Electron affinity atoms

Electron affinity is the energy change that occurs when an electron is added to a gaseous atom (or ion). Since the electron affinity process usually is exothermic for atoms, most atomic electron affinities are negative. 

Symbol B atomic number 5 atomic weight 10.811 a Group III A (Group 13) metalloid element atomic volume 4.70 cc/g-atom electron affinity 0.277 eV electronic configuration Is22s22pi valence state +3 naturally occurring stable isotopes are B-10 and B-11 and their abundance 19.57% and 80.43%, respectively. 

Halogen, X Atomic ionization potential (eV) Atomic electron affinity (eV) Atomic radius (A) Atomic polarizabilityc (A3) Bond length of x2 molecule, i e(A) Dissociation energy of X2 molecule, Dc (kcal mol-1)... 

The formalisms described in Secs. 8.2 and 8.3 have found several recent applications. Apart from the IP results / 99, iOO, 132/, there have also been calculations of some atomic electron affinities/134/ all of which accompany EE computations. There are a few molecular EE results of N, CO, CH N, CH CO,CH by Pal et aj /139,140/, Rittby et al/141/ and Mattie et a 1/141/. The results so far have been obtained mostly... 

Jorge Ayala determined the rate constants for thermal electron attachment to aliphatic halides and the halogen molecules to confirm values measured by other techniques. The electron affinities of the halogen molecules had been determined by endothermic charge transfer experiments [57-59]. In the case of the halogen molecules, the ECD results lead to the rate constant for thermal electron attachment rather than the electron affinity of the molecule. Two-dimensional Morse potentials for the anions were constructed based on these data. Freeman and Ayala searched for a nonradioactive source for the ECD. In 1975 the data on the electron affinities of atoms were summarized and correlations examined between these values and the position of the atoms in the Periodic Table [60]. A large number of the atomic electron affinities were measured by photoelectron spectroscopy [61]. A similar compilation of the electronegativities of elements was carried out. In this case some of the values were obtained from the work functions of salts [62], These results will be updated in Chapter 8. 

Recent reviews on alkali metal beam studies, theoretical and experimental determinations of electron affinities using photon methods, and atomic electron affinities and an Internet source for electron affinities all give compilations [113-117]. The evaluation of molecular electron affinities is a major objective of this book. 

Figure 5.13 A precision and accuracy plot of the atomic electron affinities determined before 1967 versus the current best values. The deviations from the unit slope and zero intercept line result from random and systematic errors.

The values of the atomic electron affinities determined before 1967 are plotted against the current EvV in Figure 5.13. This is a precision and accuracy (or P and A) plot. The plot of the electron affinities of the charge transfer acceptors (Figure 4.15) was also a P and A plot. It is used to concisely illustrate the quality of the experimental data. By comparing the data to a line with unit slope and zero intercept, an immediate picture of the precision (random) errors and accuracy (bias) errors can be visualized and outliers identified. By inspection the electron affinities of O (2 eV), Cl (4 eV), and F (4 eV) are suspected outliers. The higher values deviate... 

Two general quantum mechanical procedures were applied to the calculation of atomic electron affinities prior to 1960. These were the variational method and Hartree Fock semi-empirical method. Prior to 1960 there were no accurate calculations for elements other than hydrogen. Indeed, only the electron affinities of C and... 

The isoelectronic equivalence is the simplest procedure for estimating electron affinities. It was applied to H2 and I2 and to the atomic electron affinities. Species with the same outer electronic configuration should have similar electron affinities and bond dissociation energies. This results in the relative constancy of the electron affinities of a given family of atoms. The equivalence of the bond dissociation energies for the X2( ) and Rg2(+) ions is also based on this principle. The systematic variation of the electron affinities of the homonuclear diatomic molecules is another example. 

Selection, Assignment, and Correlations of Atomic Electron Affinities... 

The electron affinities Ea of the main group atoms are the most precisely measured values. Recall that the Ea is the difference in energy between the most stable state of the neutral and a specific state of a negative ion. It was once believed that only one bound anion state of atoms and molecules could exist. However, multiple bound states for atomic and molecular anions have been observed. This makes it necessary to assign the experimental values to the proper state. The random uncertainties of some atomic Ea determined from photodetachment thresholds occur in parts per million. These are confirmed by photoelectron spectroscopy, surface ionization, ion pair formation, and the Born Haber cycle. Atomic electron affinities illustrate the procedure for evaluating experimental Ea. 

The precision of a metric is determined by the random uncertainties of a method and the number of replications. The equipment, ability of the investigator, and material investigated affect the random uncertainties. It is important to know the best precision that has been attained and the number of replications used to attain that precision. In establishing the precision, it is assumed there are no systematic uncertainties. In the case of atomic electron affinities the largest systematic uncertainty is the state assignment. 

As new values were obtained, atomic electron affinities were reviewed periodically beginning in 1953 [1-13]. All the available experimental, extrapolated, and theoretical values were tabulated in 1984 [7]. Presently, experimental values are available at the NIST website [12]. Prior to 1970 the majority of the values for the main group elements were determined by the Born Haber cycle, electron impact, or relative and absolute equilibrium surface ionization techniques. However, values for C, O, and S had been measured by photodetachment [1-3]. By the mid-1970s virtually all the Ea of the main group elements in the first three rows had been measured by photon methods [4-7]. By the early 1980s values were obtained for the transition elements by photon techniques [7, 8]. In the 1990s the values of Ca, Sr, and Ba were measured [9-13]. Recently, experimental values have been reported for Ce, Pr, Tm, and Lu [14-17],... 

SELECTION, ASSIGNMENT, AND CORRELATIONS OF ATOMIC ELECTRON AFFINITIES... 

TABLE 8.1 Atomic Electron Affinities Current Best Averages to 1970, Weighted Averages to 1970, and Weighted Average of Photon Values to 1975 [3-5]... 

TABLE 8.2 Atomic Electron Affinities (in eV) Determined by Photodetachment, Photoelectron Spectroscopy and Surface Ionization Techniques [4, 7 12]... 

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