Postulates of VSEPR Theory

The Valence Shell Electron Pair Repulsion Model (VSEPR) is an acronym for Valence Shell Electron Pair Repulsion Model. It’s simply a molecular geometry prediction model.VSEPR models investigate the bonding and molecular geometry of organic molecules, particularly polyatomic ions.It is applicable to practically all compounds with a non-metal centre atom.

The VSEPR theory’s postulates :

• In polyatomic molecules (molecules made up of three or more atoms), one of the constituent atoms is designated as the central atom, to which all other atoms in the molecule are linked.
• The total number of valence shell electron pairs determines the form of the molecule.
• electron pairs seek to position themselves in such a way that the electron-electron repulsion between them is minimised while the distance between them is maximised.
• The valence shell is a sphere with electron pairs organised on its surface in such a way that their distance between them is as small as possible.
• An asymmetrically structured molecule can be expected if the central atom of the molecule is surrounded by bond pairs of electrons.
• The molecule will have a deformed shape if the core atom is surrounded by both lone pairs and bond pairs of electrons.
• The VSEPR theory is applicable to resonance structure of any molecule. 
• Two lone pairs have the strongest repulsion, while two bond pairs have the smallest.
• Electron pairs around the core atom repel each other if they are closer together. As a result, the molecules’ energy increases.
• If electron pairs are separated by a large distance, repulsions between them are reduced, and the molecule’s energy is reduced.

Two lone pairs have the strongest repulsion, while two bond pairs have the weakest:

It’s because the orbitals that hold lone pairs have a distinct form.The repulsion between two lone pairs of electrons is, in fact, larger than the repulsion between a lone pair and a bonded pair of electrons.Similarly, the repulsion between a lone pair of electrons and a bonding pair of electrons is larger than that between two bonding pairs of electrons.

The reason for this is because of the geometries of the orbitals that house these lengthy pairs.

Because bonding electrons are sandwiched between two nuclei, they have far less room to “move about” than lone pairs.

Between the nuclei of two atoms, a bond is simply an area of extremely high electron density. This indicates that the orbitals that carry bonding electrons are longer than those that retain lone pairs..When compared to the orbitals that hold bonding electrons, the orbitals that hold lone pairs are actually rounder and shorter.

The lone pair of electrons on the ammonia molecule, NH3, pushes down on the three bonds in this example.

Lone pairs of electrons take up more space than bonded pairs of electrons since they are spread out at a shorter distance from that nucleus because they are only attracted by one nucleus.Bonding electrons are farther from the nucleus, but they are more confined, thus they aren’t as dispersed.

This explains why two lone pairs repel each other more than one lone pair, and one bond pair repels two bond pairs more than two bond pairs.

Using VSEPR to Predict Molecule Shapes:

The number of electron pairs used to define the shape of the molecules can be used to find the VSEPR predicted shapes of molecules in a systematic fashion.

Draw out the Lewis structure of the molecule first to anticipate the shape of the molecule. Determine the central atom on the Lewis diagram.Phosphorus is the core atom in the molecule [PF6] .

VSEPR Rules:

1. Determine the atom at the centre.
2. Determine the number of valence electrons in it.
3. For each bonding atom, add one electron.
4. Charge is calculated by adding or subtracting electrons (see Top Tip)
5. Multiply the total number of electron pairs by two to get the total number of electron pairs.
6. Predict the shape using this number.

Conclusion :

As a result, according to VSEPR theory, repulsion by a lone pair is stronger than repulsion by a bonded pair. As a result, when a molecule has two interactions with differing degrees of repulsion, VSEPR theory predicts that lone pairs will occupy places that reduce repulsion. Although the VSEPR model is effective at forecasting molecular geometry, it is ineffective at predicting the morphologies of isoelectronic species and transition metal complexes.