0.9 Bonding in metals and metal-non-metal salts  (Page 15/9)

Introduction

We have noted that the Lewis model of chemical bonding is very powerful in predicting structures, stability, and reactivity of molecules. But there is a glaring hole in our model that you may have noticed: the metal elements are missing. Additionally, the Lewis model only applies to a handful of atoms at a time, and we have not examined what happens in solids that have huge numbers of atoms bonded in vast networks.

The Lewis model is based on the “octet rule” and the concept of a covalent bond as a sharing of an electron pair. These were developed based on the molecules formed by elements in Groups 4 to 8, and most specifically, the group of elements we call the “non-metals.” This name clearly says that the properties of the non-metal elements are very different from the properties of metal elements. We will look at these differences in this study. But even without analyzing those differences, we can say immediately that the octet rule does not seem to apply to these elements. Remember that the octet rule says that the number of valence electrons plus the valence of the atom (the number of bonds the atom typically forms) commonly equals 8 for compounds formed by the non-metal elements. Rather than being the general rule for metals, this is very rarely true. This means that we need a new model for bonding in metals and in compounds that contain metal atoms.

To develop this model, we will examine the specific properties of metallic elements, which differ significantly from the non-metals. By considering these properties carefully, we will be able to build a model which accounts for these properties.

Of course, to be more complete, we also need to consider compounds formed from combinations of metal atoms and non-metal atoms. These also have properties which differ greatly from either metals or non-metals. Again, by looking closely at these properties, we will be able to build a model for metal-non-metal bonding, which is different from that in metal bonding.

This means that we will develop models of two new types of bonding in addition to the one we have already developed for covalent bonding. It would be very helpful to find a way to tie these three types of bonding together, to give a simple understanding of why the bonding is different for different types and combinations of atoms. In the last section of this study, we will create such a model based on our understanding of the chemical concept of “electronegativity,” developed in the previous concept study.

Foundation

In this study, we will assume that we know the essential components of the structure and properties of individual atoms. Each atom has an electronic configuration which determines its physical and chemical properties, including ionization energy, electron affinity, atomic size, and electronegativity. Electron motion is described by orbitals, which give the probability for the electron in space around the nucleus. The energy of each electron is determined by a combination of its kinetic energy, its attraction to the nucleus, and its repulsion from other electrons in the atom. Our model considers the electron-electron repulsion as a “shielding” of the positive charge of the nucleus, resulting in an effective nuclear charge which is less than the actual nuclear charge, which we refer to as the core charge. By looking at the core charge experienced by an electron in an atom and at its distance from the nucleus, we can understand the ionization energy of that electron. We know and can account for the fact that the ionization energies are greatest for atoms near the right side of the periodic table with large core charges. And the ionization energies are greater for smaller atoms, where the valence electrons are closer to the nucleus.