.e., trigonal pyramidal.

Q 4: Which of the following is non-linear in shape?

A.
B.
C.
D.

Answer: (D)
has two sigma bond pairs and zero lone pairs. Thus, it has a linear shape.

has two sigma bond pairs and three lone pairs. Thus, it also has a linear shape.

has two sigma bond pairs and three lone pairs. Thus, it also has a linear shape.

has two sigma bond pairs and two lone pairs. Thus, it has a bent shape.

So, among all, has a non-linear structure.

Frequently asked questions-FAQs 

1. What is the difference between geometry and shape?
Answer: For predicting the electronic geometry of a molecule, both bond pairs and lone pairs are considered. However, for predicting the shape of a molecule, only bond pairs are taken into account.

2. What is the difference between tetrahedral and trigonal pyramidal?
Answer: The difference between tetrahedral and trigonal pyramidal structure is that all sides are equal in the tetrahedral and no lone pair is present in this structure, while they are not equal in the trigonal pyramidal. Due to the lone pair-bond pair repulsion, the bond angle decreases and it is less than 109.5°.

3. When will the geometry and shape of a molecule be the same?
Answer: When no lone pair is present on the central atom, the geometry and shape of the molecule will be the same.

4. Trigonal bipyramidal is not considered as 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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