valence bond theory, its postulates, formation of H2 molecule ,concept of overlapping and its types, sigma and pi bond

1. VALENCE BOND THEORY
Presented by : NEELU SHARMA
CLASS XI

2. POSTULATES OF VBT
1. Covalent bonds are formed when two valence orbitals
(half-filled) belonging to two different atoms overlap on
each other. The electron density in the area between
the two bonding atoms increases as a result of this
overlapping, thereby increasing the stability of the
resulting molecule.
2. The presence of many unpaired electrons in the
valence shell of an atom enables it to form multiple
bonds with other atoms. The paired electrons present in
the valence shell do not take participate in the formation
of chemical bonds as per the valence bond theory.
3. Covalent chemical bonds are directional.
4. The atomic orbitals overlap to form sigma and pi bonds.


3. APPLICATIONS OF VALENCE BOND THEORY
• The maximum overlap condition which is described by the valence bond
theory can explain the formation of covalent bonds in several molecules.
• This is one of its most important applications. For example, the difference in
the length and strength of the chemical bonds in H2 and F2 molecules can
be explained by the difference in the overlapping orbitals in these
molecules.
• The covalent bond in an HF molecule is formed from the overlap of the 1s
orbital of the hydrogen atom and a 2p orbital belonging to the fluorine atom,
which is explained by the valence bond theory.

4. FORMATION OF H2 MOLECULE
 H(g) + H(g) → H2(g) + 433 kJ/mol
 When atoms combine to form molecule,
there is always decrease in energy, which
leads to the stability of the molecule.
 When atoms approaches, two different
operate:
(a) The nucleus of one atom is attracted
towards the electrons of the other and
versa. Energy is release during attraction .
(b) The nuclei of the atoms as well as there
electrons repel each other . Energy is
needed to over come the force of

5. FORMATION OF HYDROGEN MOLECULE
 If the magnitude of attractive force is
more than repulsive force , a stable
molecule will be form .
 When to hydrogen atoms approach
each other , the overall potential
energy of the system decreases and
stable hydrogen molecule will
formed.
 When attraction and repulsion forces
balanced each other, both hydrogen
atoms get bonded to form H2
molecule.

6. H2 MOLECULE
 In H2 molecule, H ─ H bond length is
74 pm.
 The total decrease in the potential
energy when one mole bond is
formed between atoms in gaseous
state is called bond energy.The bond
energy in H2 molecule is 433 kJ/mol.
(Bond dissociation energy)

7. HELIUM MOLECULE
 New attractive force = 4
 New repulsive force = 5
The overall repulsive force is more
than attractive force, therefore He2
molecule don’t exist.

8. OVERLAPPING

9. TYPES OF OVERLAPPING

10. S-P OVERLAPPING

11. CO-AXIAL & SIDEWISE (P-P)OVERLAPPING

13. OVERLAPPING

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