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Molecular Orbital Theory of Simple Diatomic Molecules and Ions. Introduction

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Molecular Orbital Theory of Simple

Diatomic Molecules and Ions


Introduction

Molecular orbital theory (MOT) is another approach to explain the formation of covalent bonds in molecules or ions. This theory was mainly developed by Hund and Mulliken in 1932 and later by Lennerd Jones and Coulson. This theory is also sometimes called Hund-Mulliken theory. This theory is based on the Linear Combination of Atomic Orbitals (LCAO) of the atoms constituting the molecule or ion and hence is also called LCAO—MO theory.

According to this theory, all the electrons in a given molecule or ion are considered to be present in molecular orbitals. Thus these electrons move under the influence of all the nuclei of all the constituent atoms making the molecule or ion.


  • Necessary conditions to be satisfied by atomic orbitals to combine together to form molecular orbitals

  • The combining atomic orbitals must have the same or almost the same energy.

  • The combining atomic orbitals must be able to overlap to the maximum extent to form molecular orbitals, since greater is the extent of overlap between the combining atomic orbitals, greater will be the electron density in the region between the nuclei of the combining atoms and hence stronger will be the bond between them.

  • The combining atomic orbitals must have the same symmetry about the molecular axis.


Disallowed overlaps of atomic orbitals on the basis of their symmetry. The shaded portions indicate the overlap regions.


Comparison between sigma and pi molecular orbitals. symmetry. The shaded portions indicate the overlap regions.


  • Comparison between atomic orbitals and molecular orbitals symmetry. The shaded portions indicate the overlap regions.

  • Similarities.

  • The molecular orbitals in a molecule or an ion have different energy and different shape and size as atomic orbitals in an atom have.

  • The filling of atomic orbitals with electrons takes place in accordance with Aufbou principle, Pauli's exclusion principle and Hund's rule of maximum multiplicity. Molecular orbitals are also filled with electrons according to these principles.

  • Like atomic orbitals, a molecular orbital can be represented by a wave function  of an atomic orbital is called atomic orbital wave function and that for a molecular orbital is called molecular orbital wave function.

  • Each atomic orbital wave function in an atom is associated with a set of quantum numbers which determine the energy and shape of the atomic orbital. Similarly each molecular orbital in a molecule is also associated with a set of quantum numbers which determine the energy, shape and size of molecular orbital.


Bond order or bond multiplicity symmetry. The shaded portions indicate the overlap regions.

Bond order of a molecule or an ion is the measure of the strength or stability of the bond. Numerically bond order (B.O.) is equal to half of the difference between the total number of electrons present in bonding molecular orbitals (nb) and the total number of electrons present in the antibonding molecular orbitals (na). Thus

B.O. = (nb — nu)/2

(ii) Bond dissociation energy , stability and bond length. Greater is the value of bond order, greater is the value of bond dissociation energy and hence greater is the stability of the bond or the species, i.e., the species with higher value of bond order will have higher value of bond dissocia­tion energy and hence would be more stable than the species having lower value of bond order. For


  • Comparison between valence bond theory (VBT) and molecular orbital theory (MGT)

  • Similarities.

  • Both the theories explain the formation of covalent bond.

  • According to both the theories, a covalent bond (VBT) or a molecular orbital (MOT) is formed when two atomic orbitals having the same symmetry and same energy overlap with each other.

  • According to both the theories, a covalent bond (VBT) or a molecular orbital (MOT) has directional characteristics.

  • Both the theories predict that, when covalent bond is formed, an increase in the electron density between the nuclei takes place. This increased electron density holds the two atoms to- gether and thus a bond is established.

  • VBT does not explain about magnatc character & bond order but MOT explains.


LALTA PRASAD orbital theory (MGT) PGT (CHEMISTRY)KV BANAR JODHPURRAJASTHAN


THANKS orbital theory (MGT)


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