|Chemistry 8th Edition / Chang|
|Student Study Guide
POLAR MOLECULES (10.2)
Diatomic Molecules. In the preceding chapter you learned that polar bonds are those in which the centers of positive charge and negative charge do not coincide. This charge separation is called a dipole, and it results from electronegativity differences between two bonded atoms. A molecule with an electric dipole is said to be polar and to possess a dipole moment.
The dipole moment µ is represented by an arrow or vector with the tail at the positive center and the head at the negative center. The length of the arrow represents the magnitude of the dipole moment.
Quantitatively the dipole moment is calculated from the equation
µ = Qr
where Q is the magnitude of the charge in coulombs from either end of the dipole and r is the charge separation (the distance between the centers of positive and negative charge) in meters. In polar molecules this charge is never as great as one unit of charge. Rather the charge is a partial charge, d+ or d-, which signifies a charge of less than one unit. The SI unit for a dipole moment is the coulomb meter. Traditionally, dipole moments have been measured in debye units, where 1 debye (D) = 3.33 x 10–30 C m.
Molecules that possess a dipole moment are called polar molecules. Molecules without a dipole moment are called nonpolar molecules. All homonuclear diatomic molecules are nonpolar. In general, heteronuclear diatomic molecules are polar.
Polyatomic Molecules. The dipole moment of a molecule containing more than one bond depends both on bond polarity and molecular geometry. In a polyatomic molecule the arrows representing polar bonds may add together to yield a polar molecule with a resultant dipole moment µ. Conversely, the arrows may cancel each other when added, producing a zero resultant dipole moment. Table 10.3 lists dipole moments of some small molecules in Debye units.
Table 10.3 Dipole Moments
of Some Molecules
Molecule µ (D)
H2O 1.87 D H2S 1.10 D NH3 1.46 D CO2 0 SO2 1.60 D BCl3 0
An important property of the water molecule is its dipole moment of 1.85 D. In the water molecule each bond is polar. The resultant of these bond dipoles yields a dipole moment for the molecule. The center of negative charge lies closer to the O atom than does the center of positive charge.
In CO2, the two bond dipoles cancel and µ = 0.
When the bond dipoles are equal and point in opposite directions the centers of + and – charge are both on the central atom. Thus, the absence of a molecular dipole in CO2 suggests that the molecule is linear. Conversely, water has a dipole moment, and so it cannot be linear.
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