BOND POLARITY AND ELECTRONEGATIVITY

We can leam more about the chemistry of compounds by looking at how bonding electrons are shared between atoms. The bonding electrons are shared equally in a bond between identical nonmetal atoms. However, when a bond is between atoms of different elements, the electron pairs are usually shared unequally. Then the shared pairs of electrons are attracted to one atom in the bond more than the other. 

The difference in the electronegativity valnes of two atoms can be used to predict the type of chemical bond, ionic or covalent, that forms. For the H—H bond, the electronegativity difference is zero (2.1 — 2.1 = 0), which means the bonding electrons are shared eqnally. A bond between atoms with identical or very similar electronegativity valnes is a nonpolar covalent bond. However, when bonds are between h H 

FIGURE 6.8 In the nonpolar covalent bond of H2, electrons are shared equally. In the polar covalent bond of HCI, electrons are shared unequally. jj pj 

Unequal sharing of electrons in a polar covalent bond 

The polarity of a bond depends on the difference in the electronegativity values of its atoms. In a polar covalent bond, the shared electrons are attracted to the more electronegative atom, which makes it partially negative, due to the negatively charged electrons around that atom. At the other end of the bond, the atom with the lower electronegativity becomes partially positive dne to the lack of the electrons at that atom. 

Electronegativity is a measure of an atom s attraction for electrons in a bond. Thus, electronegativity indicates how much a particular atom wants electrons. The following trends m electronegativity are observed in the periodic table  

Electronegativity values are relative, so they can be used for comparison purposes only. When comparing two different elements, one is more electronegative than the other if it attracts electron density toward itself. One is less electronegative— more electr( ositive— if it gives up electron density to the other element. 

Rank the following atoms in order of increasing electronegativity. Label the most electronegative and most electropositive atom in each group, a. Li, Na, H b. O, C, Be 

Electronegativity values are used as a guideline to indicate whether the electrons in a bond are equally shared or unequally shared between two atoms. For example, whenever two identical atoms are bonded together, each atom attracts the electrons in the bond to the same extent. The electrons are equally shared, and the bond is nonpolar. Thus, a carbon-carbon bond is nonpolar. The same is true whenever two different atoms having similar electronegativities are bonded together. C-H bonds are considered to be nonpolar, because the electronegativity difference between C (2.5) and H (2.2) is small. 

Bonding between atoms of different electronegativity values results in the unequal sharing of electrons. For example, in a C-0 bond, the electrons are pulled away from C (2.5) toward O 

Bonding between atoms of different electronegativity values results in the unequal sharing of electrons. For example, in a C-O bond, the electrons are pulled away from C (2.5) toward 0 (3.4), the element of higher electronegativity. The bond is polar, or polar covtdent. The bond is said to have a dipole that is, a separation of charge. 

We can use the difference in electronegativity between two atoms to gauge the polarity of the bond the atoms form. Consider these three fluorine-containing compounds  

In p2 the electrons are shared equally between the fluorine atoms and, thus, the covalent bond is nonpolar. A nonpolar covalent bond results when the electronegativities 

The S-F and 8— (read delta plus and delta minus ) symbolize the partial positive and negative charges, respectively. In a polar bond, these numbers are less than the full charges of the ions. 

In liF the electronegativity difference is very large, meaning that the electron density is shifted far toward F. The resultant bond is therefore most accurately described as ionic. Thus, if we considered the bond in LiF to be fully ionic, we could say 5-F for Li is 1 + and 8— for F is 1—. 

The shift of electron density toward the more electronegative atom in a bond can be seen from the results of calculations of electron-density distributions. For the three species in our example, the calculated electron-density distributions are shown in 

In LiF the electronegativity difference is very large, meaning that the electron density is shifted far toward F. The resultant bond is therefore most accurately described as ionic. 

The shift of electron density toward the more electronegative atom in a bond can be seen in the results of calculations of electron density distributions. For the three species in our example, the calculated electron density distributions are shown in FIGURE 8.8. You can see that in F2 the distribution is symmetrical, in HF the electron density is clearly shifted toward fluorine, and in LiF the shift is even greater. These examples illustrate, therefore, that the greater the difference in electronegativity between two atoms, the more polar their bond. 

Each of the atoms taking part in a bond have an attraction for their shared electrons in a fashion similar to the attraction nonbonded atoms have for electrons. The attraction that a bonded atom has for its shared electrons is called its electronegativity. Linus Pauling, a prominent chemist of the twentieth century, assigned numerical values to the electronegativities of elements for the purpose of quantitatively comparing one with another. Since covalent bonds are mostly between nonmetal atoms, it is the electronegativities of nonmetals 

FIGURE 6.22 (a) An illustration of a nonpolar bond in which the shared electrons are distributed symmetrically in the space between the nuclei, (b) An illustration of a polar bond in which the electrons are not distributed symmetrically and slight charges appear on both ends. (From Kenkel, J., Kelter, P., and Hage, D., Chemistry An Industry-Based Introduction with CD-ROM, CRC Press, Boca Raton, FL, 2001. With Permission.) 

Covalent bonds between two atoms of the same element are nonpolar. Both atoms, being the same element, have the same electronegativity. Thus the H-H bond in Hj is nonpolar and the Cl-Cl bond in CI2 is nonpolar. Also, covalent bonds between atoms that have the same electronegativity value in Table 6.2 will also be nonpolar. For example, a bond between hydrogen and phosphorus or between tellurium and boron is nonpolar. 

Covalent bonds between atoms that have unequal electronegativities are polar. Examples include the H-Cl bond in HCl, the H-O bonds in water, and each of the C-H bonds in methane (CH4). 

Not only did this study identify which isomer of dichlorobenzene was which, but it also reinforced the view that benzene is a planar molecule. Alternative structures, such as Baeyer, Korner, or Ladenburg benzene, would have given different molecular dipole moments. 

Coulson, C. A. Valence Clarendon Press Oxford, England, 1952 p. 128. 

This idea was called the postulate of the additivity of normal covalent bonds by Pauling (reference 30). 

The premise that covalent bonds between atoms with different electronegativity values are stronger than the corresponding bonds between identical atoms is not always found to be true. Benson (reference 67) pointed out that the reaction of Hgz with CI2 to produce 2 HgCl, is endothermic by at least 10 kcal/mol. 

Theoretical studies have offered additional perspectives on electronegativity. Parr and co-workers ° defined a quantity, fi, as the electronic chemical potential, which measures the escaping tendency of the electrons in the system. The value of is approximately the same as (I + A)/2, the Mulliken electronegativity, so the value Xm has been termed absolute electro- 

Write Lewis structures for the following molecular formulas. ( ) N2 (b) HCN 

Circle any lone pairs (pairs of nonbonding electrons) in the structures you drew fbr Problem 1-3. 

These usual numbers of bonds might be single bonds, or they might be combined ito double and triple bonds. For example, three bonds to nitrogen might be three single bortds one single bond and one double bond, or one triple bond (=N=N ). 

A bond with the electrons shared equally between the two atoms is called a nonpolar covalent bond. The bond in H2 and the C — C bond in ethane are nonpolar covalent bonds. In most bonds between two different elements, the bonding electrons are attracted more strongly to one of the two nuclei. An unequally shared pair of bonding electrons is called a polar covalent bond. 

The Pauling electronegativities of some of the elements found in organic compounds. 

This polymerization reaction is analogous to the formation of polyethylene, which was discussed in Section 2.8. 

The concept of covalent bonding provides useful insight into the chemical properties of fluorine compounds, such as PTFE. Although it is certainly vital that a PTFE-coated pan is nonflammable and that the coating doesn t react with the food we cook, the property we appreciate when we are washing the dishes is the nonstick aspect of the polymer. As we will see by the end of this chapter, covalent bonding plays a role here, too. 

The sharing of electrons to form covalent bonds and the outright transfer of electrons that occurs in ionic bonding represent two ends of a bonding continuum. To describe compounds whose bonding falls somewhere in the middle 

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Despite our earlier enunciated electronegativity and bond polarity logic, we must forego nearly all comparison with the free (uncomplexed) carbanions. Unlike the rather stable cyclopropyl anion, the cyclobutyl and cyclopentyl ions are unbound with regard to loss of their extra electron. That is, the gas phase ionization process to form the radical from the carbanion, Ru R" -E e, is energetically favorable. 

As you might expect from the discussion of electronegativity and bond polarity in Section 6.4, the carbon-magnesium bond is polarized, making the carbon atom both nucleophilic and basic. An electrostatic potential map clearly shows the electron-rich (red) character of the carbon bonded to magnesium ... 

If covalent, ionic and metallic bonds are explained in electrical terms, students are better prepared to accept that hydrogen bonds, van der Waals forces, solvent-solute interactions etc. are also types of chemical bonding. Where learners see covalent bonds as electron pairs attracted to two different positive cores, they have a good basis for subsequently learning about electronegativity and bond polarity. 

Covalent Bonding, Electronegativity, and Bond Polarity (Sections 8.3 and 8.4)... 

Electronegativity and bond polarization. Electronegative elements pull electron density toward themselves. This introduces polarity into bonds, resulting in bond dipoles and molecu lar dipoles. 

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