Unit 1.1(Molecular Orbital Theory)
- 1. ENGINEERING CHEMISTRY DR FARHAT A ANSARI ASSISTANT PROFESSOR (AS) JETGI
- 2. Molecular Orbital Theory Band Theory of Solids Liquid Crystal & Its Application Point Defects in Solids Structure of Graphite & Fullerene UNIT-I
- 3. Molecular Orbital Theory 1. MO theory suggests that atomic orbitals of different atoms combine to create MOLECULAR ORBITALS 2. Electrons in these MOLECULAR ORBITALS belong to the molecule as whole 3. This contrasts to VB theory which suggests that electrons are shared by simple overlap atomic orbital's or hybridized atomic orbital's . 4. Molecular orbital can be constructed from linear combination of atomic orbital's MO = LCAO INTRODUCTION
- 4. Rules for linear combination 1. Atomic orbital's must be roughly of the same energy. 2. The orbital must overlap one another as much as possible- atoms must be close enough for effective overlap. 3. In order to produce bonding and antibonding MOs, either the symmetry of two atomic orbital must remain unchanged when rotated about the internuclear line or both atomic orbital's must change symmetry in identical manner. Linear combination of atomic orbitals
- 5. Rules for the use of MOs * When two AOs mix, two MOs will be produced * Each orbital can have a total of two electrons (Pauli principle) * Lowest energy orbitals are filled first (Aufbau principle) * Unpaired electrons have parallel spin (Hund’s rule) Bond order = ½ (bonding electrons – antibonding electrons)
- 6. A B A B AB = N(cA A + cBB) Linear Combination of Atomic Orbitals (LCAO) 2AB = (cA2 A2 + 2cAcB A B + cB2 B 2) Overlap integral The wave function for the molecular orbitals can be approximated by taking linear combinations of atomic orbitals. Probability density c – extent to which each AO contributes to the MO
- 7. When 2 atomic orbitals combine there are 2 resultant orbitals. low energy bonding orbital high energy antibonding orbital 1sb 1sa s1s s* E 1s Molecular orbitals Eg. s orbitals
- 8. First period diatomic molecules s1s2H Energy HH2 1s 1s sg su* Bond order = ½ (bonding electrons – antibonding electrons) Bond order: 1
- 9. s1s2, s*1s2He Energy HeHe2 1s 1s sg su* Molecular Orbital theory is powerful because it allows us to predict whether molecules should exist or not and it gives us a clear picture of the of the electronic structure of any hypothetical molecule that we can imagine. Diatomic molecules: The bonding in He2 Bond order: 0
- 10. Second period diatomic molecules s1s2, s*1s2, s2s2 Bond order: 1 Li Energy LiLi2 1s 1s 1sg 1su* 2s 2s 2sg 2su*
- 11. s1s2, s*1s2, s2s2, s*2s2 Bond order: 0 Be Energy BeBe2 1s 1s 1sg 1su* 2s 2s 2sg 2su* Diatomic molecules: Homonuclear Molecules of the Second Period
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