High electronegativity and small size

Fluorine has a number of peculiarities that stem from its high electronegativity, small size, and lack of available d-orbitals. It is the most electronegative element of all and has an oxidation number of — 1 in all its compounds. Its high electronegativity and small size allow it to oxidize other elements to their highest oxidation numbers. The small size helps in this, because it allows several F atoms to pack around a central atom, as in IF7. 

Within a year of the discovery of XePtFe and as a result of worldwide activity, it was clear that the chemistry of the noble gases would be limited to the heavier elements as set out in my Noranda Lecture (see Ref. S2). Because of the dangerous radioactivity associated with all of the radon isotopes, this meant that the bulk of noble-gas chemistry would be that of xenon. The chemistry of krypton appeared to be limited to KrF2 and compounds that could be derived from it. In all cases, it was clear, the range of accessible noble-gas chemistry was dictated by lower ionization potentials at the noble-gas atom, and high electronegativity and small size of the ligand atoms, as discussed in Ref. 45. 

According to Pearson s nomenclature hardness is associated with high electronegativity and small size of the ion. On the other hand, a soft base is one in which the donor atom is of high polarizability and of low electronegativity and is easily oxidized. The general principle of hard and soft acids and bases is that hard acids prefer to associate with hard bases and soft acids prefer to associate with soft bases (144,145). 

As we have seen in Chapter 15, fluorine plays an important role in medicinal chemistry with almost a quarter of all drugs in the pharmaceutical pipeline containing fluorine. The high electronegativity and small size (approximately equal to ojygen) of fluorine have made it a bioisosteric replacement for either hydrogen or ojygen. It has the ability to modify the pK of amines, alcohols... 

Fluorine is the most electronegative element and thus can oxidize many other elements to their highest oxidation state. The small size of the fluorine atom facihtates the arrangement of a large number of fluorines around an atom of another element. These properties of high oxidation potential and small size allow the formation of many simple and complex fluorides in which the other elements are at their highest oxidation states. 

Much of the chemistry of oxygen can be rationalized in terms of its electronic structure (2s 2p ), high electronegativity (3.5) and small size. Thus, oxygen shows many similarities to nitrogen (p. 412) in its covalent chemistry, and its propensity to form H bonds (p. 52) and p double bonds (p. 416), though the anionic chemistry of 0 and OH is much more extensive than for the isoelectronic ions N , and NH2. Simi-... 

An inner-sphere complex is formed between Lewis acids and bases, while an outer-sphere complex involves a water molecule interposed between the acid and the base. A hard Lewis acid is a molecular unit of small size, high oxidation state, high electronegativity, and low polarizability whereas a soft Lewis acid is a molecular unit of relatively large size, characterized by low oxidation state, low electronegativity, and high polarizability. Based on this characterization, hard bases prefer to complex hard acids, and soft bases prefer to complex soft acids, under similar conditions of acid-base strength. 

Thus, a molecule which has atoms of O, F, Cl, N or S of high electronegativity is small in size and can atdact the electron from a hydrogen atom in another molecule and form hydrogen bonding. 

Table VI gives the classification of a large number of Lewis acids into the three categories, hard, soft, and borderline. Hard and soft acids correspond to and are extensions of class a and class b acids in the earlier criterion of Ahrland, Chatt, and Davies.In general, acceptor atoms of hard acids are small in size, of high positive charge, and do not contain unshared electron-pairs in their valence shell (not all of these properties need be possessed by any one acid), leading to high electronegativity and low polarizability. On the contrary, soft acids have...

Schleyer and West (296) have examined infrared shifts for intra-and intermolecular hydrogen bonds from several hydroxyl donors to halogen atoms and have found by this criterion that the order of basicity runs I > Br > Cl > F. This is entirely counter to the traditional picture of the hydrogen bond in which the potency of an acceptor atom depends on small size and high electronegativity and is... 

Elemental boron has a diverse and complex chemistry, primarily influenced by three circumstances. Eirst, boron has a high ionization energy, 8.296 eV, 23.98 eV, and 37.75 eV for first, second, and third ionization potentials, respectively. Second, boron has a small size. Third, the electronegativities of boron (2.0), carbon (2.5), and hydrogen (2.1) are all very similar resulting in extensive and unusual covalent chemistry. 

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