Ionization potentials of alkali atoms

Experimental and calculated ionization potentials of alkali atoms (cm ). Errors of calculated values with respect to experiment are given. 

When is an atom heavy Ionization potentials of alkali atoms... 

Eliav, E., Vilkas, M.J., Ishikawa, Y., Kaldor, U. Ionization potentials of alkali atoms towards meV accuracy. Chem. Phys. 311, 163-168 (2005)... 

With a few exceptions, the ionization potential of an atom increases within a given period with increase in atomic number. Because of this phenomenon, the alkali and alkaline earth metals form ionic compounds with electronegative ligands such as halogens, while the metalloids and nonmetals form compounds with considerable covalent character. Exceptions to the 8 rule are encountered most frequently in ionic compounds as well as in compounds of metalloids and nonmetals. In the latter case, compounds with more than eight valence electrons may be formed. These deviations are a function of properties of the central atom and the ligands. 

Background alkali metal chemistry. The alkali metals have the lowest ionization potentials of any group in the periodic table and hence their chemistry is dominated by the M+ oxidation state. However, it has been known for some time that a solution of an alkali metal (except lithium) in an amine or ether forms not only M+ ions and solvated electrons but also alkali anions of type M (Matalon, Golden Ottolenghi, 1969 Lok, Tehan Dye, 1972). That is, although an alkali metal atom very readily loses its single s-shell electron ... 

AHa for the Adsorption of Alkali Metals. If an alkali metal atom is located at an infinite distance from a metal surface at zero potential, then the heat of adsorption comprises the work done in (1) transferring an electron from the atom to the metal, and (2) bringing the positive ion to its equiUbrium distance from the metal surface (127). In the first step, the energy change is (e0 — el), where is the work function of the metal and I is the ionization potential of the alkali metal atom. In the second, the force of attraction on the positive ion at a distance d from the metal surface, i.e., the electrostatic image force, is e /4d hence, the heat Uberated is e /4do, where do is the equilibrium distance of the adsorbed ion from the metal surface. This distance is often assumed to be equal to the ionic radius, which is 1.83 A. for the Na ion. The initial heat of adsorption, therefore, is... 

This is the famous Saha-Langmuir equation. In it, g+/g0 is the ratio of the statistical weights of the ionic and atomic states, is the work function of the surface, / is the first ionization potential of the element in question, k is the Boltzmann constant, and T is the absolute temperature. Note that gjg0 is close to 1 for electronically complex elements for simpler elements it can take on a variety of values depending on how many electronic states can be populated in the two species for alkali atoms, for example, it is often Vi. Attainment of thermodynamic equilibrium was assumed in the derivation of this equation, and it is applicable only to well-defined surfaces. 

The lattice enthalpy U at 298.20 K is obtainable by use of the Born—Haber cycle or from theoretical calculations, and q is generally known from experiment. Data used for the derivation of the heat of hydration of pairs of alkali and halide ions using the Born—Haber procedure to obtain lattice enthalpies are shown in Table 3. The various thermochemical values at 298.2° K [standard heat of formation of the crystalline alkali halides AHf°, heat of atomization of halogens D, heat of atomization of alkali metals L, enthalpies of solution (infinite dilution) of the crystalline alkali halides q] were taken from the compilations of Rossini et al. (28) and of Pitzer and Brewer (29), with the exception of values of AHf° for LiF and NaF and q for LiF (31, 32, 33). The ionization potentials of the alkali metal atoms I were taken from Moore (34) and the electron affinities of the halogen atoms E are the results of Berry and Reimann (35)4. 

This is a severe drawback in the case of equilibrium studies of metal molecules since, as a rule, such molecules are minor vapor components and maximum sensitivity is required for their thermodynamic evaluation. However, very precise ionization potentials can be measured using photoionization spectroscopy (5,28). Berkowltz (28) reviewed early work concerning alkali metal dimers. Herrmann et al. ( ) have measured the ionization potentials of numerous sodium, potassium and mixed sodium-potassium clusters. For most of these clusters the atomization energies of the neutral molecules are not known. Therefore, the dissociation energies of the corresponding positive ions cannot be calculated. 

The parameters given in Table I are based on the following IP = ionization potential of the alkali atom (14) EA = electron affinity of the hydrogen atom (1 ) Dg = bonding energy of the potential well (1 ) Rg = internuclear distance at the minimum of the potential curve. 

This equation permits the calculation of the lattice energy if we know F, the electron affinity of the halogen atom, S, the heat of sublimation of the alkali metal, /, the ionization potential of the metal and Z), the dissociation energy of the molecular halogen. These quantities are known, but to different orders of accuracy, and furthermore, the values should all refer to the same standard temperature, either absolute zero or room temperature, a condition which is not always fulfilled. However, the agreement between the calculated and observed values is sufficiently good to indicate that the theory developed for the lattice energy on the basis of ionic interaction is basically correct. 

Ionization interferences occur most commonly for alkali and alkaline earth metals. The low ionization potential of these metals can lead to their ionization in the relatively hot environment of the flame. If this occurs, no absorption signal is detected, since FAAS is a technique for measuring atoms not ions. This process can be prevented by the addition of an ionization suppressor or buffer , e.g. an alkali metal such as Cs. Addition of excess Cs... 

The alkaline earth atoms Ca, Sr, Ba have low ionization potentials, / 5-6 eV [however /(Mg) = 7-5 eV], which do not greatly exceed those of the alkali atoms. Indeed, the ionization potential of barium, /(Ba) = 5-2 eV, is less than that of lithium, /(Li) = 54 eV. Thus we might anticipate that the reactions of alkaline earth atoms would resemble those of alkali atoms, particularly in exhibiting electron jump transitions. However, the alkaline earth atoms are divalent and two valence electrons can potentially be transferred. This should introduce interesting new features into the reaction dynamics. A particularly close analogy might thus be expected with the reactions of alkali dimers discussed in Section II. 

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