Question 13.2: Bond Polarity and Dipole Moment For each of the following mo...

The HCl molecule: Because the electronegativity of chlorine (3.2) is greater than that of hydrogen (2.2), the chlorine is partially negative, and the hydrogen is partially positive. The HCl molecule has a dipole moment oriented as follows:

The Cl_{2} molecule: Because the two chlorine atoms share the electrons equally, no bond polarity occurs. The Cl_{2} molecule has no dipole moment.
The SO_{3} molecule: Because the electronegativity of oxygen (3.4) is greater than that of sulfur (2.6), each oxygen has a partial negative charge, and the sulfur has a partial positive charge:

However, the bond polarities cancel, and the molecule has no dipole moment.
The C H_{4} molecule: Carbon has a slightly higher electronegativity (2.6) than hydrogen (2.2). This leads to small partial positive charges on the hydrogen atoms and a small partial negative charge on the carbon:

This case is similar to the third type in Table 13.4. Since the bond polarities cancel, the molecule has no dipole moment.
The H_{2}S molecule: Since the electronegativity of sulfur (2.6) is greater than that of hydrogen (2.2), the sulfur has a partial negative charge, and the hydrogen atoms have a partial positive charge, which can be represented as follows:

This case is analogous to the water molecule. The polar bonds result in a dipole moment oriented as shown.

Table 13.4
Types of Molecules with Polar Bonds but no Resulting Dipole Moment

Type		Cancellation of Polar Bonds	Example	Ball-and-Stick Model
Linear molecules with two identical bonds	B—A—B		CO _{2}
Planar molecules with three identical bonds 120 degrees apart			SO _{3}
Tetrahedral molecules with four identical bonds 109.5 degrees apart			CCl _{4}