ABSTRACT
Covalent bonds involve the sharing of outer-shell electrons between two or more atoms, so that the individual atoms lose their discrete nature (Karplus and Porter, 1970; Atkins, 1994). The number of electrons in the outer shell of an atom governs its valency, that is, the optimum number of covalent bonds it can form with other atoms. Covalent bonds may
be saturated or unsaturated depending on the number of electrons involved. Unsaturated bonds tend to be shorter, stronger, and more rigid than saturated bonds (Israelachvili, 1992; Atkins, 1994). The distribution of the electrons within a covalent bond determines its polarity. When the electrons are shared equally among the atoms the bond has a nonpolar character, but when the electrons are shared unequally the bond has a polar character. The polarity of a molecule depends on the symmetry of the various covalent bonds that it contains (see Section 2.3.2.). Covalent bonds are also characterized by their directionality, that is, their tendency to be directed at clearly defined angles relative to each other. The valency, saturation, polarity, strength, and directionality of covalent bonds determine the three-dimensional structure, flexibility, chemical reactivity, and physical interactions of molecules.
