Valence Bond Theory
It was given by Heitler and London and developed by Pauling. It is based on concept of atomic orbitals and Electronic Configuration of atoms,overlap criteria
of atomic orbitals, the hybridization of atomic orbitals and principal of variation and superimposition
Formation of H2 molecule
When two H atoms come closer new attractions and repulsion begin to operate.
A)The nucleus of one atom is attracted towards other atom electron.
B) The electrons of both the atoms undergo repulsion and nuclei of both atoms also undergo repulsion
Attractive forces when become dominant, bond formation takes place and system acquires minimum energy. Since the energy gets released when the
bond is formed between two hydrogen atoms, the hydrogen molecule is more stable than that of isolated hydrogen atoms. The energy so released is called as bond enthalpy. The bond energy of H – H is 435.8 Kj/mol. Same energy is required to break the bond
The energy needed to break the molecules into atoms is known as BOND DISSOCIATION ENERGY.
Orbital Overlap Concept
- According to this concept, covalent bond formed between atoms results in the overlap of orbital’s belonging to the atoms having opposite spins of electrons.
- Stability of molecular orbital depends upon the type of overlapping between the atomic orbitals.
TYPES of OVERLAPPING: -
AXIAL OVERLAPPING/ HEAD ON OVERLAPPING
- Overlapping along the axis is known as axial overlapping. In this type of overlapping only one lobe of p – orbital participates.
- The extent of axial overlapping is more than side wise overlapping. Therefore, it forms stronger bond.
- The axial overlapping results in the formation of sigma bond(σ)
(i) S – S overlapping
There is overlap of two half filled s-orbitals along the internuclear axis
(ii) S – p overlapping
Here overlap occurs between half filled s-orbitals of one atom and half filled p-orbitals of another atom
(iii) p – p overlapping
Here overlap occurs between half filled p-orbitals of the two approaching atomsa
- In this overlapping both lobes of p- orbital participates.
- The extent of overlapping is less. Therefore, the bond is weaker then the bond formed by axial overlapping.
- It results in the formation of bond.
- The first bond formed by overlapping is always sigma bond.
Difference between Sigma & Pie bond
1. They are formed by axial overlapping.
2. Sigma bonds are stronger than pie bond.
3. Both One lobe of p – orbital participates.
4. Electron cloud is symmetrical.
5. They are rotational.
6. S – orbital participated.
7. It is the first bond.
1. They are formed by only side – wise overlapping.
2. Pie bond are less strong than Sigma bonds.
3. Both lobes of p – orbital participate.
4. Electron cloud is non – symmetrical.
5. They are non – rotational.
6. S – orbital does no participates.
7. It is formed after single bond.
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