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Shape of Molecules Using VSEPR Theory

In this Shape of molecules using VSEPR theory post we have briefly explained about geometry of molecules, shape of molecules, shape of molecules using VSEPR theory structure.

Geometry of Molecules

The term “molecular geometry,” also referred to as the molecular shape, is the three-dimensional shape or arrangement of atoms within molecules. Knowing the molecular structure of a substance can aid in determining the polarity and reactivity, the nature of matter, colour magneticity, and biology.

Shape of Molecules

To identify the shape of molecules, it is necessary to learn about what is known as the Lewis electron dot pattern. While the Lewis theory is not able to define the shape of molecules however it is the first step in predicting the shapes of molecules. 

The Lewis structure can help us determine bonds and isolated pairs. With an understanding of the Lewis structures, we use the valence-shell electron-pair repulsion (VSPER) theory to establish the molecular structure and electro-group geometrics.

To determine and get an accurate description of the three-dimensional structure of molecules, it is necessary to understand the bonds and their angles and. Lewis Electron Dot Structures play vital roles in determining structure of molecules as they help us determine the electrons in the valence.

Shape of Molecules Using VSEPR Theory

It is believed that the Valence Shell Electron Pair Repulsion Theory often abbreviated to VSEPR theory structure is built on idea that there exists an attraction between the pairs of electrons that are valence across all atoms and atoms always prefer to be arranged in a way where this repulsion of electrons between pairs is minimized. The arrangement of an atom is the basis for the shape of the molecule that results.

The various geometries molecules may assume in accordance to the VSEPR theory structure are evident in the figure below.

Geometry of Molecules VSEPR Theory

Different Geometries that Molecules can Assume

The two main creators of VSEPR theory structure are VSEPR theory structure is Ronald Nyholm and Ronald Gillespie. This theory is also referred to as the Gillespie-Nyholm hypothesis in honour of these chemists.

Hypothesises of VSEPR Theory

Polyatomic molecules (i.e. molecules that are composed of more than three atoms) one of the atoms that compose it is known as being the central one which all the other atoms to the molecules are connected.

The number of electron pairs in the valence shell determines the form of the molecular. Electron pairs have a tendency to align themselves in a manner that reduces the repulsion between electrons and electrons between them and also increases how far they are.

The valence shell could be considered as a sphere, where the electron pairs are placed onto the surface that their distance is increased. If the central atom of the molecule be covered by electron bond pairs which is symmetrically formed molecule is likely to be.

If the central atom is enclosed by single pairs and bond electron pairs the molecule is likely to be distorted. The VSEPR theory structure is applicable to every resonance structure of molecules. The force of the resonance is the strongest in two pairs of lone pairs, and weakest in bond pairs.

If the electron pairs around an atom’s central region are close to one another and are in a close proximity, they will be at odds with each the other. This leads to a rise in the energy of the molecules. If the electron pairs are away from one another. The repulsions between them will decrease and, eventually molecules’ energy will decrease.

VSEPR Theory Structure

The atom that is least electronegative should be chosen as the central element (since this is the atom with the greatest capacity to share electrons with the other atoms of the molecules).

The number of electrons in the outermost part in the central atom has to be measured.

The amount of electrons that belong to other atoms that are used to create connections with central molecules should be determined.

These two values need to be added to determine the valence electron pair number or the number for VSEP. 

What is VSEP Number?

The VSEP number describes the shape of the molecule, as described in the table provided below.

Geometry of Molecules VSEPR Theory

The VSEP number describes the shape of the molecule

Each of these corresponding shapes can also be found in the illustration provided earlier. However, the VSEPR theory structure cannot be used to obtain the exact bond angles between the atoms in a molecule.

1. Linear Shape of Molecule

In this kind of molecule, we can find two spots within the valence shell of the central atom. They must be organized in such a way that repulsion is reduced (pointing towards reverse direction). Example: BeF2.

2. Trigonal Planar Shape of Molecule

In this kind of molecule, there are three molecules that are attached to a central atom. They are organized in an arrangement that they can be reduced (toward one of the edges in a triangular equilateral triangular). Example: BF3.

3. Tetrahedral Shape of Molecule

In two-dimensional molecules, atoms lie in the same plane and if we place these conditions on methane, we will get a square planar geometry in which the bond angle between H-C-H is 900. Now, if we consider all these conditions 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.

4. Trigonal Bipyramid Shape of Molecule

Let’s take an example of PF5. Here, repulsion can be minimized by the even distribution of electrons towards the corner of a trigonal pyramid. In the trigonal bipyramid, three positions lie along the equator of the molecule. The two positions lie along an axis perpendicular to the equatorial plane.

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.

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.

The number of Bond pair = Total number of atoms linked to central atom by single bonds.

Number of lone pairs = Total number of electrons – No of shared pair. The electron pairs around the central atom repel each other and move so far apart from each other that there are no greater repulsions between them. This results in the molecule having minimum energy and maximum stability.

Summary

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 tells 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 determining 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 possible for molecules with polar covalent bonds.

Further Readings