chem161sg_chapter10

chem161sg_chapter10 - Chapter 10 Bonding Theory and...

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Chapter 10 Bonding Theory and Molecular Structure Consider: BF 3 P F 3 Trigonal Planar Trigonal Pyramidal Lewis structures do not tell 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 pairs are 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 Geometry Molecular geometry or the shape of the molecule takes into account the direction of the bonding electron pairs between the nuclei and the bond angles generated between the bonding electron pairs in the molecule. Electron Group Geometries The electron group geometry takes into account all the 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 geometry takes into account only bonding pairs. Electron group geometry takes into account both bonding and non-bonding pairs. 2 electron pairs Æ linear 3 electron pairs Æ trigonal planar 4 electron pairs Æ tetrahedral 5 electron pairs Æ trigonal bipyramidal 6 electron pairs Æ octahedral 7 electron pairs Æ pentagonal bipyramidal 2
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The model works well also with molecules with multiple bonds . Rules: 1) Double or triple bonds count as one valence shell electron pair. 2) Bond angles that include multiple bonds are generally longer than predicted. 3) The non-bonding electron pairs occupy more space than the bonding electron pairs. The relative repulsion of electron pairs are as follows: lone pair lone pair bonding pair repelling repelling repelling lone pair bonding pair bonding pair Class of Molecules # VSEP # Bonding Pair # Lone Pair Geometry Example AX 2 2 2 0 Linear BeF 2 AX 3 3 3 0 Trigonal planar BF 3 AX 2 E 3 2 1 Angular GeF 2 , SO 2 AX 4 4 4 0 Tetrahedral CH 4 AX 3 E 4 3 1 Trigonal pyramidal NH 3 AX 2 E 2 4 2 2 Angular H 2 O AX 5 5 5 0 Trigonal bipyramidal PF 5 AX 4 E 5 4 1 Seesaw SF 4 , IF 4 + AX 3 E 2 5 3 2 T-shaped ClF 3 AX 2 E 3 5 2 3 Linear I 3 , ICl 2 AX 6 6 6 0 Octahedral SF 6 AX 5 E 6 5 1 Square pyramidal BrF 5 AX 4 E 2 6 4 2 Square planar XeF 4 , ICl 4 AX 7 7 7 0 Pentagonal bipyramidal IF 7 X=bonding atom; E=lone pair.
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chem161sg_chapter10 - Chapter 10 Bonding Theory and...

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