Barrons SAT Subject Test Chemistry, 13th Edition

Figure 12. Polar Covalent Bonds in Nonpolar Molecules

In all the examples in Figure 12 the bonds are polar covalent bonds, but the
important thing is that they are symmetrically arranged in the molecule. The result
is a nonpolar molecule.
In the covalent bonds described so far, the shared electrons in the pair were
contributed one each from the atoms bonded. In some cases, however, both
electrons for the shared pair are supplied by only one of the atoms. Two examples

are the bonds in NH 4 + and H 2 SO 4. (See Figure 13.)

Figure 13. Covalent Bonds (both electrons supplied by one atom)

The formation of a covalent bond can be described in graphic form and
related to the potential energies of the atoms involved. Using the formation of the
hydrogen molecule as an example, we can show how the potential energy changes
as the two atoms approach and form a covalent bond. In the illustration that
follows, frames (1), (2), and (3) show the effect on potential energy as the atoms
move closer to each other. In frame (3), the atoms have reached the condition of
lowest potential energy, but the inertia of the atoms pulls them even closer, as
shown in frame (4). The repulsion between them then forces the two nucleii to a
stable position, as shown in frame (5).