Fluorine: chemical bonding

On the other hand, fluorine s high electronegativity and its ability to form mostly ionic chemical bonds, provide materials with several useful properties. First, compared to oxides, fluoride compounds have a wide forbidden zone and as a result, have low electroconductivity. In addition, fluorides are characterized by a high transparency in a wide optical range that allows for their application in the manufacturing of electro-optical devices that operate in the UV region [42,43]. 

At the same time, the relatively low energy and ionic character of the chemical bonds between metal and fluorine cause some difficulties in the application of fluoride compounds. First, fluorides typically have a tendency towards thermolysis and hygroscopicity. In addition, fluoride compounds usually display relatively low temperatures of electrostatic and magnetic ordering. 

Our explanation of chemical bonding is of value only if it has wide applicability. Let us examine its usefulness in considering the compounds of the second-row elements, beginning with fluorine. 

Under normal conditions of temperature and pressure, fluorine is a gas. From gas density experiments we discover that a molecule of fluorine contains two atoms. There is a chemical bond between the two fluorine atoms. Let us see if our expectations agree with these experimental facts. 

In the electron dot method of showing chemical bonds it is necessary to show only the valence electrons. In fluorine there are seven—the pair of electrons in the Is orbital is so tightly bound... 

Now we can say why the chemical bond forms between two fluorine atoms. First, the electron affinity of a fluorine atom makes it energetically favorable to acquire one more electron. Two fluorine atoms can realize a part of this energy stability by sharing electrons. All chemical bonds form because one or more electrons are placed so as to feel electrostatic attraction to two or more positive nuclei simultaneously. 

We have already treated the bonding in an F2 molecule. Since neither fluorine atom can pull an electron entirely away from the other, they compromise by sharing a pair of electrons equally. How does the chemical bonding in the lithium fluoride molecule compare ... 

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 ... 

Fluorine, Fs, oxygen, 02, and nitrogen, N2, all form molecular crystals but the next member of this row of the periodic table, carbon, presents another situation. There does not seem to be a small molecule of pure carbon that consumes completely the bonding capacity of each atom. As a result, it is bound in its crystal by a network of interlocking chemical bonds. 

Each nucieus of a hydrogen moiecuie has a charge of+1. Consequently, both nuclei attract electrons equally. The result is a symmetricai distribution of the eiectron density between the atoms. Each nucleus of a fluorine molecule has a charge of +9, and again the eiectrons experience the same net attraction toward both nuclei. In a chemical bond between identical atoms, the two nuclei share the bonding eiectrons equally. 

Examples of fluorine chemical shift and coupling constant data are given in Scheme 5.56. Note the significant four- and five-bond F—F and F—H coupling in these compounds, which no doubt is due in part to through-space coupling. 

Sodium, a metal, replaces iron, another metal. Fluorine, a nonmetal, replaces chlorine, another nonmetal. (In some high-temperature reactions, a nonmetal can displace a relatively inactive metal from its compounds.) The formulas for F2, NaCl, NaF, and Cl2 are written on the basis of the rules of chemical bonding (Chap. 5). 

The fact that fluorination had taken place was established by various analytic methods ESCA, IR and FTIR spectroscopy, bulk anlaysis,NMR, and DSC. The presence of chemically bonded fluorine in the surface layer of treated samples were uniquely determined by analysis data. Further details can be found elsewhere.22... 

To validate our idea that treating rubber products with gaseous XeF, causes them to become surface-fluorinated, i.e., that chemically bonded fluorine atoms are incorporated in the surface layer, we did ESCA analyses ofthe samples treated in various modes. The results are shown in Table 15.4. 

Chemical bonds are classified into two groups transfer of electrons creates an ionic bond while the sharing of electrons leads to a covalent bond. Before studying chemical bonds we need to become familiar with their representation. Chemical bonds may be represented in several ways. We are going to study orbital representation, electron dot representation and line representation. Let s examine these three types using the example of the fluorine molecule, F2. 

In 1906, the Nobel Prize in chemistry was awarded to a French chemist, Ferdinand Moissan, for isolating fluorine in its pure elemental form. Why would this achievement be deserving of such a prestigious honour Use your understanding of atomic properties as well as chemical bonds to explain your answer. 

The second class includes those elements that form only one chemical bond at a time. These elements terminate some aspect of the molecular structure, since, once they have bonded, there is nowhere else to go. Hydrogen provides an obvious example. We have already encountered three other elements in this class fluorine, chlorine. 

Hydrogen bond a weak chemical bond formed by sharing a hydrogen atom between two atoms chosen from oxygen, nitrogen, and fluorine. 

Electronegativity refers to tendency of an atom to pull electrons towards itself in a chemical bond. Nonmetals have high electronegativity, fluorine being the most electronegative while alkali metals possess least electronegativity. Electronegativity difference indicates polarity in the molecule. 

The Fluorine-19 NMR spectroscopy has been used to confirm the structures of fiuoroalkylamino, fluoroalkyl and fluoroalkoxycyclophosphazenes. The fluorine chemical shifts (relative to CFCI3) span a wide range from — 18.0 ppm to — 71.9 ppm. It has been observed that the magnitude of J(P-F) is sensitive to the cis and trans orientation of the relevant P-F bonds. Thus, in general, in cis isomers J(P-F) is ca. —1.0 Hz while, in the trans isomer it is larger ( -M2.0 Hz) [25]. 

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