Valance Bond Theory

This ppt explains the formation of molecules on the basis of Valence Vond theory.

1. Bonding in Molecules Part-II: Valence Bond Theory (Remedial Teaching Lecture-2) Prepared by D. R. Shinde Associate Professor in Chemistry Department of Chemistry P.D.E.A’s. Prof. Ramkrishna More Arts, Commerce and Science College Akurdi , Pune-411044

2. 02/08/2020 D. R. Shinde, Asociate Prof., P.D.E.A's. Prof Ramkrishna More College, Akurdi, Pune 2

3. To overcome Limitations of Lewis Concept, Valence Bond Theory (VBT) with concept of Hybridization was put forth. Assumptions of VBT : Valence shell orbitals of atoms in molecule overlap with each other to form bond. Half filled orbitals overlap with each other to form bond. Geometry of molecule can be explained on the basis of hybridization of atomic orbitals. Number bonds formed are equal to number of half filled orbital in an atom. Force of attractions is due to orbital overlap. Linear / Axial overlap of orbitals form sigma bond. Lateral / side wise overlap of orbitals form pi bond. Sigma bond is very stronger than pi bond. Orbitals overlap with each with similar sign of wave function. 02/08/2020 D. R. Shinde, Asociate Prof., P.D.E.A's. Prof Ramkrishna More College, Akurdi, Pune 3

4. p z orbitals of two atoms can overlap linearly to form sigma ( σ ) bond ( z axis is considered as internuclear axis – a convention by IUPAC ) + - + - Internuclear axis Side-wise overlap of orbitals Axial overlap of orbitals + + + + + s Orbital overlap linearly to form sigma bond Internuclear axis + + - z + - z P y orbitals of two atoms can overlap side-wise to form Pi ( π ) bond + Internuclear axis + - y + - y + - y + - y 02/08/2020 D. R. Shinde, Asociate Prof., P.D.E.A's. Prof Ramkrishna More College, Akurdi, Pune 4

5. Electronic configuration of ‘H’ atom is 1s 1 Example of VBT: Formation of H 2 Molecule Half filled 1s orbital of one H atom overlap with Half filled 1s orbital of another H atom to form H-H sigma bond. H atom consists of half filled 1s orbital which take part in bonding. + + + + + 1s Orbital of H atom overlap linearly to form sigma bond Internuclear axis H 2 Molecule consists of H– H sigma bond It has linear structure. 02/08/2020 D. R. Shinde, Asociate Prof., P.D.E.A's. Prof Ramkrishna More College, Akurdi, Pune 5

6. Electronic configuration of 'O’ atom is 1s 2 , 2s 2 , 2p x 2 2p y 1 2p z 1 Example of VBT: Formation of O 2 Molecule If the p z orbital of one ' O ' atom overlaps with the p z orbital of other ' O ' atom along internuclear axis to form σ bond. p y orbital and p z orbital are half filled. p z – p z orbital overlap is along internuclear axis. This type of overlap results into the formation σ bond between two oxygen atoms. P z – p z axial overlap - + _ + + - + p z - + p z 02/08/2020 D. R. Shinde, Asociate Prof., P.D.E.A's. Prof Ramkrishna More College, Akurdi, Pune 6

7. If the p y orbital of one 'O' atom overlaps with the p y orbital of other 'O' atom perpendicular to internuclear axis, p y – p y overlap and π bond is formed. In in oxygen molecule double bond is formed (O=O). One is σ bond another is π bond. P y orbitals of two O atoms overlap side-wise manner to form Pi ( π ) bond + Internuclear axis + - y + - y + - y + - y + σ - bond π - bond Overall orbital overlapping is - p y p z + - + - p y p z + - + - - + _ + + - + - 02/08/2020 D. R. Shinde, Asociate Prof., P.D.E.A's. Prof Ramkrishna More College, Akurdi, Pune 7

8. Geometry of some molecules such as O 2, N 2 , H 2 , HF, etc. can be explained on the basis of VBT. But geometry of many molecules cannot be explained on the basis of orbital overlap of pure atomic orbitals. Examples are - BF 3 have trigonal planar structure. CH 4 have Tetrahedral Structure. PCl 5 have trigonal bipyramidal structure, etc. BF 3 CH 4 To explain structure of these molecules the concept of hybridization is introduced in VBT. Since bond angel is different than angle between pure orbitals. Examples: In BF 3 bond angle is 120˚ while angle between orbitals is 90˚. In CH 4 bond angle is 109˚ 28˝ while angle between orbitals is 90˚. PCl 5 02/08/2020 D. R. Shinde, Asociate Prof., P.D.E.A's. Prof Ramkrishna More College, Akurdi, Pune 8

9. 02/08/2020 D. R. Shinde, Asociate Prof., P.D.E.A's. Prof Ramkrishna More College, Akurdi, Pune 9