The chemical elements in soil minerals occur typically as charged species arranged in spatial configurations held together by ionic bonds. Ionic bonds involve atoms that retain their unique “electron clouds” and, therefore, they are weaker than covalent bonds, which involve significant mixing of the electron clouds of the bonding atoms, leading to the electron sharing that makes covalent bonds stronger. However, ionic and covalent bonds are idealizations that real chemical bonds only approximate. A real chemical bond shows some degree of ionic character, which maintains the electronic identity of the bonding partners, and some degree of electron sharing, which blurs their electronic identity. The Si—O bond, for example, is said to be an even partition between ionic and covalent character, and the Al—O bond is thought to be about 40% covalent and 60% ionic. Aluminum, however, is exceptional. Almost all the metal–oxygen bonds that occur in soil minerals are ionic. For example, Mg—O and Ca—O bonds are considered to be 75% to 80% ionic whereas Na—O and K—O bonds are 80% to 85% ionic. Covalence thus plays a minor role in determining the atomic structures of soil minerals, aside from the important feature that Si—O bonds, being 50% covalent, impart mineral resistance to weathering, as discussed in Section 1.3. Given this perspective on bonding, the two most important properties of the chemical elements in soil minerals should be their ionic valence and radius. Valence is the ratio of the electric charge on an ionic species to the charge on the proton. Ionic radius is a less direct concept, because the radius of a single ion cannot be measured. Accordingly, ionic radius is a defined quantity based on the following three assumptions: (1) the radius of the bivalent oxygen ion (O2-)in all minerals is 0.140 nm, (2) the sum of radii of the cation and anion participating in a chemical bond equals the measured interatomic distance between the two, and (3) the ionic radius has the same value in all mineral structures containing an ion with a given coordination number (CN).
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