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!
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.
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
The VSEPR theory has a number of severe limitations, including
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.
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.
Related topics
MOT | Bond parameters |
Covalent bonding | VBT |
Coordinate bonding | Metallic bonding |