Difference between Valence Bond Theory and Molecular Orbital Theory

In Chemistry difference between valence bond theory and molecular orbital theory, Electrons in particle process nuclear orbitals rather than atomic orbitals. The nuclear orbitals cross-over on the bond arrangement and the bigger the cross-over the more grounded the bond. 

OR, Valence bond proportion(theory) is one of the two basic (introductory) theories, along with molecular orbital theory, that we developed to use the process(methods) of quantum mechanics to explain of chemical bonding.   

Valence bond theory explain:- Strong ligand- Good electron donor it will always pair the unpaired electrons in the CMA. Exp. CO, CN

Average ligand:- Average electron donor it can act as weak as well as strong ligands. Exp. NO ¯, H2O, NH3, EDTA, SCN, OH¯.

Weak ligand:- Poor electron donor, never pair unpaired electron Emp. X¯ (Halogens) F¯, Cl¯, Br¯, I¯, S²¯, SCN¯.

Properties of valence bond theory:- 

Hybridization, Magnetic Moment, Spin Multiplicity(2S+1), Magnetic nature n(n+2).

Example:- Q. Find out the complex formed when nickel is heated with CO gas.?

Ans:-  Nickel (Ni)       ⟶     Ni (CO )4 ( blue flame)

Hybridization = Sp²

Magnetic moment = √n(n+2) = √0(0+2) = 0

Spin multiplicity = 2S+1 = 2(0)+1 = 1

Molecular Orbital Theory

VBT considered O2 as diamagnetic .Experimentally O2 was found to paramagnetic. Magnetic behavior of this molecule was explained by. this theory. 

{tocify} $title={What information can be obtained from molecular orbital theory (MOT)}

Postulate of molecular orbital theory

The electron in molecules are present in various molecular orbital as the electron of atom are present in various atomic orbital. The atomic orbital of comparable energies and proper symmetry combine to from molecular orbital.

1s + 1s = ✓  , 1s + 2s = ㄨ This mean that 1s can combine with 1s, 1s cannot combine with 2s, because 2s has higher energy. 

Postulate of molecular orbital theory

An electron in an atomic orbital is influenced by one macules but molecular orbital. So atomic orbital was monocentric and molecular orbital are polycentric.

The number of molecular orbital formed is equal to the number of combining atomic orbital. When two atomic orbital combine, two molecular orbital are formed. One is know as bonding molecular orbital while the other is called anti boding molecular. The bonding molecular orbital has lower energy and hence greater stability than the corresponding antibonding molecular orbital.

Postulate of molecular orbital theory

Just as the electron probability distribution around a nucleus is an atom is given by an atomic orbital the electron probability distribution around a group of nucleus in a molecule is given by a molecular orbital. The molecular orbital like atomic orbitals are filled in accordance with Aufbau principle obeying the Pauli exclusion principle and the Hund's rule.  

Note: S orbital forming sigma bond and P orbital forming sigma and Pi bond. 

What information can be obtained from molecular orbital theory (MOT)

Bond order:- 

Bond order is the number of chemical bonds a pair of atoms. 

Bond Order (BO) = 1/2(Nb - Na)

Nb = number of electron in bonding orbitals

Na = number of electron in antibonding orbitals.

Nature of the bond

If bond order = 1 single covalent bond e.g. H2, F2 molecules. If bond order = 2 double covalent bond e.g. O2 molecules. If bond order = 3 triple covalent bond e.g. N3 molecules.

Stability of molecules

Case:-I If Nb > Na molecules is stable more bonding orbitals are occupied e.g. O2, N2. Case:-II If Nb < Na molecules is unstable antibonding in fluence hence an unstable molecule is formed. Case:- III If Nb = Na molecules is unstable in fluence of antibonding electron is slightly more than bonding electrons. 

Bond length

Bond length is inversely proportional to bond order. More is the bond order, lesser is the bond length.

C 一  C , C  二 C , C ☰ C      B.L 1/ B.O

Bond length is inversely proportional to bond order. If more is the bond order lesser is the bond length.

Magnetic nature

If all molecules orbitals in a molecules are doubly occupied the substance is diamagnetic. If one or more molecules orbital are singly occupied the substance is paramagnetic.

Magnetic nature

Paramagnetic:- When total number of electron in diamagnetic specie are Odd , 10,16.

Diamagnetic:- When total number of electron are Even.

>Chemistry basics for beginners


Hi I'm Banti Singh, a Chemical Engineer! Welcome all of you to my blog. If you got the information right? Share the information. All of you Thank you

Thanks to visit this site.

Post a Comment (0)
Previous Post Next Post