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2.4.1 Ionic Bonding

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The electrons in each ion occupy orbitals of complicated shape but, at an approximate level, we can assume that the ions have a spherical shape. Thus, in ionic bonding, one ion can be attracted to the other in any direction and, for the same interatomic distance, the force of attraction does not depend on the direction. We say that that ionic bond lacks directionality or is nondirectional. In packing to form a three‐dimensional structure, one type of ion will surround itself with as many ions of the other type as far as possible to maximize the attraction between cations and anions and to minimize the repulsion between ions of the same charge (Chapter 3). In this way, the attractive potential energy of the system is maximized. Another feature of ionic bonding is that in the creation of ions, an electron transferred from one atom to the other atom remains localized at this second atom and cannot move freely. These features of the ionic bond provide a basis for understanding the characteristic properties of ionic solids (Section 1.4):

 High strength, high elastic modulus, high melting point, and chemically unreactive due to the strong interatomic bonding

 Low electrical and thermal conductivity due to the localized electrons

 Brittleness: Permanent deformation of an ionic solid due to a large enough mechanical force or stress requires motion of ions from their equilibrium sites to adjacent equilibrium sites. For this to occur, ions in one plane of the solid must move over or near to ions of the same sign in the adjacent plane. This leads to a strong repulsion and to catastrophic fracture of the solid into two or more pieces

 High hardness: As hardness is related to the resistance of the material to be permanently deformed, ionic solids typically show high hardness

Ceramics are held together by ionic or covalent bonds, or by bonds with a mixture of ionic and covalent character. Although they are not useful as biomaterials, sodium chloride (NaCl), and magnesium oxide (MgO) are common examples of ceramics that show ionic bonding. Ceramic biomaterials that show predominantly ionic bonding include aluminum oxide (Al2O3) used as bearings in some hip implants, and the calcium phosphates hydroxyapatite, Ca10(PO4)6(OH)2, and tricalcium phosphate, Ca3(PO4)2, used to repair bone defects (Table 1.1).

Materials for Biomedical Engineering

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