Valence Bond Theory (Vbt) | Postulates Of Valence Bond Theory |

Valence Bond Theory (VBT).

Heitler as well as F. London calculated theoretically the bond loose energy as well as bond length of H2  with the aid of moving ridge mechanics. This handling became known every bit the valence bond theory (VBT). Later on, it was modified yesteryear L. Pauling as well as J.E. Slater.

Consider the formation of H2 from ii hydrogen atoms HA  as well as HB. Their moving ridge functions are ψA (1) ψB (2) respectively. The numbers i as well as 2 stand upwards for their electron.

In social club to empathise the formation of the bond betwixt ii H-atom nosotros volition speak over the next ii situations:

1. When ii atoms are far apart.
2. When ii atoms are unopen plenty to shape a bond. Their intermolecular distance is equal to that of bond lengths.

First Situation:

When the ii atoms are far apart, at that topographic point is no interaction betwixt the electrons as well as the atoms. The moving ridge business office for such a scheme is,

φmo     =       ψA (1) . ψB (2)      .........   (i)

When this equation is solved for loose energy as well as bond length, it is observed that the total loose energy of the scheme is equal to the amount of energies of private atoms. It agency that no bond is formed because inwards the formation of a bond the loose energy of the scheme must travel less than the loose energy of combining atoms.

Second Situation:

When ii atoms are brought closer so that their internuclear distance is equal to the bond length. At such a unopen distance the interactions of nuclei as well as electrons receive got to travel considered. Heitler as well as London pointed out that it is non possible to restrain the electrons to their ain nuclei. One electron is every bit a proficient share of an atom every bit it is of the other atom as well as thence the restriction of the electron is non possible. They suggested an guess moving ridge business office which allows both arrangements:

φmo     =       ψA (1) . ψB (2)  +  ψA (2) . ψB (1)

The loose energy is calculated amongst (ii) is the minimum of value 303 KJ/mole for a bond length of 0.80 Aº. The lowering of loose energy is due to the fact that electron is attracted yesteryear both nuclei as well as thence they receive got a greater book to motility about.

Can travel a farther improvement tin travel made inwards moving ridge business office (ii) yesteryear considering the possibility that both electrons may occupy the same molecular orbital i.e., the electron i of HA  may occupy the orbital of an atom HB and vice versa. This is a province of affairs inwards which the bond is ionic,

for this situation, the moving ridge business office of the H2  system volition be

φionic     =       ψA (1) . ψA (2)  +  ψB (1) . ψB (2)          ........... (iii)

If the loose energy is calculated taking into consideration the ionic bonding also, so the overall moving ridge business office volition become:

φMO   =  φCOV  +  λφionic               ......    (iv)

Where λ = contribution of ionic graphic symbol inwards H2 molecule. It is 103 KJ/mole. The bond length comes out to travel 0.75ºA as well as the total loose energy of φMO is nigh to the actual value, 435 KJ/mole.

In this discussion, elections are supposed to obey Pauli's exclusion Principle, i.e., the electron is paired inwards a molecular orbital. If they receive got the same spins, the moving ridge business office for the scheme volition travel some that different

φMO   =  ψA (1) . ψB (2)  +  ψA (1) . ψB (2)  ..... (v)

However, this business office does non Pb to bonding.

Postulates of valence bond theory.

"Postulates of valence bond theory"

1. Only one-half fill upwards orbitals accept share inwards bond formation.
2. No of bonds formed yesteryear an atom is equal to its seat out of unpaired electrons.
3. A molecular orbital is formed yesteryear the linear overlap of atomic orbitals.
4. The pairing of electron results inwards bond energy.
5. The electron twain is localized betwixt ii nuclei.
6. The overlap is a mensurate of the bond strength. Greater the overlap, shorter as well as stronger volition travel bond length.
7. Resonance is permissible inwards VBT.
8. It likewise accounts for the ionic graphic symbol of a covalent bond as well as the hybridization of atomic orbitals.
9. Co-ordination bonding is non explainable on the footing of this theory.