Fluorine ionization energy

Bafighian ED, Liebman JF (2002) How anomalous tire the anomalous properties of fluorine Ionization energy and electron affinity revisited. J Fluor Chem 116 35-39... 

Until about 40 years ago, these elements were referred to as "inert gases" they were believed to be entirely unreactive toward other substances. In 1962 Neil Bartlett, a 29-year-old chemist at the University of British Columbia, shook up the world of chemistry by preparing the first noble-gas compound. In the course of his research on platinum-fluorine compounds, he isolated a reddish solid that he showed to be 02+(PtFB-). Bartlett realized that the ionization energy of Xe (1170 kJ/mol) is virtually identical to that of the 02 molecule (1165 kJ/mol). This encouraged him to attempt to make the analogous compound XePtF6. His success opened up a new era in noble-gas chemistry. 

Experiment shows that a gaseous fluorine atom can acquire an electron to form a stable ion, F (g). We can discuss the energy of formation of this ion in the same way that we treated ionization energies. The first ionization energy of fluorine atom is the energy required to remove an electron from a neutral atom in the gas phase. We shall call this energy Ei. Then the heat of reaction can be written in terms of Ei. 

Thus we can expect a stable molecular species, LiF. The term stable again means that energy is required to disrupt the molecule. The chemical bond lowers the energy because the bonding electron pair feels simultaneously both the lithium nucleus and the fluorine nucleus. That is not to say, however, that the electrons are shared equally. After all, the lithium and fluorine atoms attract the electrons differently. This is shown by the ionization energies of these two atoms ... 

The trend in bond type shown in Table 16-11 has important influence on the trend in properties of the fluorine compounds. The trend arises because of the increasing difference between ionization energies of the two bonded atoms. 

The ionization energy of the hydrogen atom, 313.6 kcal/mole, is quite close to that of fluorine, so a covalent bond between these two atoms in HF is expected. Actually the properties of HF show that the molecule has a significant electric dipole, indicating ionic character in the bond. The same is true in the O—H bonds of water and, to a lesser extent, in the N—H bonds of ammonia. The ionic character of bonds to hydro-... 

The lithium fluoride bond is highly ionic in character because of the large difference in ionization energies of lithium and fluorine. Consequently, gaseous lithium fluoride has an unusually high electric dipole. 

Consider the fluorides of the second-row elements. There is a continuous change in ionic character of the bonds fluorine forms with the elements F, O, N, C, B, Be, and Li. The ionic character increases as the difference in ionization energies increases (see Table 16-11). This ionic character results in an electric dipole in each bond. The molecular dipole will be determined by the sum of the dipoles of all of the bonds, taking into account the geometry of the molecule. Since the properties of the molecule are strongly influenced by the molecular dipole, we shall investigate how it is determined by the molecular architecture and the ionic character of the individual bonds. For this study we shall begin at the left side of the periodic table. 

Ionization energies usually increase on going from left to right across the periodic table. The ionization energy for oxygen, however, is lower than that of either nitrogen or fluorine. Explain this anomaly. 

A neutral atom can add an electron to form an anion. The energy change when an electron is added to an atom is called the electron affinity (EA). Both ionization energy (IE) and electron affinity measure the stability of a bound electron, but for different species. Here, for example, are the values for fluorine ... 

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). 

The threshold wavelength for the appearance of F+ in (iii) has an energy equal to the ionization energy plus the enthalpy of fluorine. However, because some F+ is generated in (ii) via an excited F2 state, one needs to subtract this quantity of F+ (measured by observing the F-ion) in order to obtain the true threshold at 652.5 A [equivalent to D0(F2) of 157.6 1.0 kJ],... 

Heats of formation for a complete set of Group VILA fluorides are unavailable, but a set of xenon fluoride cations, isoelectronic with iodine fluorides, exhibits the alternating pattern expected for odd- and even-electron molecules. The original energy-level diagram for stepwise fluorine dissociation is shown in Fig. 5. The tabulated values were derived from the ionization energies of XeF and the threshold values for XeFJ — XeF, - + F, where n is even (27), together with heats of formation obtained by reaction calorimetry (137). 

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