VSEPR Theory - Shapes of Molecules

The VSEPR Theory (Valence Shell Electron Pair Repulsion Theory) is based on the fact that there is a repulsion between the pairs of valence electrons in all the atoms, and the atoms will always try to arrange themselves in a manner in which the electron pair repulsion is minimized. This arrangement of the atom shapes the geometry of the resulting molecule. This theory is also known as the Gillespie-Nyholm theory to honor Ronald Nyholm and Ronald Gillespie who laid its foundation.

VSEPR theory suggests that the repulsion between two electrons is caused by the Pauli exclusion principle that has greater importance than electrostatic repulsion in the determination of a molecule's geometry.

VSEPR Theory as a Tool

The VSEPR theory is a tool that is used for predicting the shape of a molecule from the electron pairs that surround the central atoms of that molecule. The theory was given by Sidgwick and Powell in the year 1940. It is based on the assumption that the molecule will take such a shape that will minimize electronic repulsion in the valence shell of that atom.

Atoms come together to form molecules and molecules come together and bind by chemical bonds to form elements or compounds. Sometimes, atoms of particular elements easily bond with different atoms to form molecules. Chemical bonds can result from three types of interactions between atoms: 

• Electron exchange.

• Sharing

• Co-ordinate

The strength of a chemical bond varies from strong or primary bonds and weak or secondary bonds. Strong chemical bonding is due to the sharing or transfer of electrons between the atoms involved. Quantum theory explains all kinds of bonding, but the simpler explanation is given by the octet rule and VSEPR theory. 

Electron sharing involves the “sharing” of one or more electrons between the atoms involved in the bond formation, whereas electron exchange is the exchange of electrons between atoms and not the sharing. Co-ordinate bond is a type of covalent bond where the electrons being shared are contributed from one atom only.

Molecules, as mentioned earlier, are made up of atoms. They have different sizes and structures. They vary in complexity as well. For example, an oxygen molecule is made of two atoms of oxygen, but sometimes under different environmental conditions, oxygen forms a molecule consisting of three atoms of oxygen, called Ozone (O3). 

To find the types and number of atomic bonds in a substance and also to show which atoms have lone pairs of electrons, the Lewis electron-pair theory can be used. However, this theory provides no information on how the atoms are arranged. The VSEPR model helps to understand the different shapes and arrangement of molecules. But this model does not say anything regarding the multiple bonds present or the bond length. It is just a representative model.

The VSEPR model is a straightforward yet useful way to understand and explain the shapes and structure of molecules. To reduce the electrostatic repulsion between electron pair is what the theory is based on. Before starting to use the VSEPR model, the Lewis dot picture is considered to determine the electron domain. Electron domain is nothing but the number of bonds and lone pairs in an atom. Knowing this, the electron geometry can be obtained. Follow the below steps, 

• Find out the sum of the total number of lone pairs and the number of binding domains.

• The sum called the steric number determines the electronic shape of the molecule. For example, a steric number of two gives a linear electronic structure.

• The electronic geometry also determines the angles between the electron domains.

• The arrangement of angles from highest to lowest is determined by the hierarchy of repulsion where lone pair-lone pair is the highest then comes lone pair-bonding which is slightly lower, and bonding-bonding is the least.

The 3-D shapes and structures of several molecules cannot be determined by Lewis electron-pair theory, and therefore the VSEPR model is used. This model helps in finding the structures of both molecules, having a central metallic atom and a non-metallic central atom. The main proposition of the VSEPR model is that the binding domains and the lone pairs will repel each other and this causes them to take up a geometry that separates each other as far apart as possible. 

According to this theory, the valence electrons in an atom can form a single bond, a double bond, a triple bond, a lone pair of electrons, or even a single unpaired electron and is counted as a lone pair. The arrangement of electron groups where the electron repulsion is minimized is the most stable arrangement. A molecule or polyatomic ion is represented as AXmEn. 

Here, A is the central atom, X is the bonded atom, and E is the nonbonding valence electron with m and n being integers. Groups around the central atom are present as a bonding pair (BP) or lone pair (LP). The interactions of these two will help predict the positions of the atoms and the bond angles.

How to Predict Molecular Geometry Using VSEPR?

Predicting the Shapes of Molecules:

In order to determine the shape of a molecule the following steps must be followed:

• The least electronegative atom must be considered the central atom since it has the highest ability to share its electrons with other atoms belonging to the molecule.

• Then Count the total number of electrons belonging to the outermost shell of the central atom.

• Count the total number of electrons belonging to the other atoms and use them in bonds with the central atom.

• Add these two values in order to obtain the valence shell electron pair number or the VSEP number.

(This procedure is better understood with the help of examples mentioned below showing different molecular geometry)

Linear Shape of Molecule

1. CO2

A molecule that has two bonded atoms and no lone pair of electrons (around the central atom): This is an example of AX2.

In this molecule, Carbon is contributing 4 electrons for bond formation, and each oxygen atom is donating a pair of electrons. Both the electron groups around the central atom, i.e. Carbon are BP. The model focuses only on the central atom and hence the lone pair of electrons on the oxygen atoms does not play a role in determining the molecular geometry. 

2. BeF₂

• This type of molecule has two places in the valence shell of the central atom.

• They should be arranged pointing in the opposite direction in order to minimize the repulsion

In this case, to reduce repulsion the molecular geometry is linear.

3. BF3

A molecule that has three bonded atoms and no lone pair of electrons: This is an example of AX3.

Boron donates three electrons, and fluorine has seven valence electrons. Here, the molecular geometry is trigonal planar.

4. NH3

A molecule that has three bonded atoms and one lone pair of the electron (around the central atom): This is an example of AX3E.

In this molecule, NH₃ has four electron groups, three BP and one LP.

Nitrogen donates six electrons for bond formation, and each hydrogen atom gives one electron. To reduce repulsion in this case, trigonal pyramidal is the molecular geometry here.

5. OF2

A molecule that has two bonded atoms and two pairs of lone electrons: This is an example of AX 2E2.

In this molecule, oxygen has four electron groups, two BP and two LP.

Oxygen donates two electrons for bond formation, and two pairs are lone pairs. In this case, to reduce the repulsion, the molecular geometry is bent or V-shaped.

Tetrahedral Shape of Molecule:

      1.CH4

• Atoms lie in the same plane in a two-dimensional molecule and if we place the same conditions on molecules like methane, we will get a square planar geometry in which the bond angle between H-C-H is 900.

• Now, if the same conditions are applied for a three-dimensional molecule, we will get a tetrahedral molecule in which the bond angle between H-C-H is 109028’ (toward the corners of an equilateral triangle) CH4.

2. NH₄⁺ 

A molecule with four bonded atoms and zero pairs of lone electrons: This is an example of AX4.

In this molecule, ammonia has four valence electrons, and all four are involved in bond formation whereas hydrogen contributes one electron in bond formation. The molecular geometry observed, in this case, is tetrahedral.

Trigonal Bipyramid Shape of Molecule:

1. PCl5

A molecule with five bonded atoms and zero pairs of lone electrons: This is an example of AX5.

In this molecule, Phosphorus has five electron groups, and all five are BP.

Phosphorus has five electrons which it donates for bond formation, and each chlorine gives a single atom for bond formation. The total valence electrons in phosphorus are five, and in chlorine, there are seven.

The molecular geometry here is trigonal bipyramidal.

2. PF5

Now, consider the example of PF5. In this case repulsion can be minimized by even distribution of electrons towards the corner of a trigonal pyramid. In a trigonal bipyramid, three positions lie along the equator of the molecule. The second positions lie along an axis perpendicular to the equatorial plane.

3. SF6

A molecule with six bonded atoms and zero pairs of lone electrons: This is an example of AX6.

In this molecule, the sulfur atom contributes six electrons for bonding, and each fluorine atom contributes one electron for bonding. This molecule has six electron groups, and all six are BP. To reduce electronic repulsion, the molecular geometry is octahedral.

4. XeF4

A molecule with four bonded atoms and two pairs of lone electrons: This is an example of AX 4E2.

In this molecule, Xenon has eight valence electrons, and all four are involved in bond formation whereas the two remaining pairs are lone pairs. In this case, it has seven valence electrons. The molecular geometry observed, in this case, is square planar.

The VSEPR model can be used to find out the structures of molecules having no central atoms, which is much more complex in comparison to the structures mentioned above.

Conclusion

It can be concluded that the Lewis electron-pair theory cannot be used to find out the structure of molecules and the number of lone pairs in a molecule whereas the VSEPR model is beneficial in determining the structure of molecules. It also says that the structure that proves to minimize repulsion is the one that has the least energy. , and they can be of two types, bonding pair or lone pair. Based on the interaction between BP and LP one can tell the position of atoms and the bond angle in a molecule and hence determine the molecular geometry.

The dipole moment is the asymmetrical distribution of charge which results in the aligning of molecules in an applied magnetic field and this is possible for molecules with polar covalent bonds.