Molecular Orbital Theory

Molecular Orbital Theory

It was given by Hund and Mulliken.
The main features are
  1. Electrons of atoms are present in atomic orbitals in the same way electrons of molecules are present in molecular orbitals.
  2. As atomic orbital’s is influenced by 1 nucleus in the same way molecular orbitals are influenced by two nuclei.
  3. In homonuclear molecules, two atomic orbital’s of equal energy combines to form molecular orbitals. In case of hetro-nuclear molecule, two atomic orbitals of comparable energy combine to form molecular orbital’s.
  4. The number. of molecular orbitals formed is equal to the number of combining atomic orbital.
  5. When two atomic orbital’s combines two molecular orbitals are formed one is known as bonding molecular orbitals while other is known as anti – bonding molecular orbital.
  6. The bonding molecular orbital has lower energy (more stability) as compared to anti - bonding molecular orbital.
  7. The filling of Molecular takes place in accordance with Hund’s, Pauli and Aufbau’s principle.  

Formation of Molecular Orbital’s

L C A O (Linear Combination of Atomic Orbitals)


When hydrogen is bonded with highly electronegative atoms like N, O, F then the e- of covalent bond are shifted towards more electronegative atom. Thus, partial charge develops due to which hydrogen forms bond with other electronegative atom. This is known as hydrogen bonding. It is of two types:-

  1. Intermolecular hydrogen bonding- It is formed b/w two different molecules of same or different compounds. Example:-
  2. Intermolecular hydrogen bonding In this type hydrogen atom is in b/w two highly electronegative atoms present within the same molecule
Give reason:-
  1. H2 O is liquid while H2 S is a gas.
  2. NH3 is more steam volatile then phosphine PH3.
  3. Ortho – nitro phenol has lower boiling point then pera – nitro phenol.
  4. HCl doesn’t show hydrogen bonding yet Cl is highly electronegativity.  
Resonance Energy:- It is the energy difference between resonating structure with lowest energy resonance hybrid.

It is used to determine the covalent and ionic character of a bond
Smaller the size of calion, greater the charge of calion and larger the anion, more the polarization more the covalent character.
Smaller the size of cation, greater the size of anion more the polarizibility an anion & more the Polarization covalent and more the character.
Eg:- Increasing order of covalent character is:-
Na F < Na Cl < Na I
Increasing order of ionic character:-
Li Cl > Na Cl > K CL       Covalent character
Li Cl < Na Cl < K Cl         Ionic character

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