Bonding - Geol 318: Mineralogy El-Shazly, A. K., 2003 1...

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Geol 318: Mineralogy El-Shazly, A. K., 2003 1 IIb-Bonding Bonding is a process of combining two or more elements to form a compound. This process usually results in attaining a lower energy for the whole system than previously existed before the combination of the elements. There are several types of bonds, the most important of which (from a mineralogical point of view) are: 1- Ionic bonds: (a.k.a. electrostatic bonds): Involves loss of one or more electrons from the atom of the electropositive element to become a cation , and gain of the same electrons by the atom of the electronegative element to become the anion (Fig. 1). Importance of the ionization potential and electron affinity values for the combining elements (please pay special attention to the relationship between the atomic number and the first ionization potential as discussed in our last lecture!). Ionic bonding takes place (or prevails) when the differences in electronegativities of the two atoms to be bonded are large. The % of ionic character for any bond is therefore determined by the difference in electronegativity values for the two elements in question (Fig. 2). Figure 3 shows the range in bond character observed in common minerals with bonds of “mixed character”. For ionic bonding to take place, the attractive forces between the cation and anion should at least neutralize the repulsive forces between like ions (cf. p. 47 of your text, as well as Fig. 4; noting the slight difference! ). This means that the sizes of the individual ions play a critical role in controlling this type of bond, as well as the resulting internal structure of the compound. The sizes of the individual ions control the structure of the crystal through determining the “ coordination number ” for each ion. This will be discussed in more detail later. Because the resulting compound has to be electrically neutral (this applies to all
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Bonding - Geol 318: Mineralogy El-Shazly, A. K., 2003 1...

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