A molecule possesses a permanent electric dipole moment if it has a net separation of positive and negative charge, which results from the presence of polar bonds and an asymmetrical molecular geometry. If a molecule has polar bonds but a symmetrical geometry, the individual bond dipoles may cancel out, leading to a zero net dipole moment.

Carbon disulfide (${\mathrm{CS}}_2$) has a linear molecular geometry. Although the C-S bonds are polar (due to the difference in electronegativity between carbon and sulfur), the two bond dipoles are equal in magnitude and opposite in direction, causing them to cancel each other out. Therefore, ${\mathrm{CS}}_2$ has a net dipole moment of zero.

• A) ${\mathrm{H}}_2\mathrm{S}$: Hydrogen sulfide has a bent molecular geometry (similar to water) due to two lone pairs on the sulfur atom. The S-H bonds are polar, and their dipoles do not cancel out, resulting in a net dipole moment.
• B) ${\mathrm{SO}}_2$: Sulfur dioxide has a bent molecular geometry due to one lone pair on the sulfur atom. The S-O bonds are polar, and their dipoles do not cancel out, resulting in a net dipole moment.
• C) ${\mathrm{NH}}_3$: Ammonia has a trigonal pyramidal molecular geometry due to one lone pair on the nitrogen atom. The N-H bonds are polar, and their dipoles do not cancel out, resulting in a net dipole moment.
• D) ${\mathrm{CS}}_2$: Carbon disulfide has a linear molecular geometry. The two C=S bond dipoles are equal in magnitude and opposite in direction, leading to a net dipole moment of zero.

Correct Answer: (D)