Chapter 10 Chemical Bonding II Molecular Geometry and Hybridization of Atomic Orbitals Structure Determines Properties Molecules 3D objects The properties of molecular substances depend on the structure of the molecule o Skeletal arrangement of the atoms o Type of bonds between the atoms Ionic polar covalent or covalent o Shape of molecule VSEPR Valence Shell Electron Pair Repulsion Theory VSEPR Theory Electron groups around the central atom will be the most stable when they are as far apart as possible o Because they re negatively charged in the molecule The resulting geometric arrangement will allow us to predict the shapes and bond angles Electron Groups atoms The Lewis structure predicts the arrangement of valence electrons around the central Each lone pair of electrons constitutes one electron group on a central atom Each bond constitutes ONE electron group on a central atom o Regardless of whether it is single double or triple Molecular Geometries 5 basic arrangements of electron groups around a central atom o Based on a maximum of 6 bonding electron groups 1 o May be more than 6 on very large atoms but it s very rare Each of these 5 basic arrangements results in 5 basic molecular shapes o Perfect geometric figure all electron groups must be bonds all bonds must be equivalent For molecules that exhibit resonance it doesn t matter which resonance for you use the molecular geometry will be the same Linear Geometry opposite each other around the central atom Bond angle 180 When there are 2 electron groups around the central atom they will occupy positions Trigonal Planar Geometry When there are 3 electron groups around the central atom they will occupy positions in the shape of a triangle around the central atom Bond angle 120 Not Quite Perfect Geometry ideal ones Because the bonds are not identical the observed angles are slightly different from the Tetrahedral Geometry When there are 4 electron groups around the central atom they will occupy positions in the shape of a tetrahedron around the central atom 2 Bond angle 109 5 Trigonal Bipyramidal Geometry When there are 5 electron groups around the central atom they will occupy positions in the shape of two tetrahedrons that are base to base with the central atom in the center of the shared bases Bond angle equatorial positions 120 Bond angle axial equatorial positions 90 Equatorial positions in the same base plane as the central atom Axial positions above and below the central atom Octahedral Geometry When there are 6 electron groups around the central atom they will occupy positions in the shape of 2 square base pyramids that are base to base with the central atom in the center of the shared bases 3 All positions equivalent Bond angle 90 Called octahedral because the geometric figure has 8 faces The Effect of Lone Pairs Lone pair groups occupy more space on the central atom o Their electron density is exclusively on the central atom rather than shared like bonding electron groups Area of negative charge takes more space Relative sizes of repulsive force interactions o Lone pair lone pair lone pair bonding pair bonding pair bonding pair This affects the bond angles making them smaller than expected o Molecules with lone pairs or different kinds of surrounding atoms will have distorted bond angles and different bond lengths o The shape will be a derivative of one of the basic shapes Derivative of Trigonal Geometry Trigonal Planar Bent o o 3 electron groups around the central atom 1 lone pair o Bond angle 120 4 Derivatives of Tetrahedral Geometry Tetrahedral Pyramidal o o 4 electron groups around central atom 1 lone pair o Bond angle 109 5 Tetrahedral Bent o o 4 electron groups around central atom 2 lone pairs o Bond angles 109 5 o Planar looks similar to the trigonal planar bent shape except with smaller angles Derivatives of the Trigonal Bipyramidal Geometry When there are 5 electron groups around the central atom and some are lone pairs they will occupy the equatorial positions because there is more room Trigonal Bipyramidal See Saw o Distorted tetrahedron o 5 electron groups around central atom 1 lone pair o 5 o Bond angles equatorial 120 o Bond angles axial equatorial 90 Trigonal Bipyramidal T Shaped o o 5 electron groups around central atom 2 lone pairs o Bond angles 90 o o o Trigonal Bipyramidal Linear 5 electron groups around central atom 3 lone pairs o Linear shape 180 axial to axial 6 Derivatives of Octahedral Geometry When there are 6 electron groups around the central atom and some are lone pairs each even number lone pair will take a position opposite of the previous lone pair Octahedral Square Pyramid Shape o o 6 electron groups around central atom 1 lone pair o Bond angles axial equatorial 90 Octahedral Square Planar Shape o o 6 electron groups around central atom 2 lone pairs o Bond angles equatorial 90 7 8 Predicting the Shapes around Central Atoms 1 Draw the Lewis structure 2 Determine the number of electron gorups around the central atom 3 Classify each electron group as bonding or lone pair and count each time a Remember that multiple bonds count as one group single double etc 4 Determine the shape bond angles Practice Problem 10 1 o Determine the shape of the following derivative shapes o o o F FAs F Pyramidal 1 O OCl Tetrahedral bent F F Cl F T shape Practice Problem 10 2 o Predict the molecular geometry and bond angles in SiF5 1 9 Practice Problem 10 3 o Predict the molecular geometry and bond angles in ClO2F Multiple Central Atoms Many molecules have larger structures with many interior atoms 10 Practice Problem 10 4 o Predict the molecular geometries in H3BO3 Polarity of Molecules For a molecule to be polar it must have a net dipole moment Electronegativity difference along the bond non zero dipole moment o Have polar bonds along the bond o Unsymmetrical shape Polarity affects the intermolecular forces of attraction o Intermolecular forces of attraction between molecules of the same substance determines boiling melting point 11 o Intermolecular forces of attraction between molecules of different substances determines solubility Nonbonding pairs affect molecular polarity Strong pull in its direction o H Cl bond polar Bonding electrons are pulled toward the Cl end of the molecule Net result polar molecule Vector Addition 12 Molecule Polarity O C bond polar Bonding electrons are pulled equally toward both O ends of the molecule Net result nonpolar molecule 13 H O bond polar Both sets of bonding electrons
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