Chemical bonding Structure determines properties Properties of molecular substances depend on structure of the molecule Bonding theory should allow you to predict shapes of molecules Skeletal arrangement Kind of bonding Shape of molecule Lewis theory predicts electron groups Predicts there are regions of electron groups in an atom Some regions result from placing shared pairs of valence electrons between bonding nuclei Other result from placing unshared electrons on single nucleus Electron groups in an atom should repel each other Electron groups around central atom are most stable when far apart as possible valence shell electron pair repulsion theory Resulting geometric arrangement allow us predict shape and bond angles in molecule Vespr theory Electron groups Lewis theory predicts number of valence electron pairs around central atoms Each lone pair constitutes one electron group on central atom Each bond constitutes one electron group on central atom regardless of single double or triple bond Three electron groups on N Electron group geometry 5 basic arrangements of electron groups around central atom assuming max of six bonding electron groups each of these ve arrangements result from ve different basic electron geometries For molecules that exhibit resonance it doesn t matter which resonance form you use electron geometry will be same Linear electron geometry Two electron groups around central atom Bond angle is 180 degrees trigonal planar electron geometry Three electron groups around central atom will result in triangle around central atom Bond angle is 120 degrees Tetrahedral electron geometry 4 groups around central atom Bond angle is 109 5 degrees Trigonal bipyramidal electron geometry 5 groups around central atom shape of two tetrahedra are baseto bases Bond angle between equatorial is 120 degrees and between axial and equatorial is 90 degrees positions above and below central atom are axial positions Positions in same baseline as central atom are called equatorial positions Octahedral electron geometry Six electron groups around central atom occupy positions in shape of two square base pyramids that are base to base with central atom in center of shared bases all positions equivalent bond angle is 90 degrees Molecular geometry The actual geometry of the molecule may be different from electron geometry Lone pairs affect molecular geometry They occupy space on central atom by are not seen as points on molecular geometry When electron groups are attached to atoms of different size or when bonding to one atom is different than bonding of another atom geometry is distorted Bent molecular geometry derivative of trigonal planar electron geometry When there are three electron groups around central atom and one is lone pair resulting shape of molecule is not trigonal planar but bent shape Bond angle is less than 120 degrees Pyramidal and bent molecular geometries derivatives of tetrahedral electron geometry Derivatives of trigonal bipyramidal electron geometry 4 electron groups and one is lone pair result is pyramid 2 lone pairs tetrahedral bent shape 5 electron groups and some lone pairs the lone pairs will occupy equatorial positions because there is more room One lone pair seesaw shape Two lone pairs T shape Three lone pairs linear shape Derivatives of octahedral geometry One is lone pair square pyramid Two lone pair square planar Predicting the shapes around central atoms Example predict geometry and bond angles of PCl3 Draw Lewis structure Determine number of electron groups around central atom Classify each electron group as bonding or lone pair and count each type Use table 10 1 to determine shape and bond angles 26 valence e Draw Lewis structure Determine number of electron groups around central atom Four electron groups around P Three bonding groups One lone pair Classify electon groups Determine shape and bond angles Tetrahedral Multiple central atoms Describe shape around each central atom in sequence Polarity of molecules Boiling points and solubilities Polar molecule must Have polar bonds Have unsymmetrical shape Polarity affects intermolecular forces of attraction Linear non polar Bent polar Trigonal planar non polar Tetrahedral non polar Trigonal pyramidal polar Predicting polarity of molecules Draw Lewis dot structure and determine molecular geometry Determine whether bonds in molecule are polar Determine whether polar bonds cancel out Does not give numerical predictions Can t be used to get actual angle Problems with Lewis theory Valence bond theory Utilizes principles of quantum mechanics to predict molecular shapes Bonds between atoms occur when orbitals on those atoms interact to make a bond The kind of interaction depends on whether orbitals align along axis between nucleus or outside axis Orbital interaction As two atoms approach the half lled valence atomic orbitals interact to form molecular orbitals Molecular orbitals are regions of high probability of nding shared electrons in molecule Molecular orbitals would be more stable than separate atomic orbitals Valence bond theory hybridization Valence atomic orbitals could hybridize before bonding takes place One hybridization of C is to mix all 2s and 2p orbitals Valence bond theory main points Orbitals can be standard s p d f orbitals or may be hybrid combos Chemical bond results when these atomic orbitals interact and there is total of two electrons in new atomic orbital Degenerate orbitals sp sp2 sp3 sp3d sp3d2 Same type of atom can have different types of hybridization Number of standard atomic orbitals combined number of hybrid orbitals formed Need one that yields lowest overall energy for that molecule Atom w four electron groups around it Atom uses hybrid orbitals for all bonds and lone pairs Tetrahedral geometry 109 5 degree angles between hybrid orbitals Hybridization Sp3 hybridization Carbon hybridizations Unhybridized Sp hybridized Sp2 hybridized Sp3 hybridized Bonding with valence bond theory Bonding takes place between atoms when their atomic or hybrid orbitals interact or overlap Sp2 To interact the orbitals must either be aligned along axis between the atoms or the orbitals must be parallel to each other and perpendicular to interatomic axis Types of bonds A sigma bond results when interacting atomic orbitals point along axis connecting 2 bonding nuclei A pi bond results when bonding atomic orbits are parallel to each other and perpendicular to axis connecting two bonding nuclei Sigma