Electron affinity for

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. 

Some physical constants for selenium are given in Table 1. More extensive data and many sources are available (1 5). For a selenium atom, the covalent radius is ca 0.115 nm, the electron affinity for two electrons is ca —2.33 eV, ie, energy absorbed, and the first ionization potential is 9.75 eV. 

The ground-state electronic structure of As, as with all Group 15 elements features 3 unpaired electrons ns np there is a substantial electron affinity for the acquisition of 1 electron but further additions must be effected against considerable coulombic repulsion, and the formation of As is highly endothermic. Consistent with this there are no ionic compounds containing the arsenide ion and... 

Figure 5.3 Reactivity scale as function of electron affinity for dianhydrides and ionization potential for diamines.

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... 

Finally, the expression for the electron affinity for the uptake of one electron into the singly occupied orbital (m) should be mentioned (106) ... 

C08-0066. According to Appendix C, each of the following elements has a positive electron affinity. For each one, constmct its valence orbital energy level diagram and use it to explain why the anion is unstable N, Mg, and Zn. 

Table 1. Values of electron affinity for few aromatic hydrocarbons andfor graphite. 

After an electron is added to an atom, the "affinity" that it has for the electron is known as the electron affinity. Because energy is released when an electron is added to most atoms, it follows that to remove the electron would require energy, so the quantity is positive for most atoms. The electron affinities for most of the main group elements are shown in Table 1.1. It is useful to remember that 1 eV per atom is equal to 96.48kj/mol. 

The electron affinities for the halogens are the highest of any group of elements. 

In fact, it may be impossible to measure the heat associated with an atom gaining two electrons, so the only way to obtain a value for the second electron affinity is to calculate it. As a result, the Born-Haber cycle is often used in this way, and this application of a Born-Haber cycle will be illustrated later in this chapter. In fact, electron affinities for some atoms are available only as values calculated by this procedure, and they have not been determined experimentally. 

If we compare the values of the first ionization energy and electron affinity for the Period 3 elements, we have... 

F (g) —> F(g) + e To accomplish this, we need to calculate Zerr for the species in the left hand column and plot the number of protons in the nucleus against Zeff. By extrapolation, we can estimate the first ionization energy for F (g). The electron affinity for F is equal to the first ionization energy of F multiplied by minus one (i.e., by reversing the ionization reaction, one can obtain the electron affinity). 

The Electron affinity for F must equal the reverse of the first ionization energy or -654.5 kJ. The actual experimental value found for the first electron affinity of F is -328 kJ/mol. 

The Electron affinity for O must equal the reverse of the first ionization energy, or - 449 kJ. Again we will use the same method of solution as was used for part (a). To find the electron affinity for the process N(g) + e —> N (g), we first need to calculate the I.E. for N (g). This is accessible from a plot of the number of protons in the nucleus vs. Zeff for the four species in the last column. 

Z. Peng and M.E. Galvin, Polymers with high electron affinities for light-emitting diodes, Chem. Mater., 10 1785-1788, 1998. 

The G2 set. Calculations of ionization energies and electron affinities for molecules and ions from the G2 set [47] were performed with P3 methods. The diversity of bonding in this set presents a convenient standard for testing the new methodology introduced here, such as electron affinity formulae and procedures for electron binding energies of open-shell systems. 

Electron affinities for 35 substituted nitrobenzenes have been reported and provided a comprehensive data set for the examination of substituent effects38. The data were used to derive Taft gas-phase substituent parameters and discussed qualitatively based on frontier orbital molecular theory38. The rate constants for the exo-energetic electron-transfer reactions were found to be close to those predicted by the ADO (average dipole orientation) theory38. 

Electron affinities for the main group elements. Negative values mean that anions form with the release of energy. Positive values mean that energy is ahsorhed in order to form the anion. The positive values are estimated, because the anions formed for atoms of groups 2 and 18 are unstable. 

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