In Section 3.12 we saw that a polar covalent bond in which electrons are not
evenly distributed has a nonzero dipole moment. A polar molecule is a molecule
with a nonzero dipole moment. A diatomic molecule is polar if its bond is polar.
An HCl molecule, with its polar covalent bond (HßCl), is a polar molecule.
Its dipole moment of 1.1 D is typical of polar diatomic molecules (TABLE 4.1). All
diatomic molecules composed of atoms of two different elements are at least
slightly polar. A nonpolar molecule is a molecule that has no electric dipole
moment. A homonuclear diatomic molecule, a diatomic molecule built from two
atoms of the same element, such as O2, N2, and Cl2, is nonpolar, because its bond
is nonpolar.
A polyatomic molecule may be nonpolar even if its bonds are polar. For
example, the two CßO dipole moments in carbon dioxide, a linear molecule,
point in opposite directions, and so they cancel each other (25). Consequently,
CO2 is a nonpolar molecule. The electrostatic potential diagram (26) illustrates
this conclusion. In contrast, the two HßO dipole moments in H2O lie at
104.5 to each other and do not cancel, and so H2O is a polar molecule (27).
This polarity is part of the reason why water is such a good solvent for ionic
compounds.
THINKING POINT Can you justify that last remark?