Chapter 10Bonding Theory and Molecular StructureConsider:BF3PF3Trigonal Planar Trigonal Pyramidal Lewis structures do nottell what the shapes of the molecules are. The shapes are important to know because some of the physical and the chemical properties of the molecules depend on their shapes. How to explain these different molecular shapes? Molecular structure takes into account: -bond length (the distance between two atoms) -bond angles (angle between bonds around an atom in the molecule) Experimentally, these parameters can be determined by x-ray diffraction method. The simplest method to predict the molecular structure is to use the valence shell electron pair repulsion(VSEPR) model. In this method, bonding is directional and the model predicts the shapes of molecules and ions in which the valence shell electron pairsare arranged about each atom so that electron pairs are kept as far away from one another as possible thus minimizing electron pair repulsions.1
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Molecular GeometryMolecular geometry or the shape of the molecule takes into account the direction of the bonding electron pairsbetween the nuclei and the bond angles generated between the bonding electron pairsin the molecule. Electron Group GeometriesThe electron group geometry takes into account allthe valence electrons,(bonding and non-bonding…and these could be single electron, lone pairs, single, double or triple bonds) and arranges them around the central atom in such a geometry as to minimize the repulsions between these electrons. In essence… Molecular geometrytakes into account only bonding pairs. Electron group geometrytakes into account both bonding and non-bonding pairs. 2 electron pairs Ælinear 3 electron pairs Ætrigonal planar 4 electron pairs Ætetrahedral 5 electron pairs Ætrigonal bipyramidal 6 electron pairs Æoctahedral 7 electron pairs Æpentagonal bipyramidal 2