Nonmetal electron affinities

Elements at the right of the p block have characteristically high electron affinities they tend to gain electrons to complete closed shells. Except for the metalloids tellurium and polonium, the members of Groups 16/VI and 17/VII are nonmetals (Fig. 1.62). They typically form molecular compounds with one another. They react with metals to form the anions in ionic compounds, and hence many of the minerals that surround us, such as limestone and granite, contain anions formed from non-metals, such as S2-, CO,2-, and S042-. Much of the metals industry is concerned with the problem of extracting metals from their combinations with nonmetals. 

As we have seen, several atomic properties are important when considering the energies associated with crystal formation. Ionization potentials and heats of sublimation for the metals, electron affinities, and dissociation energies for the nonmetals, and heats of formation of alkali halides are shown in Tables 7.1 and 7.2. 

Nonmetals follow the general trends of atomic radii, ionization energy, and electron affinity. Radii increase to the left in any row and down any column on the periodic table. Ionization energies and electron affinities increase up any column and towards the right in any row on the periodic table. The noble gases do not have electron affinity values. Ionization energies are not very important for the nonmetals because they normally form anions. Variations appear whenever the nonmetal has a half-filled or filled subshell of electrons. The electronegativity... 

B) Metals have ionization energies (or ionization potentials) that are much lower than those of the nonmetals since a smaller amount of energy is required to remove an electron from a metal than from a nonmetal. Removal of an electron(s) is called oxidation. Nomnetals, on the other hand, have high electron affinities compared to metals and tend to gain electrons. Gain of electrons is called reduction. 

We assume that we have a solid metal M which reacts with a diatomic, gaseous nonmetal X2 (e.g., CI2, F2, 02). Similar cycles can be written for solid elements such as sulfur as the nonmetal. In either case, before we can connect U with AHf we must form gaseous ions of M and X. We need not only the relevant ionization potentials (IP) and electron affinities (EA), but also the heats of atomization of solid M and gaseous X2. These atomization energies are traditionally referred to as heats of sublimation AHsuh of M(s) and of dissociation AHd ss of X2. For NaCl itself, we have... 

The alkali halogenide gas molecules MX present a still more extreme case, the bonds being essentially ionic with only a small amount of covalent character. For cesium chloride, involving the most electropositive of the metals and one of the most electronegative of the nonmetals, the electron affinity of the nonmetal (86 kcal/mole) is about as great as the ionization potential of the metal (89 kcal/mole), so that at large intemuclear distances the ionic structure Cs+Cl is about as... 

Electron affinity is conventionally defined as the energy released when an electron is added to the valence shell of an atom. Unfortunately, this is in contradiction to the universal thermodynamic convention that enthalpies of exothermic reactions shall be assigned negative signs. Since it seems impossible to overthrow the election affinity convention at this late date without undue confusion, one can adopt one of two viewpoints to minimize confusion. One is to let the electron affinities of the most active nonmetals be positive, even though in thermodynamic calculations the enthalpies are negative ... 

FIGURE 2.1 Considerable energy is needed to produce cations and anions from neutral atoms the ionization energy of the metal atoms must be supplied, and it is only partly recovered from the electron affinity of the nonmetal atoms. The overall lowering of energy that drags the ionic solid into existence is due to the strong attraction between cations and anions that occurs in the solid. 

Step 3 Form the gaseous anion of the nonmetal. This step releases an energy equal to the electron affinity of the element. If the electron affinity is... 

The ionization energy of M, or the dissociation energy and electron affinity of the nonmetal, for the incorporation of either component in the crystal lattice. 

The bond strength" thus obtained refers, of course, to the dissociation of the ion pair to the separated ions, M X - M + X . It is somewhat easier to dissociate an ion pair into the uncharged constituent atoms, M X" — M + X, because the ionization energy of the metal is greater than the electron affinity of the nonmetal. 

Ionize (anion) Attach electron(s) to the gaseous nonmetal atom to form the gaseous anion. Several electron affinity values may be required. Recall that for electron gain, A//= - ea for each electron added. 

We have seen that the metallic character of the elements decreases from left to right across a period and increases from top to bottom within a group. On the basis of these trends and the knowledge that metals usually have low ionization energies while nonmetals usually have high electron affinities, we can frequently predict the outcome of a reaction involving some of these elements. 

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