Valence bond theory is mainly use for the formation of strong covalent bond between two atoms. strong covalent bond is formed between two atoms when the extent of overlapping is greatest between two atoms.
Read More – Why should chemical equation be balanced?
What is valence bond theory?
Valence bond theory is to explain about chemical bonding and covalent bond. valence bond theory uses for explanation for the formation of covalent bond between two hydrogen atoms with the help of overlapping. the chemical bonding of two hydrogen atoms as per the valence bond theory. according to valence bond theory, a covalent bond is formed by the overlapping of two half filled atomic orbitals having electrons with opposite spins. it is based on wave nature of electrons.
The best example is: – H2
This covalent bond formation is called s – s overlapping.
Valence bond theory gives the following points such as,
- Covalent bond between hydrogen atom.
- Structure of the molecules.
- Electronic configuration.
- Hybridization of atomic orbitals.
- Sigma bond.
- PI (π) bond.
According to Lewis theory, Lewis theory failed to explain the formation of covalent bond. it mean, Lewis theory have not able to explain about exact season for formation of covalent bond between atoms.
Lewis theory does not explain why molecules have different bond dissociation enthalpy and bond length and shape of polyatomic molecules such as, NH3, H2O.
But VSEPR theory have also limited explanation about geometry of molecules. VSEPR has not able to explain properly (with reason) about geometry of molecules.
To overcome these type of limitations valence bond theory was introduced. Valence bond theory initiated by “Heitler” in London in 1927. these are the basic history of VBT.
Types of overlap
Sigma Bond
A covalent bond is formed by the overlapping of two half filled atomic orbitals having electrons with opposite spins.
- S-S overlap
- S-P overlap
- P-P overlap
1. S-S overlapping
Sigma bond are form covalent bond between two half filled atomic orbitals. this is know as s-s overlapping. this type of overlapping occur in only sigma bond.
This is the best example for sigma bond – H2
You know that, the single bond is formed between two hydrogen atoms is called a sigma bond.
for example – two hydrogen atoms (H2)
2. S-P overlapping
These are the second example of sigma bond. If i m talking about HF, you know that hydrogen has S orbitals but F has P orbitals. the main relation between S-P overlapping is it is always head to head overlap. After overlapping between two atoms (H and F) , only sigma bond is formed. it is clear with the help of following structure.
Remember – In this case, PI (π) bond will never be made, only sigma bond is formed between HF atoms.
You know that, the single bond is formed between two different atoms such as (HF) is called sigma bond.
3. P-P overlapping
In case of p-p overlapping, both sigma bond and pi bond can be formed. let’s know about it with examples.
for example – F2
first draw the electron configuration of F.
F – 1s2 , 2s2, 2p5
F – 1s2, 2s2, 2p5
In case of first (F) 2p5 mean – In case of p orbitals only one is unpaired electron. It mean only 1p is empty.
In case of second F 2p5 mean – in case of p orbitals only 1p is empty.
therefore, both atoms are overlapping each other on x axis. this overlapping is called head on overlapping. According to this overlapping sigma bond is formed. these are the best example of sigma bond.
Whenever, you find single bond between two atoms such as, H2, HF, FF that is called sigma bonds. It cannot be pi bond.
Pi (π) bond cannot form without sigma bond. this mean that, first sigma bond will be formed then PI bond will be formed.
As a result, the strength of pi bond depend upon the extent of overlapping between atomic orbitals.
You know that, according to better overlapping, the extent of overlapping is greeter then strongest to the pi bond.
PI Bond
It is formed by lateral overlapping. and side ways parallel overlap between pi (π) atomic orbitals. In this case, some times it is formed by perpendicular overlap between pi atomic orbitals. You know that, π bond is a weaker bond then sigma bond.
let’s know about it with example.
the best example is O2. the concept of π bond will clear with the help of this example.
first draw the electrons configuration of both atoms. Such as, O2.
O – 1s2 2s2 2p4
O – 1s2 2s2 2p4
In both case, in p orbitals 2p is empty.
You know that, each oxygen has two half filled in p orbitals.
where, one is know as px – x axis
and other is know as py – y axis
Now, Both oxygen atoms overlap each other on x axis. due to this sigma bond is formed as shown in figer. After that π bond also formed with lateral overlapping.
Formation of sigma bond
- Sigma bond is formed from axial overlapping.
- Sigma bond is formed from One head to another head overlapping.
- Sigma bond is also formed End to End overlap.
- Sigma bond is formed along the inter nuclei axis.
Formation of PI (π) bond
- PI bond is formed Lateral overlapping.
- PI bond is formed with the help of parallel overlapping. If two atoms are overlapping with parallel. this is know as PI bond.
- Overlapping perpendicular to the inter nuclei axis.
Diffrence between sigma bond and PI (π) bond
- Hybridization depend upon sigma bond whereas hybridization is not depend upon Pi bond.
- Shap of molecules is determined by sigma bond whereas shap of molecules is not determine by pi bond.
- Free rotation is possible in case of sigma bond whereas free rotation is not possible in pi bond.
- sigma bond is localized whereas pi bond is not localized.
Postulates of Valence bond theory
- Valence bond theory is mainly use for the formation of covalent bond but if half filled orbitals of two atoms overlap each other.
- In case of each overlapping, atomic orbitals must contain an unpaired electron with opposite spin.
- The unpaired electron are converted to paired electron by overlapping of atomic orbitals.
- Electron density increase between two nuclei and repulsion between two nuclei decrease during overlap of atomic orbitals.
- The overlapping atomic orbitals must have same activities.
- A bond is formed at equilibrium distance when system has minimum potential energy and maximum stability.
- the extent of overlapping orbitals between two atoms is greatest then the strength of covalent bond also greatest. it mean, the strength of covalent bond is depend upon the extent of overlapping between two atoms.
- More overlapping gives strong covalent bond between two atoms.
Limitations of valence bond theory
- This theory is not able to explain about kinetic stabilities and thermodynamics for coordination compounds.
- This theory does not explain about coordination compounds.
- This theory is not able to explain about ligand.
- This theory is not able to explain about exact value of magnatic movement.
- it does not properly explain about effect of temperatures on magnatic movement.
- This theory is not able to explain about inner or outer orbitals.
- This theory is failed to explain about colour of coordination compounds.
- Not explain about orbitals contribution to magnatic movement.
Application of valence bond theory
- Valence bond theory can be use for explain about the formation of covalent bond between two atoms. for example H2 atoms. and covalent bond is formed by s-s overlapping between two hydrogen atoms. this type of overlapping (s-s) called sigma bond. sigma bond is show head to head overlapping.
Importance of valance bond theory
• The main theory is formation of strong bonds. this theory is indicated development of covalent bond in molecules. It also develop overlap of atomic orbitals.
• This theory is also indicate development of structure of molecules and also focus on electron configuration of molecules.
Valence bond theory is mainly use for the formation of strong covalent bond between two atoms. strong covalent bond is formed between two atoms when the extent of overlapping is greatest between two atoms.
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