Anomalies Associated with Fluorine

Note that the product formed by reaction with pyridine (reaction c) contains the tri-iodide ion, also present in solutions containing both iodine and potassium iodide thus, solutions of iodine in pyridine and iodine in aqueous KI have about the same color. 

As with the other first-row elements, much of the chemistry of fluorine stands distinctly apart from that of its congeners. Anomalous effects are due partially to the extreme electronegativity of fluorine, partially to the inability for the fluorine octet to expand, and partially to the small size of the atom. The low F—F bond energy, coupled with the high energy of bonds formed between fluorine and other elements, makes many of the reactions of the F2 greatly exothermic, and a number take place with almost explosive violence. (Fluorination of organic compounds with undiluted fluorine, for instance, can be particularly hazardous.) 

Fluorine also differs from its congeners in its apparent inability to form oxy-acids and oxy-salts. On the other hand, there are some fluorine-oxygen compounds that have no chlorine, bromine, or iodine homologs. Among these are fluorine nitrate, FONO2 fluorine perchlorate, FOCIO3  

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Upon cooling. These changes are associated with the anomalies of the specific heat at the ferroelectric transition. The results show that the disorder of the high-temperature paraelectric phase (T < Tc) is of dynamical origin. Fluorine-19 spin-lattice relaxation was also investigated. For measurements at 9.14 MHz the observed Ti appears to be dominated by the dynamics of the amorphous phase and exhibits no anomaly through the phase transition. However, from measurements at 20 MHz, well-defined minima in Ti were observed, and associated with the ferroelectric transition. 

Note Fj is also destabilized by repulsive interactions between lone pairs. Politzer, P. (1969). Anomalous properties of fluorine. J. Am. Chem. Soc., 97(23), 6235-6237. Tt appears rather that the anomaly involves the fluorine atom, in whatever circumstances, and its interaction with an external electron which enters its outer shell. [...] In any case, it is suggested that because of the exceptionally small size of the fluorine atom (see Table I), the entering electron feels unusually large forces of attraction from the nucleus and repulsion from the electrons already associated with the atom. It appears that the magnitude of the anomaly in the repulsive forces is greater than in the attractive forces. For an alternative perspective, see also the Charge-Shift Bond Section. 

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