Alkaline earth metals, electron affinity

As a consequence of its closed-shell electron configuration, zinc has a negative electron affinity, that is, the removal of an electron from Zn is exothermic. The electronegativity of zinc (1.588 PU) is intermediate between those of the alkaline earth metals and the first row transition metals and remarkably similar to that of beryllium (1.57 PU). 

The reaction involves the transfer of an electron from the alkali metal to naphthalene. The radical nature of the anion-radical has been established from electron spin resonance spectroscopy and the carbanion nature by their reaction with carbon dioxide to form the carboxylic acid derivative. The equilibrium in Eq. 5-65 depends on the electron affinity of the hydrocarbon and the donor properties of the solvent. Biphenyl is less useful than naphthalene since its equilibrium is far less toward the anion-radical than for naphthalene. Anthracene is also less useful even though it easily forms the anion-radical. The anthracene anion-radical is too stable to initiate polymerization. Polar solvents are needed to stabilize the anion-radical, primarily via solvation of the cation. Sodium naphthalene is formed quantitatively in tetrahy-drofuran (THF), but dilution with hydrocarbons results in precipitation of sodium and regeneration of naphthalene. For the less electropositive alkaline-earth metals, an even more polar solent than THF [e.g., hexamethylphosphoramide (HMPA)] is needed. 

The alkaline earth metals and noble gases are the only groups of elements with negative (first) electron affinities. ... 

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. 

The oxides of the alkaline earth metals crystallize in a sodium chloride lattice although in SrO and BaO the radius ratio is greater than 0 732. It has been proposed that the crystals are constructed from the ions M + and the electron affinity of the oxygen atom calculated on this assumption by the Born-Haber cycle for the different oxides give rather... 

In the sulphides, selenides, tellurides and arsenides, all types of bond, ionic, covalent and metallic occur. The compounds of the alkali metals with sulphur, selenium and tellurium form an ionic lattice with an anti-fluorite structure and the sulphides of the alkaline earth metals form ionic lattices with a sodium chloride structure. If in MgS, GaS, SrS and BaS, the bond is assumed to be entirely ionic, the lattice energies may be calculated from equation 13.18 and from these values the affinity of sulphur for two electrons obtained by the Born-Haber cycle. The values obtained vary from —- 71 to — 80 kcals and if van der Waal s forces are considered, from 83 to -- 102 kcals. 

Why are the electron affinities of the alkaline earth metals, shown in Table 8.4, either negative or small positive values ... 

Explain why alkali metals have a greater affinity for electrons than alkaline earth metals. 

The elements most likely to form ionic compounds have low ionization energies (such as the alkali metals and the alkaline earth metals, which form cations) or high electron affinities (such as the halogens and oxygen, which form anions). 

J.C. Wheeler (1997) Journal of Chemical Education, vol. 74, p. 123 Electron affinities of the alkaline earth metals and the sign convention for electron affinity . 

Gajewski studied the electronic structure and geometry of 18-crown-6 (C12H24O6), hexaaza[18]annulene (CnH Ne), and their complexes with cations of the heavier alkali (Rb+ and Cs" ") and alkaline earth metals (Sr + and Ba +) [245]. She showed that the ions bind more strongly to hexaaza[18]annulene than to 18-crown-6, with affinity greater by 8-23 kcal/mol. Gajewski extended her work on host-guest chemistry to complexes between ethylenediamine tetraacetate and alkali (Na+, K+, and Rb+) and alkaline earth cations (Mg +, Ca +, and Sr +) [246]... 

Mineral acids such as hydrochloric acid or nitric acid are employed as eluents, regardless of whether the background conductivity is chemically suppressed or electronically compensated. While alkali metak are eluted within 10 min under the chromatographic conditions in Figure 4.2, alkaline-earth metals are strongly retained due to their high affinity toward the stationary phase. When the samples to be analyzed contain alkaline-earth metals, too, alkali metal analysis will be interfered with by late-eluting alkaline-earth metals. 

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