0.7 Covalent bonding and electron pair sharing  (Page 16/8)

Introduction

The previous concept studies have provided us a detailed model for the structure and energetics of each atom of each element. This model is extremely useful in helping us understand how the chemical and physical properties of the elements are related to the properties of the individual atoms.

Chemistry, though, is mostly about molecules and how they react with one another, so we now turn out attention to understanding molecules. We should begin by reviewing what we know so far. Recall that there are over 50 million known compounds in our world, each made up from less than roughly 90 commonly occurring elements. From the atomic molecular theory, we understand what it means to form a compound from its component elements. A compound consists of identical molecules, with each molecule made up of the atoms of the elements in a simple whole number ratio. We call this ratio of atoms the molecular formula, and from our work on mass ratios, we also know how to determine what the molecular formula is for any compound of interest. From our work on chemical algebra, we can quantify chemical reactions, determining how much product can be produced from a given amount of reactant.

However, there are a great many fundamental questions about molecules we have not addressed or answered. Perhaps most interestingly, although we know what the molecular formula is for any compound, we don’t know what determines the numbers of atoms which combine to form a molecule. Some combinations are observed (e.g. H ₂ O, H ₂ O ₂ ) and others are never observed (e.g. H ₆ O, HO ₆ ). We need to understand the principles which govern what combinations will work to form stable molecules and what combinations will not.

In order to answer these questions, we will need to develop an understanding of the forces which hold molecules together. Since atoms are neutral, the forces cannot simply be attractions of oppositely charged atoms. We know that there are diatomic molecules like H ₂ and O ₂ . Why would identical atoms attract each other? Our knowledge of the charges contained inside atoms will be very helpful in understanding forces which bond atoms together. These forces must also be essential in determining the reactivity of a molecule, since these forces will determine how readily the atoms can be separated and recombined with atoms from other molecules. Predicting the reactivity of a substance is one of the great powers of Chemistry. If we wish to predict chemical reactivity of a particularly substance, we must understand the forces which bond atoms together in a molecule.

Foundation

In this study, we will assume that we know the postulates of the Atomic Molecular Theory and our measurements of relative atomic masses. We know that an element is composed of individual atoms with identical masses, and we know that the atoms of different elements have different masses, which have been measured. From these masses, we can determine the molecular formula of any substance or compound of interest. As such, we’ll assume that these are known.