0.8 Molecular structures  (Page 24/5)

Introduction

We have developed a model by which we can understand molecular formulas. In the previous concept development study, we began by asking what determines which combinations of atoms will form stable molecules and compounds and why some combinations are never observed. We combined our knowledge of the electronic structures of atoms with our knowledge of molecular formulas and the common valences of atoms to develop the octet rule. The rule tells us that the most common valence of each main group atom is equal the number of spaces for electrons in the valence shell of the atom, which is eight minus the number of valence electrons. Thus, according to the octet rule, atoms tend to bond such that they have eight valence electrons in the bonded molecule. The Lewis structure model implements the octet rule and reveals which atoms are bonded to which other atoms in a molecule and whether these bonds are single, double, or triple bonds. Thus, Lewis structures provide additional information about molecules that we cannot learn just from molecular formulas.

This model of molecular structure, like most good scientific models, poses at least as many new questions as the questions it was designed to answer. What new information can be gleaned from our knowledge of the structure of a molecule? It seems reasonable for us to assume that the properties of a compound are related to the properties of the individual molecules of the compound. It also seems reasonable to assume that the properties of individual molecules are related to their structures. If we could identify the properties of molecular structures which are related to their chemical or physical properties, we could understand these properties and perhaps even design molecules which have interesting or valuable properties. This is one of the very most important questions in chemistry. Before we can pursue it, we need more detail and insight about molecular structures.

In this concept development study, we will examine various arrangements of atoms in a variety of molecules and look for common structures. We will find both variety and common features. We will also examine how the electrons are arranged in these molecular structures. This will allow us to interpret molecular structures accurately.

Foundation

We will assume that we know the valences of the common main group elements, and from this, we know the octet rule. This assumes knowledge of the valence shell model of the electronic structure of atoms. We know that we can combine these atoms in the Lewis structure model to build molecular structures which satisfy the valences of the main group elements and fit the octet rule for valence electrons. We have found that molecules which have Lewis structures that fit the octet rule typically form stable compounds. And we found that, for combinations of atoms where we cannot construct Lewis structures that fit the octet rule, the compounds are either unstable or non-existent.

By comparing our molecular structures to experimental data, we observed that double bonds are stronger and shorter than single bonds, and triple bonds are stronger and shorter than double bonds.