Ionic bonding fluorine compounds

As opposed to the oxides, fluoride compounds are characterized by the formation of mostly ionic bonds. This peculiarity is related to fluorine s high electronegativity. 

Lewis and many other chemists had recognized the shortcomings of the ionic bond. When diatomic molecules, such as or Cl, were considered, there was no reason why one atom should lose an electron and an identical atom should gain an electron. There had to be another explanation for how diatomic molecules formed. We have seen how the octet rule applies to the formation of ionic compounds by the transfer of electrons. This rule also helps explain the formation of covalent bonds when molecules (covalent compounds) form. Covalent bonds result when atoms share electrons. Using fluorine, F, as a representative halogen, we can see how the octet rule applies to the formation of the molecule. Each fluorine atom has seven valence electrons and needs one more electron to achieve the stable octet valence configuration. If two fluorines share a pair of electrons, then the stable octet configuration is achieved ... 

Unsaturated fluorinated compounds are fundamentally different from those of hydrocarbon chemistry. Whereas conventional alkenes are electron rich at the double bond, fluoroal-kenes suffer from a deficiency of electrons due to the negative inductive effect. Therefore, fluoroalkenes react smoothly in a very typical way with oxygen, sulfur, nitrogen and carbon nucleophiles.31 Usually, the reaction path of the addition or addition-elimination reaction goes through an intermediate carbanion. The reaction conditions decide whether the product is saturated or unsaturated and if vinylic or allylic substitution is required. Highly branched fluoroalkenes, obtained from the fluoride-initiated ionic oligomerization of tetrafluoroethene or hexafluoropropene, are different and more complex in their reactions and reactivities. 

Examine Figure 3.11. In an ionic bond, calcium tends to lose two electrons and fluorine tends to gain one electron. Therefore, one calcium atom bonds with two fluorine atoms. Calcium loses one of each of its valence electrons to each fluorine atom. Calcium becomes Ca2+, and fluorine becomes F . They form the compound calcium fluoride, CaF2. 

The ionically bonded sodium bromide receives two electrons, and loses two +ve sodium cations to form sodium tri-bromide (see Porterfield, 1993). An alternative homogeneous reaction route to tri-bromide is to bubble bromine (Br2) through aqueous sodium bromide. Accordingly, the electrochemically formed tri-bromide has an associated atmosphere of bromine from homogeneous dissociation. Such bromine is corrosive, but is contained by the fluorinated plastic tanks and by one side of the (Na ) Nafion perm-selective membrane. Nafion is itself a compound with many fluorine atoms. Bromine is very much lower than fluorine in chemical reactivity, so that the bromine containment strategy is complete, and the selection of the bromide electrolyte seen to be a well-considered choice. 

Another difference is that the 5/ orbitals have a greater spatial extension relative to the 6,s and 6

electron-spin resonance spectrum of UF3 in a CaF2 lattice shows structure attributable to the interaction of fluorine nuclei and the electron spin of the U3+ ion. This implies a small overlap of 5/ orbitals with fluorine and constitutes an/covalent contribution to the ionic bonding. With the neodymium ion a similar effect is not observed. Because they occupy inner orbitals the 4/ electrons in the lanthanides are not accessible for bonding purposes and virtually no compound in which 4f orbitals are used can be said to exist. 

The halogens, particularly fluorine, have very high electronegativity values (see Table 20.17). They tend to form polar covalent bonds with other nonmetals and ionic bonds with metals in their lower oxidation states. When a metal ion is in a higher oxidation state, such as +3 or +4, the metal-halogen bonds are polar and covalent. For example, TiCl4 and SnCl4 are both covalent compounds that are liquids under normal conditions. 

The compound which forms has the formula LiF, and the chemical bond which unites the ions is an ionic bond. An ionic bond is identified as a bond which unites ions of opposite charge. Ions are formed by an electron(s) leaving the valence shell of one atom and entering the valence shell of its partner. This is most likely to happen when the difference in ability to attract electrons of the two partners is large, as in the case of lithium and fluorine. 

FIGURE 5.7 Nonpolar, polar, and ionic bonds, (a) In a nonpolar molecule such as H, the valence electron density is equally shared by both atoms, (b) In a polar molecule like HF, the valence electron density is shifted toward the fluorine atom. An arrow is used to show the direction of molecular polarity, with the arrowhead pointing toward the negative end of the molecule and the plus sign at the positive end of the molecule, (c) In ionic compounds such as NaCI, the valence electron or electrons of the metal are transferred completely to the nonmetal to give ions. 

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