Lecture Presentation Chapter 9 Molecular Geometries and Bonding Theories John D Bookstaver St Charles Community College Cottleville MO 2012 Pearson Education Inc Molecular Shapes The shape of a molecule plays an important role in its reactivity By noting the number of bonding and nonbonding electron pairs we can easily predict the shape of the molecule 2012 Pearson Education Inc Molecular Geometries and Bonding What Determines the Shape of a Molecule Simply put electron pairs whether they be bonding or nonbonding repel each other By assuming the electron pairs are placed as far as possible from each other we can predict the shape of the molecule 2012 Pearson Education Inc Molecular Geometries and Bonding Electron Domains We can refer to the electron pairs as electron domains In a double or triple bond all electrons shared between those two atoms are on the same side of the central atom therefore they count as one electron domain Molecular Geometries and Bonding The central atom in this molecule A has four electron domains 2012 Pearson Education Inc Valence Shell Electron Pair Repulsion Theory VSEPR The best arrangement of a given number of electron domains is the one that minimizes the repulsions among them 2012 Pearson Education Inc Molecular Geometries and Bonding Electron Domain Geometries Table 9 1 contains the electron domain geometries for two through six electron domains around a central atom Molecular Geometries and Bonding 2012 Pearson Education Inc Electron Domain Geometries All one must do is count the number of electron domains in the Lewis structure The geometry will be that which corresponds to the number of electron domains 2012 Pearson Education Inc Molecular Geometries and Bonding Molecular Geometries The electron domain geometry is often not the shape of the molecule however The molecular geometry is that defined by the positions of only the atoms in the molecules not the nonbonding pairs Molecular Geometries and Bonding 2012 Pearson Education Inc Molecular Geometries Within each electron domain then there might be more than one molecular geometry 2012 Pearson Education Inc Molecular Geometries and Bonding Linear Electron Domain In the linear domain there is only one molecular geometry linear NOTE If there are only two atoms in the molecule the molecule will be linear no matter what the electron domain is 2012 Pearson Education Inc Molecular Geometries and Bonding Trigonal Planar Electron Domain There are two molecular geometries Trigonal planar if all the electron domains are bonding Bent if one of the domains is a nonbonding pair 2012 Pearson Education Inc Molecular Geometries and Bonding Nonbonding Pairs and Bond Angle Nonbonding pairs are physically larger than bonding pairs Therefore their repulsions are greater this tends to decrease bond angles in a molecule 2012 Pearson Education Inc Molecular Geometries and Bonding Multiple Bonds and Bond Angles Double and triple bonds place greater electron density on one side of the central atom than do single bonds Therefore they also affect bond angles 2012 Pearson Education Inc Molecular Geometries and Bonding Tetrahedral Electron Domain There are three molecular geometries Tetrahedral if all are bonding pairs Trigonal pyramidal if one is a nonbonding pair Bent if there are two nonbonding pairs Molecular Geometries and Bonding 2012 Pearson Education Inc Trigonal Bipyramidal Electron Domain There are two distinct positions in this geometry Axial Equatorial 2012 Pearson Education Inc Molecular Geometries and Bonding Trigonal Bipyramidal Electron Domain Lower energy conformations result from having nonbonding electron pairs in equatorial rather than axial positions in this geometry 2012 Pearson Education Inc Molecular Geometries and Bonding Trigonal Bipyramidal Electron Domain There are four distinct molecular geometries in this domain Trigonal bipyramidal Seesaw T shaped Linear 2012 Pearson Education Inc Molecular Geometries and Bonding Octahedral Electron Domain All positions are equivalent in the octahedral domain There are three molecular geometries Octahedral Square pyramidal Square planar Molecular Geometries and Bonding 2012 Pearson Education Inc Larger Molecules In larger molecules it makes more sense to talk about the geometry about a particular atom rather than the geometry of the molecule as a whole 2012 Pearson Education Inc Molecular Geometries and Bonding Polarity In Chapter 8 we discussed bond dipoles But just because a molecule possesses polar bonds does not mean the molecule as a whole will be polar 2012 Pearson Education Inc Molecular Geometries and Bonding Polarity By adding the individual bond dipoles one can determine the overall dipole moment for the molecule 2012 Pearson Education Inc Molecular Geometries and Bonding Polarity 2012 Pearson Education Inc Molecular Geometries and Bonding Overlap and Bonding We think of covalent bonds forming through the sharing of electrons by adjacent atoms In such an approach this can only occur when orbitals on the two atoms overlap 2012 Pearson Education Inc Molecular Geometries and Bonding Overlap and Bonding Increased overlap brings the electrons and nuclei closer together while simultaneously decreasing electron electron repulsion However if atoms get too close the internuclear repulsion greatly raises the energy 2012 Pearson Education Inc Molecular Geometries and Bonding Hybrid Orbitals Consider beryllium In its ground electronic state beryllium would not be able to form bonds because it has no singly occupied orbitals 2012 Pearson Education Inc Molecular Geometries and Bonding Hybrid Orbitals But if it absorbs the small amount of energy needed to promote an electron from the 2s to the 2p orbital it can form two bonds 2012 Pearson Education Inc Molecular Geometries and Bonding Hybrid Orbitals Mixing the s and p orbitals yields two degenerate orbitals that are hybrids of the two orbitals These sp hybrid orbitals have two lobes like a p orbital One of the lobes is larger and more rounded as is the s orbital 2012 Pearson Education Inc Molecular Geometries and Bonding Hybrid Orbitals These two degenerate orbitals would align themselves 180 from each other This is consistent with the observed geometry of beryllium compounds linear 2012 Pearson Education Inc Molecular Geometries and Bonding Hybrid Orbitals With hybrid orbitals the orbital diagram for beryllium would look like this Fig 9 15 The sp orbitals are higher in energy than the 1s orbital but lower than