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Candidates who are preparing for their CSIR NET exam might need to get some short study notes and strategies to apply while preparing for the key exam of their life. At this point in time, We at BYJU'S Exam Prep come up with short notes on the VSEPR Theory which comes under the Inorganic Chemistry section of the Chemical Science syllabus.
This set of short notes on VSEPR Theory has been meticulously designed by our experienced subject-matter experts to give you the most standard set of study materials to be focused upon. In this cut-throat competitive world, students need to prepare themselves with the best study materials to help them in the learning process and for their future. Here we are offering the best study notes that are reliable and can be used by the students during their preparations for the upcoming CSIR-NET 2021 exam.
Short Notes on VSEPR Theory
The main postulates of VSEPR theory are:
- The geometry of a molecule can be predicted with the help of the total number of valence shell electron pairs present.
- If there are only bond pairs present, the molecule will be symmetrical in shape.
- If lone pairs along with bind pairs are present around the central atom, molecules will be distorted in shape.
- The relative order of repulsion is:
lp – lp>lp – bp > bp – bp.
Molecules in this theory are divided into two categories, depending on whether the central atom has a lone pair of electrons or not. In this article, we will discuss the molecules in which the central atom has no lone pairs.
a. AB2 Type Molecule
An example of this type of molecule is BeCl2. Its Lewis structure in the gaseous state is:
As bonding pairs repel each other, they must be present at opposite ends of a straight line, so that they remain as far apart as possible. Thus, the Cl-Be-Cl bond angle is predicted to be 180° and its geometry is linear.
b. AB3 Type Molecule
An example of this type of molecule is BF3. It contains three bonding pairs that are present at the corners of an equilateral triangle having a boron at the centre of the triangle.
This geometry is known as trigonal planar in which the FBF bond angle is around 120°.
c. AB4 Type Molecule
An example of this molecule is Methane (CH4). Its Lewis structure is:
The four bonding pairs in CH4 are arranged in the form of a tetrahedron. A tetrahedron has four faces, all of which are equilateral triangles. In a tetrahedral molecule, the central atom (carbon) is located at the centre and all other four atoms (H) are present at the corners. The HCH bond angles are all 109°48’.
d. AB5 Type Molecule
The general formula AB5 is represented by the molecule PCl5. Its Lewis structure in the gas phase is:
If all the five bonding pairs are arranged in the form of a trigonal bipyramid, repulsion will be minimum. Atoms above and below the triangular plane occupy axial and equatorial positions, respectively. Between two equatorial bonds, the bond angle is around 120°, between two axial, the bond angle is 180° and between an axial bond and an equatorial bond is 90°.
e. AB6 Type Molecule
An example of this type of molecule is SF6 and its Lewis structure is:
For this molecule, the most stable arrangement is the octahedron in which S will be present at the centre of the square base and F atoms will be present at six corners. All bond angles will be 90° except the one which is between the central atom and the pairs of atoms that are diametrically opposite each other, which is 180°. As all the bonds are equivalent in an octahedral molecule, the terms axial and equatorial cannot be used here.
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