Concept of Electronegativity

Effective charges of atoms are known only for a small minority of polar molecules and crystals, therefore it is important to find a dependence of these values on such characteristics of atoms which allows to estimate the polarity of bonds a priori. Such characteristic is the electronegativity of atom (EN) which, according to Pauling who introduced this concept in 1932, is the measure of the power of an atom in a molecule to attract electrons. 

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The concepts of electronegativity, hardness, and polarizability are all interrelated. For the kind of qualitative applications we will make in discussing reactivity, the concept that initial interactions between reacting molecules can be dominated by either partial electron transfer by bond formation (soft reactants) or by electrostatic interaction (hard reactants) is a useftxl generalization. 

In 1923. Lewis published a classic book (later reprinted by Dover Publications) titled Valence and the Structure of Atoms and Molecules. Here, in Lewis s characteristically lucid style, we find many of the basic principles of covalent bonding discussed in this chapter. Included are electron-dot structures, the octet rule, and the concept of electronegativity. Here too is the Lewis definition of acids and bases (Chapter 15). That same year, Lewis published with Merle Randall a text called Thermodynamics and the Free Energy of Chemical Substances. Today, a revised edition of that text is still used in graduate courses in chemistry. 

In Section 2.12, when we have met the concept of electronegativity, we shall see that another way to express this rule is to say that the central atom is usually the element with lowest electronegativity. 

Similar to the concept of electronegativity is the electropositivity of an element, which is the power of its atoms (when part of a compound) to lose an electron. The most electropositive elements are the metals on the far-left of the periodic table, particularly Groups 1(a) and 11(a), which prefer to exist as cations. Being the opposite concept to electronegativity, electropositivity is not employed often. Rather, we tend to say that an atom such as sodium has a tiny electronegativity instead of being very electropositive. 

The starting point of the DFT of chemical reactivity is the identification of the concept of electronegativity, y, with the chemical potential, through the relationship [7]... 

The concept of electronegativity is widely used in geochemistry to define the combinational properties of the various elements in a qualitative manner. However, it is important to stress that this concept is largely empirical, because it varies from author to author and cannot be univocally quantified. 

Much effort is still being spent on the derivation and application of various such scales in research papers and text books. However, MuUiken whose electronegativity scale of 1934 is often designated as well founded, recently raised the question whether the concept of electronegativity... 

The earlier sections on ionic bonding and covalent bonding refer to the concept of electronegativity, or the tendency of an atom to draw electrons toward itself. Ionic bonds form between atoms with large differences in electronegativity, whereas covalent bonds form between atoms with smaller differences in electronegativity. In truth, there s no natural distinction between the two types of bonds (ionic and covalent) they lie on opposite sides of a spectrum of polarity, or unevenness in the distribution of electrons within a bond. 

The concept of electronegativity is easy to visualize but difficult to apply quantitatively. Muffiken s definition, based on free-atomic properties, does not account for the influence of the local atomic environment on the ionic state that is reflected, for example, in the fact that the degree of ionicity... 

Bond Energies (Enthalpies) and Bond Lengths. The Concept of Electronegativity... 

Closely related to ionization energy and electron affinity is the concept of electronegativity, a measure of the ability of an atom to attract shared electrons in a bond. We usually use electronegativities in a qualitative way, for example, to predict bond polarities (see Chapter 8). Electronegativities, on a dimensionless scale ranging from 0 to 4, are given in Table 7-2. 

From this viewpoint the electronegativity of a species is the drop in energy when an infinitesimal amount (infinitesimal so that it remains the same species) of electronic charge enters it. It is a measure of how hospitable an atom or ion, or a group or an atom in a molecule (Section 5.5.4), is to the ingress of electronic charge, which fits in with our intuitive concept of electronegativity. 

Linus Pauling developed the concept of electronegativity and its relationship to different types of bonds in the 1930s. 

The concept of electronegativity (see Table 3.2) of substituents near the proton in question is a dependable guide, up to a point, to chemical shifts. It tells us... 

We could make a number of good estimates as to chemical shifts, using concepts of electronegativity and proton acidity. For example, the values found in Table 3.3 are reasonable solely on the basis of electronegativity. 

We point out again that an appreciation of the concepts of electronegativity (inductive effects) and of electron delocalization—combined with an understanding of diamagnetic anisotropy—permits both rationalization and prediction of approximate chemical shift. Several examples make the point ... 

It is interesting that the three different criteria that we have considered give us electronegativity scales that are very similar. This indicates that, despite the difficulty in defining it, the concept of electronegativity is a meaningful one. 

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