chem1030 chapter 9 notes: Molecular Geometries and Bonding Theories

Chapter 9: Molecular Geometries and Bonding Theories  Molecular Shapes o The shape of a molecule plays an important role in its reactivity. o By noting the number of bonding and nonbonding electron pairs we can easily predict the shape of the molecule.  What Determines the Shape of a Molecule? o Simply put, electron pairs, whether they be bonding or nonbonding, repel each other. o By assuming the electron pairs are placed as far as possible from each other, we can predict the shape of the molecule.  Electron Domains o We can refer to the electron pairs as electron domains. o 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.  Valence Shell Electron Pair Repulsion Theory (VSEPR) o “The best arrangement of a given number of electron domains is the one that minimizes the repulsions among them.”  Electron-Domain Geometries o All one must do is count the number of electron domains in the Lewis structure. o The geometry will be that which corresponds to the number of electron domains.  Molecular Geometries o The electron-domain geometry is often not the shape of the molecule, however. o The molecular geometry is that defined by the positions of only the atoms in the molecules, not the nonbonding pairs. o Within each electron domain, then, there might be more than one molecular geometry.  Linear Electron Domain o 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.  Trigonal Planar Electron Domain o There are two molecular geometries:  Trigonal planar, if all the electron domains are bonding,  Bent, if one of the domains is a nonbonding pair.  Nonbonding Pairs and Bond Angle o Nonbonding pairs are physically larger than bonding pairs. o Therefore, their repulsions are greater; this tends to decrease bond angles in a molecule.  Multiple Bonds and Bond Angles o Double and triple bonds place greater electron density on one side of the central atom than do single bonds. o Therefore, they also affect bond angles.  Tetrahedral Electron Domain o 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.  Trigonal Bipyramidal Electron Domain o There are two distinct positions in this geometry:  Axial  Equatorial o Lower-energy conformations result from having nonbonding electron pairs in equatorial, rather than axial, positions in this geometry. o There are four distinct molecular geometries in this domain:  Trigonal bipyramidal  Seesaw  T-shaped  Linear  Octahedral Electron Domain o All positions are equivalent in the octahedral domain. o There are three molecular geometries:  Octahedral  Square pyramidal  Square planar  Larger Molecules o 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. o This approach makes sense, especially because larger molecules tend to react at a particular site in the molecule.  Polarity o Just because a molecule possesses polar bonds does not mean the molecule as a whole will be polar. o By adding the individual bond dipoles, one can determine the overall dipole moment for the molecule.  Overlap and Bonding o We think of covalent bonds forming through the sharing of electrons by adjacent atoms. o In such an approach this can only occur when orbitals on the two atoms overlap. o Increased overlap brings the electrons and nuclei closer together while simultaneously decreasing electron-electron repulsion. o However, if atoms get too close, the internuclear repulsion grea