Читать книгу Materials for Biomedical Engineering - Mohamed N. Rahaman - Страница 58

Two factors are mainly responsible: (i) the charge of the ions and (ii) the size of the ions, taken as the ionic radius, which determines the equilibrium interatomic spacing.

1 Charge of the ions: The sodium atom (atomic number = 11; ground state configuration = 1s22s22p63s1) loses one electron to become a sodium ion (Na+) while the chlorine atom (atomic number = 17; ground state configuration = 1s22s22p63s23p5) gains one electron to become a chlorine ion (Cl−). In comparison, the magnesium atom (atomic number = 12; ground state configuration = 1s22s22p63s2) loses two electrons to become a magnesium ion (Mg2+) while the oxygen atom (atomic number = 8; ground state configuration = 1s22s22p4) gains two electrons to become an oxygen ion (O2−). The attractive potential energy (Eq. (2.3)) between the ions depends on the product of the ionic charges. For NaCl, this is (1 × 1) whereas for MgO, this is (2 × 2). Thus, for a given distance of separation, the attractive potential energy for the MgO bond is four times that for the NaCl bond.

2 Size of the ions: The electronic configuration of the Na+ and Mg2+ ions are the same (1s22s22p6). On the other hand, the Mg2+ ion has a higher nuclear charge than the Na+ ion and, thus, it has a smaller ionic radius due to the greater attraction between the nucleus and the electrons remaining in the ion. The O2− ion has an electronic configuration (1s22s22p6) composed of one less shell than the Cl− ion (1s22s22p63s23p6). Although the oxygen atom gains two electrons to form an O2− ion whereas the chlorine atom gains one electron to form a Cl− ion, it might be expected that the O2− ion will have a somewhat smaller radius due to it having one less electron shell. Thus, the equilibrium interatomic separation of the molecule, taken approximately as the sum of the ionic radii, is smaller in MgO than in NaCl.

We see from Eq. (2.3) that the potential energy of attraction is proportional to the product of the charges and inversely proportional to the interatomic distance. Thus, based on the ionic charges and interatomic separation, the attractive potential energy at a separation approximating the equilibrium separation distance for MgO is at least four times higher than for NaCl. Although we have not taken into account the repulsive potential energy, Figure 2.2 indicates that it is far smaller than the attractive potential energy. Overall, it will take a much larger amount of energy to disrupt the MgO bond and, thus, the melting point of MgO is much higher than that of NaCl.