Atomic Structure and Atomic Bonds
Since atomic structure is covered in introductory chemistry courses, the student should be aware that the atom consists of a nucleus containing neutrons and protons around which electrons orbit. Figure 3.1 shows, for example, the electron configuration of a sodium atom, in which the nucleus consists of 11 protons and neutrons and the outer orbits contain 11 electrons. Orbits are more or less confined to certain radii. We will not be concerned here with the subatomic particles such as quarks and neutrinos. The smaller the diameter of the orbit, the greater the attractive force between the electron and the nucleus and the greater the absolute binding energy. Binding energies, by convention, are considered to be negative, so we are speaking of a large negative binding energy for the innermost orbit. The electrons in the outer orbits are bound less tightly, and the outermost electrons can to some extent be considered to be loosely bound and not necessarily residing in well-defined orbits. The outer electrons are the valence electrons and are involved in the bonding together of the atoms and hence strongly affect all physical, mechanical, and chemical properties of materials. The nature of this bond is what determines whether the substance is a metal (iron, aluminum, and so forth), in which the bond is often between like atoms (Al atoms are bonded to Al atoms in metallic Al), a ceramic (e.g., alumina, silicon nitride), or a polymer material (e.g., polyethylene, polycarbonate), where the bond is between dissimilar atoms (Si to N in silicon nitride or carbon to hydrogen in polyethylene). Before describing these various bond types, we will briefly review the electron configuration and the related periodicity of the elements.