Lithium electron affinity

Self-Test 1.15A Account for the large decrease in electron affinity between lithium and beryllium. 

Which element of each of the following pairs has the higher electron affinity (a) oxygen or fluorine (b) nitrogen or carbon (c) chlorine or bromine (d) lithium or sodium ... 

Return to the case of LiF. Lithium ionizes readily, but has little affinity for electrons (I = ionization energy = 5.4 eV and A = electron affinity = 0eV.). On the other hand, fluorine is difficult to ionize, but has considerable electron affinity (I = 17.4eV. and A = -3.6eV.). Thus, when Li and F atoms are close neighbors, electrons can transfer to make Li+ and I. These then attract electrostatically until compression of their ion-cores prevent them from contracting further. In a solid crystal, there are both attractive +/- pairs, and repulsive (+/+ as well as -/-) pairs. However, for large arrays, there is a net attraction. This can be shown most simply by examining a linear chain of +q, and -q charges (Kittel, 1966). 

Figure A.l 1 shows the change in density of states due to chemisorption of Cl and Li. Note that the zero of energy has been chosen at the vacuum level and that all levels below the Fermi level are filled. For lithium, we are looking at the broadened 2s level with an ionization potential in the free atom of 5.4 eV. The density functional calculation tells us that chemisorption has shifted this level above the Fermi level so that it is largely empty. Thus, lithium atoms on jellium are present as Li, with 8 almost equal to 1. Chemisorption of chlorine involves the initially unoccupied 3p level, which has the high electron affinity of 3.8 eV. This level has shifted down in energy upon adsorption and ended up below the Fermi level, where it has become occupied. Hence the charge on the chlorine atom is about-1.

The enthalpy of reaction 2.45 cannot be determined directly. As shown in figure 2.5, it is calculated by using several experimental quantities the standard enthalpy of formation of the solid alkoxide, the standard sublimation enthalpy and the ionization energy of lithium, and the standard enthalpy of formation and the adiabatic electron affinity of gaseous methoxy radical (equation 2.47). 

The determination of electron affinities (EAs) is one of the most serious problems in quantum chemistry. While the Hartree-Fock electron affinity can be easily evaluated, most anions turn out to be unbound at this level of theory. Thus, the correlation effects are extremely crucial in evaluating EAs. At this point, lithium hydride and lithium hydride anion make up a very good benchmark system because they are still small enough yet exhibit features of more complicated systems. Four and five electrons, respectively, give rise to higher-order correlation effects that are not possible in H2. 

Wc have seen, tn Chapter 2, that platinum hexafluoride has an electron affinity more than twice as great us fluorine. Yet when lithium metal reacts with platinum hexafluoride, the crystalline product is Li F. not Li PlFJ- Explain... 

Flowever, the electrons of a covalent bond are not necessarily shared equally by the bonded atoms, especially when the affinities of the atoms for electrons are very different. Thus, carbon-fluorine and carbon-lithium bonds, although they are not ionic, are polarized such that the electrons are associated more with the atom of higher electron affinity. This is usually the atom with the higher effective nuclear charge. 

We will investigate the stability of the anionic lithium clusters in Section 5. A relevant quantity is the dependence of the binding energy per atom on the number of atoms, as well as the electron affinity of neutral Lin clusters. From the latter, we evaluate whether these neutral clusters are able to receive an extra electron and to form an anionic system. 

Adiabatic electron affinities (EAt) (kcal/mol) of the small neutral lithium clusters. 

Calculate the difference between the ionization energy of lithium and the electron affinity of bromine. Deduce whether the transfer of an electron from one to the other in the gas phase is spontaneous. 

The small affinities of lithium and sodium are of little importance, but copper, silver and gold, with completed d shells, possess marked electron affinity and whereas the alkaline earth metals, with completed s levels, have negative electron affinities, mercury, with a completed d and s level, has a high positive electron affinity. 

Electron-donating substituents on the aromatic rings decrease the electron affinity of the oligosilanes. l-Chloro-2-[p-(dimethyIamino)phenyl]tet-ramethyldisilane thus reacts with lithium to form exclusively the tetrasi-lane that is stable in the presence of excess Li metal [Eq. (45)]. 

Only in the lithium catalysed copolymerization of styrene with the dienes in hydrocarbons do the monomers show an unexpected order of reactivity, an anomaly which disappears for polar solvents. The preference for the diene in the copolymer vanishes even in hydrocarbon solvents if sodium is used as initiator [231]. With the dienes, however, an added complication exists which makes simple experiments on electron affinity measured in solvents such as dioxane—water a poor guide to reactivity. They can react in more than one way to give a 3,4 (or 1,2) structure or alternatively a 1,4 structure. There appears to be good correlation between the amount of styrene in the copolymer and the percentage of... 

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