Monatomic ions formation

One factor that favors an atom of a representative element forming a monatomic ion in a compound is the formation of a stable noble gas electron configuration. Energy considerations are consistent with this observation. For example, as one mole of Li from Group LA forms one mole of Li+ ions, it absorbs 520 kj per mole of Li atoms. The IE2 value is 14 times greater, 7298 kj/mol, and is prohibitively large for the formation of Li + ions under ordinary conditions. For LE+ ions to form, an electron would have to be removed from the filled first shell. We recognize that this is unlikely. The other alkali metals behave in the same way, for similar reasons. 

The hydration shell formation was estimated from the translational entropy loss, 0.615.S °transi. of inert gas atoms isoelectronic with monatomic ions on dissolution in water, but 0 < ) < 1 was an unknown numerical coefficient. Tables of values of Astmc for many cations and anions, based on 6 °°(H+, aq) = —8.8 J mol were shown in Krestov s book (Krestov 1991). 

In the previous section, we described the formation of ions from atoms. Often you can understand what monatomic ions form by looking at the electron configurations of the atoms and deciding what configurations you would expect for the ions. 

The formation of a monatomic ion from an atom can be expressed in a chemical equation. For example, a magnesium atom forms a magnesium ion by losing two electrons ... 

We have been discussing the formation of monatomic ions as neutral atoms that gain or lose electrons. For most elements this is not a common event, but an accomplished fact. The natural occurrence of many elements is in ionic compounds—compounds made up of ions—or solutions of ionic compounds. Nowhere in natme, for example, are sodium or chlorine atoms to be found, but there are large natural deposits of sodium chloride (table salt) that are made up of sodium ions and chloride ions. The compound, along with other ionic compounds, may also be obtained by evaporating seawater, which contains the ions in solution. 

Let us return to simple electrode reactions. Large displacements of heavy particles should occur during the discharge of simple monatomic ions resulting in the formation of adsorbed atoms. These displacements are of the order of the difference between ionic and covalent (metallic) radii. 

The standard free energy of formation of a gaseous metal ion, AGj (M+, g), can be viewed as the sum of the standard free energy of sublimation, AGj (M , g), and the free energy of ionization, AG j. The standard entropy of a monatomic gas is very nearly equal to the standard entropy of its corresponding monatomic gaseous... 

The enthalpies of formation of aqueous ions may be estimated in the manner described, but they are all dependent on the assumption of the reference zero that the enthalpy of formation of the hydrated proton is zero. In order to study the effects of the interactions between water and ions, it is helpful to estimate values for the enthalpies of hydration of individual ions, and to compare the results with ionic radii and ionic charges. The standard molar enthalpy of hydration of an ion is defined as the enthalpy change occurring when one mole of the gaseous ion at 100 kPa (1 bar) pressure is hydrated and forms a standard 1 mol dm-3 aqueous solution, i.e. the enthalpy changes for the reactions Mr + (g) — M + (aq) for cations, X (g) — Xr-(aq) for monatomic anions, and XOj (g) —< XO (aq) for oxoanions. M represents an atom of an electropositive element, e.g. Cs or Ca, and X represents an atom of an electronegative element, e.g. Cl or S. 

The conventional standard entropy of an monatomic aqueous anion is calculated from the change in standard entropy for its formation and the standard entropies of the element and that of dihydrogen, as shown in the following worked example for the aqueous chloride ion. 

The basis of the estimations of the absolute enthalpies of hydration of the main group ions is dealt with extensively in Chapter 2. In this section, the same principles are applied to the estimation of the enthalpies of hydration of the monatomic cations of the transition elements, i.e. those of the ions M" +. The standard enthalpies of formation of the aqueous ions are known from experimental measurements and their values, combined with the appropriate number of moles of dihydrogen oxidations to hydrated protons, gives the conventional values for the enthalpies of hydration of the ions concerned. Table 7.4 contains the Gibbs energies of formation and the enthalpies of formation of some ions formed by the first-row transition elements, and includes those formed by Ag, Cd, Hg and Ga. 

The kinetics by which UPD layers form are qualitatively the processes already discussed. There are the electron transfer kinetics from the metal substrate to the depositing ion and the surface diffusion of the adions formed to edge sites on terraces. Complications occur, however, for there is the adsorption of ions to take care of and that brings up questions of which isotherm to use (Section 6.8). Three kinds of UPD formations are shown in Fig. 7.146. Thus Fig. 7.146 (c) shows ID phase formation along a monatomic step in the terraces on the single ciystal Fig. 7.146 (b) shows 2D nucleation at a step, and Fig. 7.146 (a) shows 2D nucleation on an atomically flat plane. 

The present paper reviews and assesses the current information about the formation and identification of X , AZ , and other doubly-charged negative ion species that have been reported. It also presents mechanisms and explanations for the production of the monatomic, diatomic and polyatomic doubly-charged negative ions that have been observed. Finally, some potentially fruitful paths for the further study of these ions are indicated. 

The discovery of doubly-charged negative ions was first reported in 1966 by Stuckey and Kiser The species observed were the monatomic 0, F, Cl , and Br , and the diatomic CN ions. Their studies also included an estimate of the lifetime of these species based on the ion flight times and a discussion of the electronic configurations and possible modes of formation of the observed ions. 

Since the formation of a wide variety of doubly-charged negative monatomic ionic species is possible with a Penning ion source, and since nA beam currents of the T species may be obtained after momentum/charge and kinetic energy analysis with magnetic and electric sectors, photodetachment studies of the species... 

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