Electron affinity values

The electron affinity values for many of the elements shown in Figure 8-17 appear to lie on the x axis. Actually, these elements have positive electron affinities, meaning the resulting anion is less stable than the neutral atom. Moreover, the second electron affinity of every element is large and positive. Positive electron affinities cannot be measured directly. Instead, these values are estimated by other methods, as we show in Section 8-1. 

Although electron affinity values show only one clear trend, there is a recognizable pattern in the values that are positive. When the electron that is added must occupy a new orbital, the resulting anion is unstable. Thus, all the elements of Group 2 have positive electron affinities, because their valence S orbitals are filled. Similarly, all the noble gases have positive electron affinities, because their valence a p orbitals are filled. Elements with half-filled orbitals also have lower electron affinities than their neighbors. As examples, N (half-filled 2 p orbital set) has a positive electron affinity, and so does Mn (half-filled 3 d orbital set). 

By CNDO calculation BE is an increasing function of cluster size for Ag clusters and for Ni clusters [54]. The calculations for Ni clusters showed that the contribution an atom makes to the total BE is proportional to its coordination number [54]. The orbital energies of Ni follow a smooth function of cluster size. As size increases, LUMO decreases and HOMO increases. This represents a convergence of IP and electron affinity values with increase in size. 

When parameters of the Pariser-Parr-Pople configuration interaction molecular orbital (PPP-CI MO) method were modified so as to reproduce the Aol)s values for l,3-di(5-aryl-l,3,4-oxadiazol-2-yl)benzenes 16 and 17, the calculated HOMO and LUMO energy levels corresponded with the experimental ionization potential and electron affinity values. The relationships between the electrical properties and molecular structures for the dyes were investigated. The absorption maximum wavelengths for amorphous films were found to be nearly equal to those for solution samples <1997PCA2350>. 

The early agreement between calculated and experimental heats for fluorides was fortuitous because the high value given to D(F2) was compensated by the large electron affinity value [A/f F ) = lD(Ft) -.EA(F)]. The drop in value of AH°(F >) over the years (see Table I) vitiates some of the more elaborate lattice-energy calculations and Ka-pustinskii s semiempirical method seems adequate (138), but see reference (126). 

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... 

It is not yet possible to measure lattice energy directly, which is why the best experimental values for the alkali halides, as listed in Table 1.16, are derived from a thermochemical cycle. This in itself is not always easy for compounds other than the alkali halides because, as we noted before, not all of the data is necessarily available. Electron affinity values are known from experimental measurements for... 

FIGURE 2 The electron affinity versus alloy concentration for AlxGai xN alloys. The values were deduced from UV photoemission spectra excited with 21.2 eV light. Note that the technique cannot determine the electron affinity value for cases which exhibit a negative electron affinity. These samples are indicated with points at 0 with an arrow pointing towards larger negative values [6],... 

Positive electron affinity values signify that energy Is liberated. In contrast to Ionization energy values and values of other variables to be Introduced In Chapter 14. 

The transition from the atom to the cluster to the bulk metal can best be understood in the alkali metals. For example, the ionization potential (IP) (and also the electron affinity (EA)) of sodium clusters Na must approach the metallic sodium work function in the limit N - . We previously displayed this (1) by showing these values from the beautiful experiments by Schumacher et al. (36, 37) (also described in this volume 38)) plotted versus N". The electron affinity values also shown are from (39), (40) and (34) for N = 1,2 and 3, respectively. A better plot still is versus the radius R of the N-mer, equivalent to a plot versus as shown in Figure 1. The slopes of the lines labelled "metal sphere" are slightly uncertain those shown are 4/3 times the slope of Wood ( j ) and assume a simple cubic lattice relation of R and N. It is clear that reasonably accurate interpolation between the bulk work function and the IP and EA values for small clusters is now possible. There are, of course, important quantum and statistical effects for small N, e.g. the trimer has an anomalously low IP and high EA, which can be readily understood in terms of molecular orbital theory (, ). The positive trimer ions may in fact be "ionization sinks" in alkali vapor discharges a possible explanation for the "violet bands" seen in sodium vapor (20) is the radiative recombination of Na. Csj may be the hypothetical negative ion corresponding to EA == 1.2 eV... 

The sign convention for electron affinity values follows the convention for energy changes used in Chapters 9 and 10. 

Because two different conventions have been used, there is a good deal of confusion in the chemical literature about the signs for electron affinity values. Electron affinity has been defined in many textbooks as the energy released when an electron is added to a gaseous atom. This convention requires that a positive sign be attached to an exothermic addition of an electron to an atom, which opposes normal thermodynamic conventions. Therefore, in this... 

The electron affinity values for atoms among the first 20 elements that form stable, isolated X ions. The lines shown connect adjacent elements. The absence of a line indicates missing elements (He, Be, N, Ne, Mg, and Ar) whose atoms do not add an electron exothermically and thus do not form stable, isolated X- ions. 

The second electron affinity values for both oxygen and sulfur are unfavorable (endothermic). Explain. 

Sources Ionization energies cited in this chapter are from C. E. Moore, Ionization Potentials and Ionization Limits Derived from the Analyses of Optical Spectra, National Standard Reference Data Series, U.S. National Bureau of Standards, NSRDS-NBS 34, Washington, DC, 1970, unless noted otherwise. Electron affinity values listed in this chapter are from H. Hotop and W. C. Lineberger, J. Phys. Chem. Ref. Data, 1985,14,731. Standard electrode potentials listed in this chapter are from A. J, Bard, R. Parsons, and... 

The enthalpy of formation at O K for HD"(g) is based on the electron affinity value from a Rydberg-Klein-Rees configuration-interaction calculation for H " (1). This study by Sharpe (1) led to EA(H ) = -2.5 eV. Assuming this value is valid for HD (g)... 

The heat of formation at 0 K for Hg"(g) is based on the electron affinity value derived from a Rydberg-Klein-Rees... 

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. 

One of the major applications of the CURES-EC procedure has been in calculating the electron affinities of the carbon clusters C [46]. Carbon clusters are common species in nature, since they are a product of combustion and contribute to environmental problems. They have been observed in the interstellar medium. The fuller-enes represent one of the most recent discoveries of a new form of carbon. Carbon clusters have many forms—linear chains, cyclic compounds and the fullerenes— because carbon can form covalent bonds. The NIST tables give 54 electron affinities values for C 14 for Si 17 for Ge 45 for Sn and 55 for Pb . Only the structures for the carbon compounds from n = 3 to 30 are identified [12]. 

Table 6-2 shows that electron affinity values generally become more negative from left to right across a period, with major exceptions at Groups HA (Be) and VA (N). They generally become more negative from bottom to top. 

Three different experimental electron affinity values have been reported for C4 (3.7, 3.88, and 3.79 eV). Exactly which measure of EA is being sampled is debatable, but it appears to be closest to AEA. The higher precision value of 3.88 eV would appear to offer the best comparison. Allowing for a correction of 0.15 eV from VEDE values as obtained in the PVTZ UHF-CCSD(T) results, the larger, more diffuse basis values cluster near 3.8 eV for this quantity. 

Larger molecules, with more degrees of freedom and longer activated complex lifetimes, should be more amenable to this type of analysis. However, as seen in the case of trifluoride, the vibrational and rotational constants of all product species have a significant influence and must be reasonably well known to derive useful electron affinity values. Few experimental frequencies are known for the larger anionic products, and theoretical methods are not well developed for frequency calculations on halide anions. Testing of various computational techniques to provide the necessary information is underway in collaboration with Professor T. M. Gilbert, also at Northern Illinois University. 

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