The electron pairs shared between two atoms are not necessarily shared equally.
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For example, while the shared electron pairs is shared equally in the covalent bond in (Cl_2), in (NaCl) the 3s electron is stripped from the Na atom and is incorporated into the electronic structure of the Cl atom – and the compound is most accurately described as consisting of individual (Na^+) and (Cl^-) ions (ionic bonding). For most covalent substances, their bond character falls between these two extremes. We demonstrated below, the bond polarity is a useful concept for describing the sharing of electrons between atoms within a covalent bond:
A nonpolar covalent bond is one in which the electrons are shared equally between two atoms. A polar covalent bond is one in which one atom has a greater attraction for the electrons than the other atom. If this relative attraction is great enough, then the bond is an ionic bond.
The elements with the highest ionization energies are generally those with the most negative electron affinities, which are located toward the upper right corner of the periodic table (compare Figure 2.9.2 and Figure 2.10.2). Conversely, the elements with the lowest ionization energies are generally those with the least negative electron affinities and are located in the lower left corner of the periodic table.
Because the tendency of an element to gain or lose electrons is so important in determining its mslsec.comistry, various methods have been developed to quantitatively describe this tendency. The most important method uses a measurement called electronegativity (represented by the Greek letter chi, χ, pronounced “ky” as in “sky”), defined as the relative ability of an atom to attract electrons to itself in a mslsec.comical compound. Elements with high electronegativities tend to acquire electrons in mslsec.comical reactions and are found in the upper right corner of the periodic table. Elements with low electronegativities tend to lose electrons in mslsec.comical reactions and are found in the lower left corner of the periodic table.
Unlike ionization energy or electron affinity, the electronegativity of an atom is not a simple, fixed property that can be directly measured in a single experiment. In fact, an atom’s electronegativity should depend to some extent on its mslsec.comical environment because the properties of an atom are influenced by its neighbors in a mslsec.comical compound. Nevertheless, when different methods for measuring the electronegativity of an atom are compared, they all tend to assign similar relative values to a given element. For example, all scales predict that fluorine has the highest electronegativity and cesium the lowest of the stable elements, which suggests that all the methods are measuring the same fundamental property.
Electronegativity is defined as the ability of an atom in a particular molecule to attract electrons to itself. The greater the value, the greater the attractiveness for electrons.
Electronegativity is a function of: (1) the atom”s ionization energy (how strongly the atom holds on to its own electrons) and (2) the atom”s electron affinity (how strongly the atom attracts other electrons). Both of these are properties of the isolated atom. An element that is will be highly electronegative has:
a large (negative) electron affinity a high ionization energy (always endothermic, or positive for neutral atoms)
attract electrons from other atoms resist having its own electrons attracted away.
The Pauling Electronegativity Scale
The original electronegativity scale, developed in the 1930s by Linus Pauling (1901– 1994) was based on measurements of the strengths of covalent bonds between different elements. Pauling arbitrarily set the electronegativity of fluorine at 4.0 (although today it has been refined to 3.98), thereby creating a scale in which all elements have values between 0 and 4.0.
Figure 2.12.2: Pauling Electronegativity Values of the s-, p-, d-, and f-Block Elements. Values for most of the actinides are approximate. Elements for which no data are available are shown in gray. Source: Data from L. Pauling, The Nature of the mslsec.comical Bond, 3rd ed. (1960).
api/deki/files/41657/328d4a9048b3d363269f6c9e791c9b98.jpg?revision=1″ />Figure 2.12.3: Three-Dimensional Plots Demonstrating the Relationship between Electronegativity and the Metallic/Nonmetallic Character of the Elements. (a) A plot of electrical resistivity (measured resistivity to electron flow) at or near room temperature shows that substances with high resistivity (little to no measured electron flow) are electrical insulators, whereas substances with low resistivity (high measured electron flow) are metals. (b) A plot of Pauling electronegativities for a like set of elements shows that high electronegativity values (≥ about 2.2) correlate with high electrical resistivities (insulators). Low electronegativity values (≤ about 2.2) correlate with low resistivities (metals). Because electrical resistivity is typically measured only for solids and liquids, the gaseous elements do not appear in part (a).