Magnesium electron affinity

Table 2.6 shows the electron affinities, for the addition of one electron to elements in Periods 2 and 3. Energy is evolved by many atoms when they accept electrons. In the cases in which energy is absorbed it will be noted that the new electron enters either a previously unoccupied orbital or a half-filled orbital thus in beryllium or magnesium the new electron enters the p orbital, and in nitrogen electron-pairing in the p orbitals is necessary. 

Given that the sum of the first two ionisation energies of magnesium is endothermic and that the sum of the first two electron affinities of oxygen is also endothermic, suggest why the reaction between magnesium and oxygen is, nonetheless, exothermic overall. (2)... 

Calculate a value for the electron affinity of oxygen for two electrons. Take magnesium oxide, MgO, as a model and devise a suitable cycle. Use the data given in Table 1.22. 

Sample Why is it that sodium has a negative electron affinity while magnesium has a positive value ... 

Calculate the difference between the sum of the first two ionization energies of magnesium and the sum of the first two electron affinities of oxygen. Deduce whether the transfer of two electrons from one to the other in the gas phase is spontaneous. (The second electron affinity of oxygen is —816kJ/mol.)... 

Fig. 4.9 Energies of free cations and of ionic compounds as a function of the oxidation state of the cation. Top Lines represent the ionization energy necessary to form the +1. +2, +3, and + 4 cations of sodium, magnesium, and aluminum. Note that although the ionization energy increases most sharply when a noble gas configuration is broken, isolated cations are always less stable in Itiifher oxidation states. Bottom Lines represent the sum of ionization energy and ionic bonding energy for hypothetical molecules MX, MXj, MXj, and MX in which the interatomic distance, r, has been arbitrarily set at 200 pm. Note that the most stable compounds (identified by arrows) arc NaX, MgXj, and AlXj. (All of the.se molecules will be stabilized additionally to a small extent by the electron affinity of X.)...

We need to construct a thermodynamic cycle involving a metal oxide for which all quantities except the second electron affinity are either known experimentally or, in the case of lattice energy, available from the Bom-Lande or Kapustinskii equation. Such a cycle for magnesium oxide is shown in Figure 8.6. MgO assumes a rock salt structure, and its lattice energy is obtained from the Born-Lande equation as shown... 

The Born-Haber q de for magnesium oxide, used to calculate the second electron affinity of oxygen. 

Write electron configurations for sodium and magnesium. Briefly explain why the electron affinities of these two elements are not in accordance with the expected general trends. 

Current-potential curves of the O2 reduction were measured [68] in concentrated KOH on alloy electrodes prepared from silver and small amounts of elements that form oxides of low electron affinity. The reactivity of a silver — 1.7w/o (weight percent) magnesium alloy was better than that of pure silver. In contrast, no improvement was found for foils of silver — Iw/o thorium, silver — Iw/o radium, and silver — 1 w/o barium. 

Let us apply these ideas to the third-row elements. On the left side of the table we have the metallic reducing agents sodium and magnesium, which we already know have small affinity for electrons, since they have low ionization energies and are readily oxidized. It is not surprising, then, that the hydroxides of these elements, NaOH and Mg(OH)z, are solid ionic compounds made up of hydroxide ions and metal ions. Sodium hydroxide is very soluble in water and its solutions are alkaline due to the presence of the OH- ion. Sodium hydroxide is a strong base. Magnesium hydroxide, Mg(OH)2, is not very soluble in water, but it does dissolve in acid solutions because of the reaction... 

---