0.6 Molecular geometry and electron domain theory

Foundation

We begin by assuming a Lewis structure model for chemical bonding based on valence shell electron pair sharing and the octet rule. We thusassume the nuclear structure of the atom, and we further assume the existence of a valence shell of electrons in each atom whichdominates the chemical behavior of that atom. A covalent chemical bond is formed when the two bonded atoms share a pair of valenceshell electrons between them. In general, atoms of Groups IV through VII bond so as to complete an octet of valence shellelectrons. A number of atoms, including C, N, O, P, and S, can form double or triple bonds as needed to complete an octet. We know thatdouble bonds are generally stronger and have shorter lengths than single bonds, and triple bonds are stronger and shorter than doublebonds.

Goals

We should expect that the properties of molecules, and correspondingly the substances which they comprise,should depend on the details of the structure and bonding in these molecules. The relationship between bonding, structure, andproperties is comparatively simple in diatomic molecules, which contain two atoms only, e.g. $H\mathrm{Cl}$ or $O_2$ . A polyatomic molecule contains more than two atoms. An example of the complexities which arise with polyatomic moleculesis molecular geometry: how are the atoms in the molecule arranged with respect to one another? In a diatomic molecule, only a singlemolecular geometry is possible since the two atoms must lie on a line. However, with a triatomic molecule (three atoms), there aretwo possible geometries: the atoms may lie on a line, producing a linear molecule, or not, producing a bent molecule. In moleculeswith more than three atoms, there are many more possible geometries. What geometries are actually observed? What determineswhich geometry will be observed in a particular molecule? We seek a model which allows us to understand the observed geometries ofmolecules and thus to predict these geometries.

Once we have developed an understanding of the relationship between molecular structure and chemical bonding, wecan attempt an understanding of the relationship of he structure and bonding in a polyatomic molecule to the physical and chemicalproperties we observe for those molecules.

Observation 1: geometries of molecules

The geometry of a molecule includes a description of the arrangements of the atoms in the molecule. At asimple level, the molecular structure tells us which atoms are bonded to which. At a more detailed level, the geometry includesthe lengths of all of these bonds, that is, the distances between the atoms which are bonded together, and the angles between pairsof bonds. For example, we find that in water, $H_{2}O$ , the two hydrogens are bonded to the oxygen and each O-H bond lengthis 95.72pm (where $1\mathrm{pm}=10^{-12}m$ ). Furthermore, $H_{2}O$ is a bent molecule, with the H-O-H angle equal to 104.5°. (The measurement of these geometric properties is difficult,involving the measurement of the frequencies at which the molecule rotates in the gas phase. In molecules in crystalline form, thegeometry of the molecule is revealed by irradiating the crystal with x-rays and analyzing the patterns formed as the x-raysdiffract off of the crystal.)