Читать книгу Materials for Biomedical Engineering - Mohamed N. Rahaman - Страница 50

The type of primary bond formed between two atoms depends on their position in the periodic table of elements. Ionic bonding involves transfer of electrons from one atom to the other, resulting in the formation of a positive ion (cation) and a negative ion (anion) as, for example, the creation of sodium (Na⁺) and chlorine (Cl⁻) ions to form a sodium chloride molecule (NaCl) (Figure 2.1a). The transferred electrons reside in the outermost shell of the anion and, thus, are almost bound to that ion. Covalent bonding, on the other hand, involves sharing of electrons between two atoms but the shared electrons are bound between the atoms as, for example, in the chlorine molecule (Cl₂) (Figure 2.1b). In the metallic bond, electrons tend to leave the parent atoms, making them cations, and combine to form a sea of highly mobile electrons, as illustrated in Figure 2.1c for sodium (Na). In contrast to the ionic bond, the electrons that leave the metal atoms are not transferred to any one ion. The electrons involved in the formation of primary bonds are typically those in the outermost orbital of the atoms, referred to as valence electrons.

Guidelines have been proposed to predict the type of primary bond that two atoms will form. We provide a description of a few of these guidelines.