- The document discusses the Valence Shell Electron Pair Repulsion (VSEPR) theory, which predicts molecular geometry based on electron pair repulsion around central atoms. - VSEPR theory postulates that molecules form shapes that minimize repulsion between electron pairs on central atoms. This determines if molecular geometries are linear, trigonal planar, tetrahedral or other shapes. - The document outlines how VSEPR theory can be applied to predict shapes of molecules containing different numbers of electron pairs through examples like methane, ammonia and water. It also addresses how double and triple bonds are handled in VSEPR predictions.

1. Name:
Muhammad Abdullah
Reg. No:
BSCSR07183031
Class & Department:
BS Chemistry V
Assignment no 4:
VSEPR Theory
THE UNIVERSITY OF LAHORE

2. VSEPR Theory
It stands for Valence Shell Electron Pair Repulsion Theory.
History:
 The idea of a correlation between molecular geometry and
number of valence electron pairs (both shared and unshared
pairs) was originally proposed in 1939 by Ryutaro Tsuchida in
Japan.
 And was independently presented in a Bakerian Lecture in 1940 by Nevil Sidgwick
and Herbert Powell of the University of Oxford.
Definition:
The VSEPR Theoryis used to predict the shape of the molecules from the
electronpairs that surround the central atoms of the molecule. The VSEPR
theory is basedon the assumption that the molecule will take a shape such
that electronic repulsion in the valence shell of that atom is minimized.
Introduction:
The Valence Shell Electron Pair Repulsion Theory abbreviated as VSEPR theory is
based on the premise that there is a repulsion between the pairs of valence

3. electrons in all atoms, and the atoms will always tend to arrange themselves in a
manner in which this electron pair repulsion is minimalized. This arrangement of
the atom determines the geometry of the resulting molecule.
The two primary founders of the VSEPR theory are RonaldNyholm and Ronald
Gillespie. This theory is also known as the Gillespie-Nyholm theory to honour
these chemists.
According to the VSEPR theory, the repulsion between two electrons is caused by
the Pauli exclusion principle that has greater importance than electrostatic
repulsion in the determination of molecular geometry.
Postulates of VSEPR Theory:
The postulates of the VSEPR theory are listed below
 In poly-atomic molecules (i.e. molecules made up of three or more atoms),
one of the constituent atoms is identified as the central atom to which all
other atoms belonging to the molecule are linked.
 The total number of valence shell electron pairs decides the shape of the
molecule.
 The electron pairs have a tendency to orient themselves in a way that
minimizes the electron-electron repulsion between them and maximizes the
distance between them.
 The valence shell can be thought of as a sphere wherein the electron pairs
are localized on the surface in such a way that the distance between them is
maximized.
 Should the central atom of the molecule be surrounded by bond pairs of
electrons, then, the asymmetrically shaped molecule can be expected.
 Should the central atom be surrounded by both lone pairs and bond pairs of
electrons, the molecule would tend to have a distorted shape.
 The VSEPR theory can be applied to each resonance structure of a molecule.

4.  The strength of the repulsion is strongest in two lone pairs and weakest in
two bond pairs.
 If electron pairs around the central atom are closer to each other, they will
repel each other. This results in an increase in the energy of the molecules.
 If the electron pairs lie far from each other, the repulsions between them will
be less and eventually, the energy of the molecule will be low.
Predicting the Shapes of Molecules:
There is no direct relationship between the formula of a compound and the shape
of its molecules. The shapes of these molecules can be predicted from their Lewis
structures, however, with a model developed about 30 years ago, known as
the valence-shellelectron-pairrepulsion (VSEPR)theory.
The VSEPR theory assumes that each atom in a molecule will achieve a geometry
that minimizes the repulsion between electrons in the valence shell of that atom
and thus giving us the following shapes according no electrons in the domains.
Following are some of the shapes given according to the no of electrons in
molecules with their examples and diagrams.
Shape of molecules with 2 electrons:
 Such molecules form a linear shape with an ideal bond angle of 180°
 In this type of molecule, we find two places in the valence shell of the
central atom.
 They should be arranged in such a manner suchthat repulsion can be
minimized (pointing in the oppositedirection).
 Example: BeF2 , C02

5. Shapes of molecules with 3 electrons:
 Such molecules make a trigonal planar shape or bent shape with a bond
angle of 120°
 In this type of molecule, we find three molecules attached to a central atom.
 They are arranged in such a manner such that repulsion between the
electrons can be minimized (toward the corners of an equilateral triangle).
 Example: BF3 , S02
Shapes of molecules with 4 electrons:
 Such molecules give a tetrahedral geometry or trigonal pyramidal. In some
cases molecules show a bent shape.
 If we considerall these conditions for a three-dimensional molecule, we will
get a tetrahedral molecule in which the bond angle between H-C-H is
109.28’ (toward the corners of an equilateral triangle) CH4
 Example: CH4 , NH3, H20
Trigonal Planar
Bent
Tetrahedral
Trigonal pyramidal

6. The Shape of H20 Molecule:
Shapes of molecules with 5 electrons:
 Such Molecules a trigonal bi-pyramidal geometry, with a bond angles of
90°, 120°, 180°
 In rare cases like SF4 molecules show a seesawshape and molecules like
CLF4 show a T-shapedstructure.
 Let’s take an example of PF5. Here, repulsion can be minimized by even
distribution of electrons towards the corner of a trigonal pyramid. In trigonal
bipyramid, three positions lie along the equator of the molecule. The two
positions lie along an axis perpendicular to the equatorial plane.
Trigonal bi-pyramidal
See-Saw Shape

7. Shapes of molecules with 6 electrons:
 These molecules an octahedral, square planer or square pyramidal with
bond angles of 90°, 180°
 Examples: SF6, BRF5
VSEPR Theory and the Shapes of Molecules:
The strength of the repulsion between a lone pair and a bond pair of electrons lies
in between the repulsion between two lone pairs and between two bond pairs. The
order of repulsion between electron pairs is as follows:
Lone Pair- lone pair > Lone Pair- bond-pair > Bond Pair- bond pair.
1. Total number of electron pairs around the central atom = ½ (number of valence
electrons of central atom + number of atoms linked to central atom by single bonds)
 For negative ions, add the number of electrons equal to the units of negative
charge on the ions to the valence electrons of the central atom.
 For positive ions, subtract the number of electrons equal to the units
of positive charge on the ion from the valence electrons of the central atom.
2. The number of Bond pair = Total number of atoms linked to central atom by
single bonds.
3. Number of lone pairs = Total number of electron – No of shared pair
The electron pairs around the central atom repel each another and move so far
apart from each another that there are no greater repulsions between them. This
results in the molecule having minimum energy and maximum stability.
 The shape of a molecule with only two atoms is always linear.

8.  For molecules with three or more atoms, one of the atoms is called the
central atom and other atoms are attached to the central atom.
 If the central atom is linked to similar atoms and is surrounded by bond pairs
of electrons only, the repulsions between them are similar as a result the
shape of the molecule is symmetrical and the molecule is said to have
regular geometry.
 If the central atom is linked to different atoms or is surrounded by bond pair
as well as a lone pair of electrons, the repulsion between them is similar. As
a result, the shape of the molecule has an irregular or distorted geometry.
 The exact shape of the molecule depends upon the total number of electron
pairs present around the central atom.
Role of Non-bonding electrons in VSEPR
Theory:
The valence electrons on the central atom in both NH3 and H2O should be distributed
toward the corners of a tetrahedron, as shown in the figure below. Our goal, however,
isn't predicting the distribution of valence electrons. It is to use this distribution of
electrons to predict the shape of the molecule. Until now, the two have been the same.
Once we include nonbonding electrons, that is no longer true.
The VSEPR theory predicts that the valence electrons on the central atoms in
ammonia and water will point toward the corners of a tetrahedron. Because we can't
locate the nonbonding electrons with any precision, this prediction can't be tested
directly. But the results of the VSEPR theory can be used to predict the positions of
the nuclei in these molecules, which can be tested experimentally. If we focus on the
positions of the nuclei in ammonia, we predict that the NH3 molecule should have a
shape best described as trigonal pyramidal, with the nitrogen at the top of the pyramid.
Water, on the other hand, should have a shape that can be described as bent,

9. or angular. Both of these predictions have been shown to be correct, which reinforces
our faith in the VSEPR theory.
Incorporating Double and Triple bonds in
VSEPR Theory:
Compounds that contain double and triple bonds raise an important point: The
geometry around an atom is determined by the number of places in the valence
shell of an atom where electrons can be found, not the number of pairs of valence
electrons. Consider the Lewis structures of carbon dioxide (CO2) and the carbonate
(CO3
2-) ion, for example.
There are four pairs of bonding electrons on the carbon atom in CO2, but only two
places where these electrons can be found. (There are electrons in the C=O double
bond on the left and electrons in the double bond on the right.) The force of
repulsion between these electrons is minimized when the two C=O double bonds
are placed on oppositesides of the carbonatom. The VSEPR theory therefore
predicts that CO2 will be a linear molecule, just like BeF2, with a bond angle of
180o.
The Lewis structure of the carbonate ion also suggests a total of four pairs of
valence electrons on the central atom. But these electrons are concentrated in three
places: The two C-O single bonds and the C=O double bond. Repulsions between
these electrons are minimized when the three oxygen atoms are arranged toward
the corners of an equilateral triangle. The CO3
2- ion should therefore have a
trigonal-planar geometry, just like BF3, with a 120o bond angle.