The Valence Shell Electron Pair Repulsion Theory (VSEPR)

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Theories of Covalent Bonds:

There are three theories of covalent bonds.

(i) VSEPRT, Resonance Theory,

(ii) VBT

(iii) MOT.

Valence Shell Electron Pair Repulsion (VSEPR) Theory is used to predict:

VSEPR stands for Valence Shell Electron Pair Repulsion.

VSEPR theory is good enough to explain

• molecule geometries
• bond angles.

A summary of molecular shapes is given in the following table:

VSEPR Theory Main Postulates

According to vsepr, two types of electron pairs around the central atom of a molecule and it has the following postulates:

• Two types of electron pairs are present around the central atom of a molecule. Electron pair which is shared between two atoms and is responsible for the covalent bond is called a “bond pair” while electron pair which is not shared between two atoms is called a “lone pair”.

Lone pair-lone pair > Lone pair-bond pair > Bond pair-bond pair

• To reduce repulsion these electron pairs move to maximum angles apart from each other and are directed in different directions in space. So they are responsible for molecular geometries.

linear Geometry

For two electron pairs, geometry is linear and the bond angle is 180^o.

Triangular Planar Geometry

For three electron pairs, the geometry is triangular planar and the bond angle is 120^o.

Tetrahedral Geometry

For four electron pairs, the geometry is tetrahedral and the bond angle is 109.5^o.

• If more than one electron pairs are there in the same region they behave like a single pair. e.g.                      

Sulphur in SO₂ shows three electron pairs behaviour.

..

           O : : S : O

• Deviation from the ideal bond angle occurs if lone pair of electrons is there.
• The effect of a bonding electron pair decreases with the increasing electronegativity of an atom forming a molecule.

Shape of molecules with two electron pairs:

e.g (i)  BeCl₂                                                                       

        Cl :  Be  :  Cl          

   or      

   Cl ─ Be ─ Cl  Linear Geometry

(  180^o )

The central atom Be has two electron pairs so the molecule is linear with a bond angle of 180^o.

            e.g (ii) CO₂       

          O  : :  C  : :  O          

  or         O = C = O       Linear Geometry

Four pairs around the central Carbon atom behave like two pairs so the CO2 molecule is linear with a bond angle of 180^o.

            e.g (iii) HCN      

          H  :  C  : : : N         

   or         H ─  C  ≡  N  Linear Geometry

Four pairs around the central Carbon atom behave like two pairs so the HCN molecule is linear with a bond angle of 180^o.

The shape of molecules with three electron pairs:

   e.g (i)  BF₃

The central atom B has three electron pairs so the molecule is a triangular planner with a bond angle 120^o.

e.g (ii) SO₂

Two electron pairs that Sulphur has shared with one of the oxygen act as single pair so SO₂ shows the behaviour of three electron pairs. Hence electron geometry in SO2 is a triangular planner while molecule geometry is an angular or bent structure. In SO2 deviation from the ideal bond, angle occurs due to the presence of a lone pair.     

   e.g (iii) SnCl₂

In stannous chloride, Sn has 5s² 5p² valence electrons. The two unpaired electrons of the 5p orbital take part in the bond formation of a covalent bond with chlorine, while the pair of electrons of 5s remains a lone pair and does not participate in bond formation. Hence electron geometry in SnCl2 is a triangular planner while molecule geometry is an angular or bent structure. In SnCl2 deviation from the ideal bond, angle occurs due to the presence of a lone pair. 

Shape of molecules with four electron pairs:

e.g (i)  CH₄          

The central atom C has four electron pairs so the molecule is tetrahedral with a bond angle of 109.5^o.

e.g (ii) NH₃                       

The central atom N has four electron pairs so electron geometry in NH₃ is tetrahedral while molecule geometry is pyramidal. In NH₃ deviation from the ideal bond angle occurs due to the presence of a lone pair of electrons and the angle reduces to 107.5 ^o.

e.g (iii) H₂O                   

The central atom O has four electron pairs so electron geometry in H₂O is tetrahedral while molecule geometry is angular or bent structure. In H₂O deviation from the ideal bond angle occurs due to the presence of two lone pairs of electrons and the angle reduces to 104.5 ^o.

Limitations of VSEPR Theory

Although this theory is good enough to explain molecular shapes and bond angles:

• it fails to explain the mechanism of the formation of covalent bonds.
• it fails to explain why two electrons of the same pair do not repel each other.
• it fails to explain the paramagnetic nature of oxygen.

Continue Reading

VB Theory

MOT

MO Diagram for Li ² ,Li⁺² , Li ^-2

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Frequently Asked Questions-FAQs

What is the basis of VSEPR theory?

The VSEPR model, also known as the “Valence Shell Electron Pair Repulsion Theory,” is based on the idea that all atoms have a repulsion between their pairs of valence electrons, and that these atoms will always seek to arrange themselves in a way that minimises this repulsion.

Why is VSEPR theory important?

It is obvious that understanding a molecule’s form is crucial if one is to comprehend its responses. A straightforward mechanism for predicting the geometries of compounds is also desirable. The VSEPR approach is the most practical prediction technique available for main group chemicals.

What is difference between VBT and VSEPR theory?

VSEPR theory and valence bond theory are different in that VSEPR theory describes how molecules are formed, whereas valence bond theory explains how molecules are joined together chemically.

What does VSEPR stand for?

Valence shell electron pair repulsion is referred to as VSEPR. It is a theory that is used to determine how molecules will be arranged.

What does VSEPR predict?

Molecules’ shapes, including bond angles, are predicted by VSEPR.

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