chapter10_ValenceTheory_MolecularOrbital

chapter10_ValenceTheory_MolecularOrbital - Molecular Shapes...

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1 Chapter 10 olecular Shape Molecular Shape, Valence Bond Theory and Molecular Orbital Theory 1 Molecular Shapes • The shape of a molecule plays an important le in its reactivity role in its reactivity. 2 Electron Domains • Electron pairs around an tom can be referred as atom can be referred as electron domains (or electron groups). • In a double or triple bond, all electrons shared between those two atoms are on the same side of the central atom; therefore, they count as one electron domain. • This molecule has four electron domains around its central atom. 3 What Determines the Shape of a Molecule? lectron pairs aro nd an atom hether the • Electron pairs around an atom, whether they be bonding or nonbonding, repel each other. • The best arrangement is the one that minimizes the repulsions among them . o minimize the repulsion electron pairs are To minimize the repulsion, electron pairs are placed as far as possible from each other around the central atom. 4
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2 Valence Shell Electron Pair Repulsion Theory (VSEPR) linear trigonal planar tetrahedral trigonal bipyramidal octahedral 5 6 7 Electron-Domain Geometries • To determine the electron domain geometry, count the number of electron domains in the Lewis structure. • The geometry will be the one that corresponds to that number of electron domains. 8
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3 Electron-Domain Geometries etermine the electron domain geometry • Determine the electron domain geometry of CH 4 , PBr 3 , and ClF 3 . 9 Molecular Geometries • The electron-domain geometry is sometimes not the shape of the molecule. • The molecular geometry (shape) is defined by the positions of only the atoms in the molecules, not the nonbonding electron pairs. 10 Molecular Geometries he simplest situation: • The simplest situation: If there are only two atoms in the molecule, the molecule will be linear no matter what the electron domain is. • Examples: O 2 , HCl 11 Linear Electron Domain Geometry • In this situation, there is only one possible molecular geometry: linear. 12
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4 Trigonal Planar Electron Domain Geometry • There are two possible molecular geometries: Trigonal planar ent Bent 13 Nonbonding Pairs and Bond Angle • Nonbonding pairs (lone pairs) are physically larger than bonding pairs. • Therefore, their repulsions are greater; this tends to decrease bond angles in a molecule. 14 Multiple Bonds and Bond Angles • Double and triple bonds place greater electron density on one side of the central atom than do y single bonds. • Therefore, they also affect bond angles. 15 Tetrahedral Electron Domain Geometry • There are three possible molecular geometries: Tetrahedral, Trigonal Pyramidal, and Bent 16
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5 Trigonal Bipyramidal Electron Domain Geometry • There are two distinct positions in this geometry: Axial quatorial Equatorial 17 Trigonal Bipyramidal Electron Domain Geometry Lower-energy conformations result from having nonbonding electron pairs in equatorial, rather than axial, positions in this geometry.
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chapter10_ValenceTheory_MolecularOrbital - Molecular Shapes...

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