Question

A "model" can be generally regarded as a representative description of a real system that cannot be fully described. Scientists use models to help understand and predict the behavior of systems under study. Valence Bond Theory and Molecular Orbital Theory are both rigorous mathematical models that describe molecular bonding. Compare and contrast the details of the two models, and discuss how a model's limitations should be understood and discussed in scientific writing.

Answer 1

Valence Bond Theory (VBT) and Molecular Orbital Theory (MOT) have the following common features:

1. The Basic principle is same in both the theories. Both involve distribution of electrons. Thus whether it is atomic or molecular cannot have more than two electrons.

2. Both the theories include the sharing of electrons by the nuclei.

3. According to both the theories the energy of overlapping orbitals must be comparable and there must be similarities in their symmetry.

Some dissimilarities include-

VBT-

1.Ineratomic orbital is produced by multiplying, exchanging and combinations of space wave functions of two electrons.

2.Orbitals of bonded atoms cannot lose their identity.

3.Atomic orbitals monocentric.

4.It explains that inert gases have orbital already spin paired, so they have no tendency to form any linkage.

5.Pauli’s exclusion principle decides the spin function combination with wave function as

Bonding Antibonding

6.VBT introduces a special form to explain the presence of ionic character in bond.

7.VBT explains the difference between bonding and antibonding orbitals as following:

Bonding Antibonding

MOT-

1. Molecular orbitals are formed by of all atomic orbitals from the valency shell of two atoms.

2. Orbitals of bonded atoms lose their individual identity.

3. Molecular orbitals are polycentric

4. Molecular orbital theory explain the non-existence of molecules of inert gases, since number of bonding and antibonding electrons are equal.

5. It explains how electrons are originally present in atomic orbitals and are distributed in new molecular orbitals.

6. MOT is not capable or predicting ionic character in a bond.

7. In MOT both bonding and antibonding orbitals can have electron pairs with opposed spins.

Limitations of a model:

1. Models are normally incomplete.

2. Models assume the future will be like the past.

3. The operational status of a model may be unclear.

4. Input data may be uncertain.

5.The usefulness of a model may be limited by its original purpose.

6.The apparent precision of model forecasts may be misleading.

Because of all these basic reasons time and again models proposed my scientists have been proven wrong because they fail when we account them in practice.