ionic compound sodium chloride is formed. In this case, ionic bond formation occurs due to the characteristic ability of chlorine to attract one extra electron and of sodium to donate one electron. This electron transfer is energetically the most favourable bond formation process between these elements and therefore is its driving force (Fig. 1.10b). This concept of ionic bonding works very well to describe the attraction forces of compounds formed by metallic elements on the left side of the periodic table, e.g. sodium (Na), potassium (K), magnesium (Mg) or calcium (Ca), with non-metals such as fluorine (F) or chlorine (Cl) on the right side (Atkins and Jones 2002) (see Table 1.2, p. 8).

Up to this point, only two extreme types of chemical bonds have been described. In a pure covalent bond, two identical atoms share electrons equally. The bond results from the mutual attraction by the two nuclei of the shared electrons (Zumdahl and Zumdahl 2000). A pure covalent bond is always non-polar. In a pure ionic bond, the atoms involved are so different that one or more electrons are completely transferred from one partner to the other. Charged particles – ions – are formed. The oppositely charged ions attract each other strongly. They are forced into regularly spaced patterns and in the solid state form crystals. Ionic bonds that are created by ionic particles of opposite charge can therefore be regarded as an extreme case of polarity.

Ionic and covalent chemical bonds do not only occur in pure forms. There are many intermediate bonding situations where the atoms involved are not so different that electrons are transferred, but are different enough that unequal sharing of the binding electrons occurs. In such a case, a so-called polar covalent bond is formed. It can be regarded as a covalent bond having partial ionic character due to the fact that one of the partners involved has a higher attraction for electrons than the other. In other words, it accumulates more electrons in its vicinity. This property of an element is called electronegativity, a property of an atom that was proposed 1932 by the American chemist Linus Pauling (1901–1994) as a improvement of the valence bond theory (Pauling 1988). Electronegativity is the ability of an atom to attract