In Chemistry difference between valence bond theory and molecular orbital theory is 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 chemical bonding.
Valence bond theory explains:- Strong ligand- Good electron donor it will always pair the unpaired electrons in the CMA. Exp. CO, CN
Average ligand:- Average electron donor 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 be paramagnetic. The magnetic behavior of this molecule was explained by. this theory.
{tocify} $title={What information can be obtained from molecular orbital theory (MOT)}
The postulate of molecular orbital theory
The electrons in molecules are present in various molecular orbitals as the electrons of atoms are present in various atomic orbitals. The atomic orbital of comparable energies and proper symmetry combine to form a 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.
An electron in an atomic orbital is influenced by one macule but molecular orbital. So atomic orbital was monocentric and molecular orbital is polycentric.
The number of molecular orbitals formed is equal to the number of combining atomic orbitals. When two atomic orbitals combine, two molecular orbitals are formed. One is known 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.
Just as the electron probability distribution around a nucleus in an atom is given by an atomic orbital the electron probability distribution around a group of nuclei in a molecule is given by a molecular orbital. The molecular orbital like atomic orbitals is filled by the Aufbau principle obeying the Pauli exclusion principle and Hund's rule.
Note: The S orbital forms a sigma bond and the P orbital forms a sigma and Pi bond.
What information can be obtained from molecular orbital theory (MOT)
Bond order:-
Bond order is the number of chemical bonds in a pair of atoms.
Bond Order (BO) = 1/2(Nb - Na)
Nb = number of electrons in bonding orbitals
Na = number of electrons 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 bonds e.g. O2 molecules. If bond order = 3 triple covalent bonds e.g. N3 molecules.
Stability of molecules
Case:-I If Nb > Na molecules are stable more bonding orbitals are occupied e.g. O2, N2. Case:-II If Nb < Na molecules are unstable antibonding in fluence hence an unstable molecule is formed. Case:- III If Nb = Na molecules are unstable influence of the antibonding electron is slightly more than bonding electrons.
Bond length
Bond length is inversely proportional to bond order. The more is the bond order, the 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 molecule orbitals in a molecule are doubly occupied the substance is diamagnetic. If one or more molecules' orbital are singly occupied the substance is paramagnetic.
Paramagnetic:- When the total number of electrons in diamagnetic species is Odd, 10,16.
Diamagnetic:- When a total number of electrons are Even.