0.4 Covalent bonding and electron pair sharing  (Page 142/8)

Foundation

We begin with our understanding of the relationship between chemical behavior and atomic structure. Thatis, we assume the Periodic Law that the chemical and physical properties of the elements are periodic functions of atomic number.We further assume the structure of the atom as a massive, positively charged nucleus, whose size is much smaller than that ofthe atom as a whole, surrounded by a vast open space in which move negatively charged electrons. These electrons can be effectivelypartitioned into a core and a valence shell, and it is only the electrons in the valence shell which are significant to thechemical properties of the atom. The number of valence electrons in each atom is equal to the group number of that element in thePeriodic Table.

Goals

The atomic molecular theory is extremely useful in explaining what it means to form a compound its componentelements. That is, a compound consists of identical molecules, each comprised of the atoms of the component elements in a simple wholenumber ratio. However, the atomic molecular theory also opens up a wide range of new questions. We would like to know what atomicproperties determine the number of atoms of each type which combine to form stable compounds. Why are some combinations observed andother combinations not observed? Some elements with very dissimilar atomic masses (for example, iodine and chlorine) form very similarchemical compounds, but other elements with very similar atomic masses (for example, oxygen and nitrogen) form very dissimilarcompounds. What factors are responsible for the bonding properties of the elements in a similar group? In general, we need to knowwhat forces hold atoms together in forming a molecule.

We have developed a detail understanding of the structure of the atom. Our task now is to apply thisunderstanding to develop a similar level of detail about how atoms bond together to form molecules.

Observation 1: valence and the periodic table

To begin our analysis of chemical bonding, we define the valence of an atom by its tendencies to form molecules. The inert gases do not tend to combine with any other atoms. Wethus assign their valence as 0, meaning that these atoms tend to form 0bonds. Each halogen prefers to form molecules by combining with a single hydrogen atom ( e.g. $HF$ , $H\mathrm{Cl}$ ). We thus assign their valence as 1, also taking hydrogen to alsohave a valence of 1. What we mean by a valence of 1 is that these atoms prefer to bind to only one other atom. The valence of oxygen,sulfur, etc. is assigned as 2, since two hydrogens are required to satisfy bonding needs of these atoms. Nitrogen, phosphorus, etc.have a valence of 3, and carbon and silicon have a valence of 4. This concept also applies to elements just following the inertgases. Lithium, sodium, potassium, and rubidium bind with a single halogen atom. Therefore, they also have a valence of 1.Correspondingly, it is not surprising to find that, for example, the combination of two potassium atoms with a single oxygen atomforms a stable molecule, since oxygen's valence of 2 is be satisfied by the two alkali atoms, each with valence 1. We canproceed in this manner to assign a valence to each element, by simply determining the number of atoms to which thiselement's atoms prefer to bind.