Loading chem110..

In the previous section, we saw that covalent bonds involve the sharing of electrons between two nuclei. The shared electrons in covalent bonds are not necessarily shared evenly. Covalent bonds in which electrons are shared evenly are known as nonpolar covalent bonds. Covalent bonds in which electrons are shared unequally are known as polar covalent bonds. In contrast, ionic bonds represent a more extreme situation in which electrons are not shared at all; they are exchanged instead. There are several factors that cause a bond to be polar or nonpolar that we will examine.

When electrons are shared evenly, electron density is spread evenly across the bond. As a result, the two atoms in the bond carry no net charge (shown as green shading surrounding the H-H bond in Figure 1).

In contrast, uneven electron sharing results in an uneven distribution of charge along the length of the bond, as shown in the Cl-H bond in Figure 1. The chlorine atom more strongly attracts the electrons in the bond towards itself, causing these electrons to reside, on average, closer to chlorine than hydrogen. As a consequence, the chlorine atom becomes slightly negatively charged because of the excess electron density that it attracts towards itself. In contrast, the hydrogen, which has a deficit of electron density, is slightly positive (from the charge of its nucleus). This small amount of charge is known as a partial charge, represented by the lower-case Greek letter delta (δ). A partial charge represents a nonzero amount of charge that is smaller in magnitude than the charge of a proton (+1) or electron (-1). Partial negative and partial positive charges are represented respectively as δ- and δ+. The partial negative and positive charges in the Cl-H bond are represented in the diagram with red and blue shading, respectively.

Whether a bond is polar or not depends on the electronegativity values of the bonded atoms. Electronegativity (abbreviated as the lower-case Greek letter chi, \chi ) describes the tendency of an atom in a bond to attract electron density towards itself. This slide shows the electronegativity values of the elements on the periodic table expressed using the Pauling electronegativity scale, which ranges from ~0.79 to about 4. Higher values mean that the element is more electronegative and has a higher tendency to attract electron density towards itself.

Whether or not a bond is polar is determined by the difference in the electronegativity values of the two bonded atoms (in terms of absolute value). When the difference in electronegativity (Δχ) is large, the more bond is more polar, and the partial charges carried by each atom in the bond are larger (there will be greater charge separation). Bonds exist along a continuum from nonpolar to polar, becoming more and more polar as Δχ increases (as shown on the graph).