Question 7.EP.3: Which bond is the most polar: C—H, O—H, or H—Cl? For each of...
Which bond is the most polar: C—H, O—H, or H—Cl? For each of these bonds, determine which atom has a partial positive charge.
Strategy
Bond polarity depends on the difference in electronegativity between the two bonded atoms. So we can look up the electronegativity values and subtract to find the difference in each case. Negatively charged electrons are drawn more strongly toward an atom with higher electronegativity, so the less electronegative atom has a partial positive charge.
From Figure 7.9, we find the following electronegativities: H = 2.1, C = 2.5, O = 3.5, Cl = 3.0. The electronegativity differences are, therefore, C—H = 0.4, O—H = 1.4, and H—Cl = 0.9. So the O—H bond is the most polar of the three. In each of the three bonds considered here, H is the less electronegative atom, so it carries the positive charge.
Discussion
This question may appear simple, because we are using electronegativity values and differences in a straightforward way. Despite this simplicity, the results can provide important insight into material properties. Our focus at this point is on single molecules, but invariably in laboratory observations in the real world, we encounter huge collections of molecules. The way these samples of molecules behave is often influenced strongly by the polarity of the bonding within the individual molecules. Therefore, understanding a bond dipole and predicting its presence using electronegativity will be useful in a number of ways as we move forward in our study of chemistry.
