0.6 Molecular structure and physical properties  (Page 7/5)

Foundation

We begin with our knowledge of the structure and properties of atoms. We know that atoms have a nuclearstructure, meaning that all of the positive charge and virtually all of the mass of the atom are concentrated in a nucleus which isa very small fraction of the volume of the atom. In addition, we know that many of the properties of atoms can be understood by amodel in which the electrons in the atom are arranged in “shells” about the nucleus, with each shell fartherfrom the nucleus that the previous. The electrons in outer shells are more weakly attached to the atom than the electrons in theinner shells, and only a limited number of electrons can fit in each shell. Within each shell are subshells, each of which can alsohold a limited number of electrons. The electrons in different subshells have different energies and different locations formotion about the nucleus. We also assume a knowledge of the a Lewis structure model for chemical bonding based on valence shell electron pair sharing and the octet rule. Acovalent chemical bond is formed when the two bonded atoms share a pair of valence shell electrons between them. In general, atoms ofGroups IV through VII bond so as to complete an octet of valence shell electrons. We finally assume the Electron Domain Model for understanding and predicting molecular geometries. The pairs of valence shell electrons arearranged in bonding and non-bonding domains, and these domains are separated in space to minimize electron-electron repulsions. Thiselectron domain arrangement determines the molecular geometry.

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. Now that we have developed an understanding of therelationship between molecular structure and chemical bonding, we analyze physical properties of the molecules and compounds of thesemolecules to relate to this bonding and structure. Simple examples of physical properties which can be related to molecular propertiesare the melting and boiling temperatures. These vary dramatically from substance to substance, even for substances which appearsimilar in molecular formulae, with some melting temperatures in the hundreds or thousands of degrees Celsius and others well below0°C. We seek to understand these variations by analyzing molecular structures.

To develop this understanding, we will have to apply more details of our understanding of atomic structure andelectronic configurations. In our covalent bonding model, we have assumed that atoms “share” electrons to form a bond.However, our knowledge of the properties of atoms reveals that different atoms attract electrons with different strengths,resulting in very strong variations in ionization energies, atomic radii, and electron affinities. We seek to incorporate thisinformation into our understanding of chemical bonding

Observation 1: compounds of groups i and ii

We begin by analyzing compounds formed from elements from Groups I and II ( e.g. sodium and magnesium). These compounds are not currently part of our Lewis structure model. For example, Sodium, with a singlevalence electron, is unlikely to gain seven additional electrons to complete an octet. Indeed, the common valence of the alkali metalsin Group I is 1, not 7, and the common valence of the alkaline earth metals is 2, not 6. Thus, our current model of bonding doesnot apply to elements in these groups.