VSEPR THEORY( shapes of molecules)

VSEPR THEORY( shapes of molecules)

VSEPR THEORY

VSEPR (valency shell electron pair repulsion) THEORY

In 1940 this theory was put forward by Sidgwick and Powell and in 1957 it was further improved by Nyholm and Gillespie. At first, we should get the basic concept of this theory, which are as follows:

The central atom is linked by covalent bonds to other atoms that are formed by the sharing of electrons. The central atom is surrounding by a sharing electron and may or may not be surrounded by non-bonding electron or we can say lone pairs, (we know, lone pairs are, which do not take part in bond formation).

To become a stable molecule, we know that molecules should have minimum energy. If the electron pairs surrounding the central atom are nearer, repulsion will occur among them, thereby energy will increase and destability as well.

But if the electron pairs are far apart, there will be a decrease of repulsive force due to which the energy of the molecule will be low. Thus the main idea of the VSEPR theory is as under:-

The electron pairs surrounding the central atom repel one another that there is no further repulsion between them. As a result, the molecules have minimum energy and maximum stability.

Electron pairs take up their definite positions around the central atom after repulsions between them, due to which the molecules have a definite shape or geometry.

The above concept leads to the following results (assumptions), which help to find the shape of molecules:

1-The shape of a molecule containing only two atoms is usually linear.

2-For molecules containing 3 or more atoms, one of the atoms is called the central atom to which other atoms are linked.

3– If the central atom is surrounded by similar atoms, (means it is surrounded by only bond pairs no lone pair electrons), the repulsions between them are similar. Because of this, the shape of the molecule is symmetrical and the molecule is known to have a regular geometry.

4– If the central atom is linked to different atoms or is surrounded by bond pairs, as well as lone pairs of electrons, the repulsions between them, are different. Because of this, the geometry of the molecule will be irregular or distorted. The order of repulsion between the electron pair is as follows:

Lone pair-Lone pair > Lone pair-Bond pair > Bond pair-Bond pair.

5– The exact shape of molecules depends upon the total number of electron pairs around the central atom.

Limitation of  VSEPR theory:

Though VSEPR theory is able to explain the shapes of simple molecules but in many cases, it is unable to predict the correct geometry of molecules.

That is why it had only a limited application. Generally, if we talk about the direction of electron pairs, this theory fails, and does not seems to be very rational. Thus Lewis approaches as well as VSEPR theory had a number of limitations.

Now we will discuss to the shape of the different molecule having bond pair only or lone pair and bond pair (depending on the VSEPR theory)-

pairs of electrons-

1-AB4 type molecules with one lone pair (AB3E)-

A simple example, in this case, is that of ammonia molecules, NH3. This molecule has three bonding and one nonbonding electron. It means nitrogen has four electron pairs (three N-H bonds and one lone pair of nitrogen).

The molecule is of AB4 type and due to the 4 electron pairs, we can assume a tetrahedral orientation. Is it correct?   No, the actual shape of ammonia molecules is trigonal pyramidal due to lp -bp repulsion. The bond angles of H-N-H are less than the normal tetrahedral angle 109.5 degrees.

This decrease in angle is due to the presence of lone pair of electron on the central nitrogen atom which repels the N-H bonds more strongly than they repel each other.

2-AB4type molecules and ions with two lone pairs(AB2E2)-

The simplest case is that of water molecules, H2O. The lewis dot structure of water molecules is the oxygen atom is the central atom around which four electron pairs are present.

Because the molecule is of AB4 type, and four electron pairs are there, so we could assume tetrahedral geometry for a water molecule. Since H2O contains only two bonding pairs, the shape of the molecule ignoring the lone pair, is termed as angular or V-shaped. In this molecule, the repulsive effect of the lone pairs on the bonding pairs is more pronounced.

The mole concept(explanation with examples)

Bond angle H-O-H is less than the tetrahedral angle and is even less than that for the H-N-H angle in the NH3 molecule. The lone pair in the H2O molecule not only repel the two bonding pairs but also repel each other.

Between the lone pairs the mutual repulsion increases, and so the electron density comes closer to the O-H bonds which results in the smaller H-O-H angle which is 104.5°.

3- AB5 type molecules or ions with one l0ne pair(AB4E):-

As we can see that five electron pairs are there so we can easily say that the geometry would be trigonal bipyramidal but would it be? we can understand it with the help of the structure of  SF4.

We can see that a single lone pair can occupy either an equatorial position or an axial position. From the above picture, we can get that, to minimize the repulsion and acquire higher stability, structure” a ” with the lone pair at equatorial position is the preferred structure. which is known as, a see-saw and distorted tetrahedral geometry.

4-AB5 type molecules or ions with two lone pairs(AB3E2):-VSEPR THEORY( shapes of molecules)

Here again, with the formula given above, we can say that the geometry would be trigonal bipyramidal.

but due to the presence of two lone pair, something is going to change, what is it???? let’s see this with the example of chlorine trifluoride, ClF3.

since there are two lone pair and three bond pairs are present in ClF3, there are three probable structures of this molecules:

For the VSEPR analysis, the interactions at 90 degrees are considered. It can be concluded that shape (a) with two lone pairs of equatorial positions is most stable. This shape is known as T-shape.

5-AB6 type molecules or ions with one lone pair(AB5E):-

In this case, we can see the total electron pairs are six and we can assume the shape of this molecule would be octahedral. But it doesn’t happen. why???? let’s understand it with the example of chlorine pentafluoride.

In this molecule 5 bond pairs and one lone pair are present. The four bond pairs placed in four corners of a square, whereas one bond pair and one lone pair placed perpendicular to the square. So this molecule will be square pyramidal in shape.

6-AB6 type molecules or ions with two lone pairs (AB4E2)-

We can see the six electron pairs related to the octahedral geometry but because of the presence of two lone pairs and four bond pairs, the extent of repulsion is different and the geometry will not be octahedral.

We can understand this with the example of Xenon tetrafluoride XeF4. There are two possible structures of Xenon tetrafluoride. In one structure four bond pairs are in one plane (form square) and two lone pairs are perpendicular to the square.

In the second structure 3 bond pairs and one lone pair, are in one plane, and one lone pair and one bond pair are perpendicular to the square. If we compare the maximum stability and minimum energy than structure one has maximum stability in which four bond pairs are on a plane and two lone pairs are perpendicular to the plane.

Structure, one is preferred because there is no lone pair- lone pair interaction, and the shape of the molecule XeF4 will be square planar.

Problem-based on VSEPR theory:-

Question – Find out the shape of XeF2 with the help of the VSEPR theory.

Solution– In XeF2 molecules two bond pairs are formed as F-Xe-F. Xenon has three lone pairs. It means the orientation of the five electron pairs would be trigonal bipyramidal. Is it?? let’s understand.

there are three possible structures of XeF2with the electron pair interaction.

structure (b) and (c) are more unstable than structure(a) because these have lp-lp repulsion. So structure (a) is preferred. The shape of the molecule is linear.

thanks for reading VSEPR THEORY( shapes of molecules)

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THE HYDROGEN BOND (hydrogen bonding)

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