Electron affinity of halogen atoms

Work on the direct determination of the electron affinities of halogen atoms was begun by Mayer,7 who, with his students, measured directly the equilibrium constant for dissociation of alkali halogenide gas molecules into ions or of gas halogenide ions into atoms and electrons. Other methods have also been used, especially some involving mass... 

Calvin continued his studies at the University of Alinnesota, where he investigated the electron affinities of halogen atoms. He received a Ph.D. degree in 1935. As a Rockefeller Foundation fellow at the University of Manchester in England (1935-1937), Calvin worked with Alichael Polanyi, who introduced him to the interdisciplinary approach, on coordination catalysis, the activation of molecular hydrogen, and metalloporphyrins. In 1937 he joined the faculty of the University of California, Berkeley, as an instmctor, and remained there for the balance of his career. 

Dissociation of hydrogen halides in plasma with production of hydrogen and halogens can successfully compete with electrolysis and in some cases can be very attractive for practical applications. The experimental data related to these processes, however, are not sufficient today for clear conclusions to be drawn about their detailed mechanism, kinetics, and energy efficiency. The dissociative attachment of electrons to hydrogen halides is usually very fast (see Chapter 2) because of the high electron affinity of halogen atoms ... 

The trend in oxidation potentials may be considered a composite trend, similar to that described for the E° values of the alkali metals (Chap. 6). For the halogens, the following quantities are involved heats of dissociation of the molecules, electron affinities of the atoms, hydration energies of the ions, heats of vaporization (for bromine and iodine only), and, finally, entropy or randomness effects. Aside from the entropy effects (which turn out to be quite small for the reactions being considered), the reduction of the halogen X to the hydrated ion X at room temperature may be represented in steps as follows ... 

The tables in Appendix IV summarize the evaluated values of the electron affinities given in this book. The electron affinities of the atoms and homonuclear diatomic molecules are given in two tables, Al.l and A1.2. The references for both tables are combined. The electron affinities of the hydrocarbons are given in Tables A2.1 and Al.l. Tables A2.3 and A2.4 provide the electron affinities of the halogenated hydrocarbons. The odd-numbered tables are ordered by value and the even-numbered tables are ordered by molecular weight. The references for the hydrocarbons are given separately from those of the CHX compounds. Tables A3.1 and A3.2 list the values for the CHNX molecules. These were combined because there are so few halogenated compounds. Tables A4.1 and A4.2 contain the electron affinities of the CHO and CHOX compounds, while Tables A5.1 and A5.2 contain those of the CHON and CHONX compounds. 

It is illuminating to compare the bond strength of NgNg with isoelectronic non-noble gas ions. Very similar D values are found when the homonuclear cations Ng2 are compared with isoelectronic XJ, especially for halogen anions. There are nearly identical D values for FJ (29.5 kcal/mol) and Nej (31.3 kcal/mol), ClJ (29.0 kcal/mol) and ArJ (29.2 kcal/mol), Br2 (26.5 kcal/mol) and Kr (26.5 kcal/mol), and J2 (24.0 kcal/mol) and XeJ (23.7 kcal/mol) [45]. H2 does not form a stable molecular anion in the gas phase, but taken the D value for Hj (103.2 kcal/mol [45]) and the electron affinity of H atom (17.4 kcal/mol... 

There he worked with George Glockler on the electron affinity of halogens (initially iodine and later bromine and chlorine as well) from space-charge effects—Calvin s problem was to measure the amount of energy released when a halogen atom captures an electron and for that he had first to devise the methods for doing so. 

Since the electron affinities of the atomic halogens are higher and the ionisation energies are lower than the corresponding values for H2, the value of m is determined by the electronic properties of the atomic halogen species (see Appendix III). 

The electronic configuration of each halogen is one electron less than that of a noble gas, and it is not surprising therefore, that all the halogens can accept electrons to form X" ions. Indeed, the reactions X(g) + e - X (g), are all exothermic and the values (see Table 11.1), though small relative to the ionisation energies, are all larger than the electron affinity of any other atom. 

By introducing reasonable values (about 2 for nitrogen, 4 for oxygen) for the electron affinity parameter relative to carbon, 8, and for the induced electron affinity for adjacent atoms (32/8i = Vio), we have shown that the calculated permanent charge distributions for pyridine, toluene, phenyltrimethylammonium ion, nitrobenzene, benzoic acid, benzaldehyde, acetophenone, benzo-nitrile, furan, thiophene, pyrrole, aniline, and phenol can be satisfactorily correlated qualitatively with the observed positions and rates of substitution. For naphthalene and the halogen benzenes this calculation does not lead to results... 

Halogens, the elements in Group 17 of the periodic table, have the largest electron affinities of all the elements, so halogen atoms (a n readily accept electrons to produce halide anions (a a. This allows halogens to react with many metals to form binary compounds, called halides, which contain metal cations and halide anions. Examples include NaCl (chloride anion), Cap2 (fluoride anion), AgBr (bromide anion), and KI (iodide anion). 

A few years ago experimental values were available for Q, S, /, and Z), but not for E the procedure adopted in testing the equation was to use the equation with calculated values of Uq (Equation 13-5) to find E, and as a test of the method to examine the constancy of E for a series of alkali halogenides containing the same halogen. The values obtained in this way were found to be constant to within about 3 kcal/mole. However, later experimental determinations of the values of the electron affinities of the halogen atoms by direct methods have shown that Equation 13-5 for the crystal energy is in general reliable only to about 2 percent. 

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