revised VBT & MOT

revised VBT & MOT - CHEM1101: CHEMISTRY (EEE/COE)...

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CHEM1101: CHEMISTRY (EEE/COE) LECTURE 6 VALENCE BOND THEORY (VBT) AND MOLECULAR ORBITAL THEORY (MOT) Valence-Bond Theory (VBT) It was proposed by Heitler and London in 1927. According to this theory- A covalent bond is formed by the sharing of electron between the participating atoms. This results if the overlapping between the half-filled atomic orbitals takes place and the strength of the bond formed depends upon the extent of overlapping between the atomic orbitals of two atoms. VBT explains the geometry of many molecules and ions. Molecular-Orbital Theory (MOT) It was proposed by Hund and Mulliken in 1932. According to this theory- All atomic orbitals of the atoms participating in molecule formation get disturbed when the concerned nuclei approach nearer. They all get mixed up to give rise to an equivalent number of new orbitals that belong to the molecule now. These are called the molecular orbitals. It explains the formation of a covalent bond in a better way. Main Features (i) A molecule is quite different from its constituent atoms, (ii) Atomic orbitals (AO’s) of individual atoms combine to form molecular orbitals (MO’s) and these MO’s are filled up in the same way as AO’s are formed. ( ) (iii) The MO’s have definite energy levels. (iv) The shapes of MO’s formed depend upon the shape of combining atomic orbitals. AIUB Last updated: 14/09/10 Course-Teacher: Shahrina Alam
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CHEM1101: CHEMISTRY (EEE/COE) LECTURE 6 Bond Order (BO) The number of bonds in a molecule is one-half of the difference of the number of electrons in the bonding molecular orbitals (BMO’s) and the number of electrons in the anti-bonding molecular orbitals (ABMO’s). Mathematically we can write, B.O. = 2 ) ' ( ) ' ( s ABMO in electrons of Number s BMO in electrons of Number - = 2 a b N N - Information given by MOT Applying the above formula the following information can be determined. (i)
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revised VBT & MOT - CHEM1101: CHEMISTRY (EEE/COE)...

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