Polar Bonds

Many substances contain bonds that are intermediate in character—between pure covalent and pure ionic bonds. Such polar bonds occur when one of the elements attracts the shared electrons more strongly than the other element. In hydrogen fluoride, for instance, the shared electrons are so much more attracted by fluorine than hydrogen that the sharing is unequal. (See Figure 1 .)

Figure 1 Unequal sharing of electrons.

Due to the unequal sharing of the bonding electrons, the 2 atoms have fractional electrical charges, represented by the Greek letter delta (Δ), in Figure 2 .

Figure 2 Sowing the partial charges of a polar bond.

Around 1935, the American chemist Linus Pauling developed a scale of electronegativity to describe the attraction an element has for electrons in a chemical bond. The values in Figure 3 are higher for elements that more strongly attract electrons, which increase the likelihood of a negative partial charge on that atom.

Figure 3 The electronegativity of the elements.

In Figure 3 , you can see that the most electronegative element is fluorine. The nonmetals in the upper right corner have a strong tendency to gain electrons. The element of lowest electronegativity is cesium (Cs), in the lower-left corner. The relatively weak attraction for electrons by the alkali metals and alkaline earths is responsible for the loss of electrons by those elements.

Two atoms of the same electronegativity will share electrons equally in a pure covalent bond; therefore, any molecule that contains atoms of only one element, like H₂ or Cl₂, has pure covalent bonding. Two atoms of different electronegativities, however, will have either the distorted electron distribution of a polar bond or the complete electron transfer of an ionic bond. Table 1 interprets the bonding between two elements as a function of the difference in their electronegativity.

Now use electronegativity to estimate the bond character in hydrogen sulfide, H₂S. The difference in electronegativities is

You can interpolate this value in the first column of Table 1 to find that such a bond is about 4% ionic and 96% covalent, which is virtually a pure covalent bond.

•      Problem 1: Use the chart of electronegativity and the chart of bond types to interpret the bonding in magnesium chloride, MgCl₂.

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