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VSEPR Theory and Valence bond theory

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Learning Outcomes At the end of this presentation the students will be able to: 1.Explain VSEPR Theory 2.State the postulates of Valence bond Theory

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Valence Shell Electron Pair Repulsion Theory A basic geometry can be assigned to each non-terminal atom based on the number of “objects” attached to it. Objects include bonded atoms (single, double, triple, partial bonds) and “lone pairs” of electrons. VSEPR theory lets us predict the shape of a molecule based on the electron configurations of the constituent atoms. It is based on maximizing the distance between points on a spherical surface. Number of Objects Geometry lineartrigonal planar tetrahedraltrigonal bipyramidal* Octahedral

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Number of Objects Geometry LinearTrigonal planar TetrahedralTrigonal bipyramidal Octahedral Formula (Shape) AX 2 AX 3 (trigonal planar) AX 2 E (bent) AX 4 (tetrahedral) AX 3 E (pyramidal) AX 2 E 2 (bent) AX 5 (trigonal bipyramidal or square pyramidal) AX 4 E (seesaw) AX 3 E 2 (T-shaped) AX 2 E 3 (linear) AX 6 (octahedral) AX 5 E (square pyramidal) AX 4 E 2 (square planar) AX 3 E 3 (T-shaped) The geometry around an atom is described by the general formula: AX m E n Where X is a bonded atom, E is a lone pair and (m+n) is the number of objects (sometimes called the steric number, SN) around the central atom A.

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Number of Objects 78 Geometry Pentagonal bipyramidal Square anti-prismatic XeF 5 - NMe 4 + Xe - FFFFF Xe is described as AX 5 E 2 and has a pentagonal planar shape derived from the pentagonal bipyramidal geometry. Less common geometries

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Valence Bond Theory Valence bond theory (VBT) is a localized quantum mechanical approach to describe the bonding in molecules. VBT provides a mathematical justification for the Lewis interpretation of electron pairs making bonds between atoms. VBT asserts that electron pairs occupy directed orbitals localized on a particular atom. The directionality of the orbitals is determined by the geometry around the atom which is obtained from the predictions of VSEPR theory. In VBT, a bond will be formed if there is overlap of appropriate orbitals on two atoms and these orbitals are populated by a maximum of two electrons. bonds : symmetric about the internuclea r axis bonds: have a node on the inter- nuclear axis and the sign of the lobes changes across the axis.

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Valence bond theory is based on the following four postulates: 1.A bond is formed when atomic valence orbitals overlap with each other. 2.Overlapping orbitals contain a pair of electrons. 3.Electron density concentrates between bonded atoms. 4.The strength of the bond depends on the degree of overlapping.

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Valence Bond Theory Valence bond theory treatment of bonding in H 2 and F 2. H A 1s 1 H B 1s 1 A A B B This gives a 1s-1s bond between the two H atoms. F 2s2p F 2s2p 2p z Z axis This gives a 2p-2p bond between the two F atoms.

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