Fluorides gaseous molecule

There is little new to be said about the bonding capacity of a lithium atom. With just one valence electron, it should form gaseous molecules LiH and LiF. Because of the vacant valence orbitals, these substances will be expected only at extremely high temperatures. These expectations are in accord with the facts, as shown in Table 16-1, which summarizes the formulas and the melting and boiling points of the stable fluorides of the second-row elements. In each case, the formula given in the table is the actual molecular formula of the species found in the gas phase. 

For the Madelung part of the lattice energy, in the case of a salt for which the structure is accurately known, we employed the method of E. F. Bertaut as modified by D. H. Templeton, and evaluated other terms as set out in Ref. 82. When applied to salts that dissociate easily into gaseous molecules, this provided us, via the vant Hofi relationship (see Refs. 80, 82, and 105) with evaluation of the enthalpies of ionization, AH gg [EFa (g) (EFi i)+(g) -I- F (g)] for a variety of F donor molecules, and the fluoride-ion affinities for well known F acceptors, e.g. Ap(BF3) = — AH29g [BF3(g) + F (g) BF4 (g)]. Such evaluations help to provide a more quantitative evaluation of possible reaction chemistry. The intercalation of graphite by one-electron oxidizers and by fluoride-ion acceptors, as covered in Chap. 10, provides examples. Such evaluations are also useful in assessing the likelihood of the existence of salts not yet known, such as [ArFJ fMFg] (see Ref. 92). 

In Eq. (5) a and ac stand, as in Eq. (4), for the gaseous ions and its main idea is that in the molecule formation these polarizabilities are decreased for anions, increased for cations. It was surprising that the correction factors, which were derived from the experimental p values for the extreme cases of Lil and CsF, proved to be satisfactory also for the other 7 molecules, the largest deviation (in %) being — 3.9 for KCl, the average one 1.3. The result of the appUcation of Eq. (5) to the other 4 fluorides is shown in line 5 of Table 3. The deviation from the experimental p is, relative to Eq. (4), more diminished for LiF and NaF than increased for KF and RbF and the average deviation (in %) for the 5 fluorides decreases from 4.5 to 3.0. 

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