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1、Chapter 10 : Chemical Bonding IIText sections 10.1 10.5 Key ConceptsVSEPR theoryPredicting molecular geometriesMolecular shape and polarityMolecular GeometryAccording to Lewis structure of water you might think the atoms are all in a straight line.OH HLewis structures tell us nothing about the shape
2、 of molecules.Experiment shows that this is not the case. Water molecules are bent not linear.Molecular geometry is determined by the electrostatic repulsions between valence shell e-pairs.OHHBut first some terminologyBond length : the distances between the nuclei of bonded atoms.Bond angle : the an
3、gles between adjacent lines representing bonds.104.5o97 pmVSEPR Theory e- groups around the central atom will be most stable when they are as far apart as possible; called valence shell electron pair repulsion (VSEPR) theory since e-s have - charge, they should be most stable when they are separated
4、 as much as possible the resulting geometric arrangement will allow us to predict the shapes and bond angles in the molecule Electron Groups the Lewis structure predicts the arrangement of valence e-s around the central atom(s) each lone pair of e-s constitutes one e- group on a central atom each ty
5、pe of bond (single, double, triple) constitutes one e- group on a central atomthere are 3 electron groups on N 1 lone pair 1 single bond 1 double bondONOMolecular Geometries 5 basic arrangements of e- groups around a central atom each of these 5 basic arrangements results in 5 different basic molecu
6、lar shapesLinear, trigonal, tetrahedral, trigonal bipyramid, octahedral Linear Geometry when there are 2 e- groups around the central atom, they will occupy positions opposite each other around the central atom this results in the molecule taking a linear geometry the bond angle is 180ClBeCl O C OTr
7、igonal Geometry when there are 3 e- groups around the central atom, they will occupy positions in the shape of a triangle around the central atom this results in the molecule taking a trigonal planar geometry the bond angle is 120FF B FNot Quite Perfect Geometry:Because the bonds are not identical,
8、the observed angles are slightly different from ideal.Tetrahedral Geometry when there are 4 e- groups around the central atom, they will occupy positions in the shape of a tetrahedron around the central atom this results in the molecule taking a tetrahedral geometry the bond angle is 109.5FF C FFTet
9、rahedral GeometryMethaneTrigonal Bipyramidal Geometry 5 e- groups around the central atom occupy positions in the shape of a two base-to-base tetrahedra with the central atom in the center of the shared bases called trigonal bipyramidal geometry positions above and below central atom called axial po
10、sitions positions in same plane as the central atom called equatorial positionsTrigonal BipyramidTrigonal Bipyramidal GeometryPCl ClClClCl Octahedral Geometry: when there are 6 electron groups around the central atom, they will occupy positions in the shape of two square-base pyramids that are base-
11、to-base with the central atom in the center of the shared bases this results in the molecule taking an octahedral geometry it is called octahedral because the geometric figure has 8 sides all positions are equivalent the bond angle is 90Octahedral Geometry:Octahedral Geometry:SFFFFFF The Effect of L
12、one Pairs: lone pair groups “occupy more space” on the central atom than shared e-s in a bond; lone pairs repel other e-s very strongly relative sizes of repulsive force interactions is:Lone Pair Lone Pair Lone Pair Bonding Pair Bonding Pair Bonding Pair this effects bond angles, making them smaller
13、 than expectedEffect of Lone Pairs:The bonding electrons are shared by two atoms, so some of the negative charge is removed from the central atom.The nonbonding electrons are localized on the central atom, so area of negative charge takes more space.Derivative Shapes the molecules shape will be one
14、of basic molecular geometries if all the electron groups are bonds and all the bonds are equivalent molecules with lone pairs or different kinds of surrounding atoms will have distorted bond angles and different bond lengths, but the shape will be a derivative of one of the basic shapesDerivative of
15、 Trigonal Geometry when there are 3 electron groups around the central atom, and 1 of them is a lone pair, the resulting shape of the molecule is called a trigonal planar - bent shape the bond angle is 120 O S O O S O O S ODerivatives of Tetrahedral Geometry 4 electron groups around the central atom, and 1 is a lone pair, called a pyramidal shape because it is a triangular-base pyramid with the central atom at the apex 4 electron groups around the central atom, and 2 are
