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SUBJECT-CHEMISTRY

CHAPTER-CHEMICAL BONDING R MOLECULAR STRUCTURE

SUBTOPIC- MOT,ORBITAL OVERLAPPING CONCEPT(LCAO),HYDROGEN BOND.

KEY NOTES :

MOLECULAR ORBITAL THEORY :
Molecular orbital (MO) theory was developed by F. Hund and R.S. Mulliken in
1932. The salient features of this theory are :

(i) The electrons in a molecule are present in the various molecular orbitals as the
electrons of atoms are present in the various atomic orbitals.

(ii) The atomic orbitals of comparable energies and proper symmetry combine to
form molecular orbitals.
(iii) While an electron in an atomic orbital is influenced by one nucleus, in a

molecular orbital it is influenced by two or more nuclei depending upon the
number of atoms in the molecule. Thus, an atomic orbital is monocentric while a
molecular orbital is polycentric.

(iv) The number of molecular orbital 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
antibonding molecular orbital.

(v) The bonding molecular orbital has lower energy and hence greater stability
than the corresponding antibonding molecular orbital.

(vi) 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.

(vii) The molecular orbitals like atomic orbitals are filled in accordance with the
aufbau principle obeying the Pauliísexclusion principle and the Hundís rule.
linear combination of atomic orbitals (LCAO):
Consider the hydrogen molecule consisting of two atoms A and B. Each hydrogen
atom in the ground state has one electron in 1s orbital. The atomic orbitals of
these atoms may be represented by the wave functions ψA and ψB.