Chemical polarity describes how electrical charge is distributed within a molecule. This property arises from the unequal sharing of electrons between atoms, creating a separation of charge known as a dipole moment. For a molecule to be polar, it must have a net positive and a net negative end. Carbon disulfide (\(\text{CS}_2\)) is a common industrial chemical and volatile liquid solvent whose overall polarity is determined by its structure.
Building the Molecule: Lewis Structure and Geometry
Understanding the polarity of carbon disulfide requires examining its atomic arrangement. The molecule consists of a central carbon atom double-bonded to two sulfur atoms. Since the central carbon uses all four of its valence electrons to form these two double bonds, it has no non-bonding electron pairs (lone pairs) remaining. This arrangement creates a highly symmetrical structure with only two electron-dense regions. Consequently, the atoms arrange themselves in a straight line, giving \(\text{CS}_2\) a linear molecular geometry with a \(180^\circ\) bond angle.
Are the Individual Carbon-Sulfur Bonds Polar?
Molecular polarity depends on both the polarity of individual bonds and the overall shape of the molecule. Bond polarity is determined by the difference in electronegativity between the bonded atoms. On the Pauling scale, carbon has an electronegativity value of approximately \(2.55\), and sulfur is \(2.58\). This small difference of \(0.03\) causes a slight unevenness in electron sharing, pulling the electrons closer to the more electronegative sulfur atom. Consequently, each individual \(\text{C-S}\) double bond is technically a polar bond.
The Final Verdict: Why Carbon Disulfide is Nonpolar
Despite having slightly polar individual bonds, carbon disulfide is classified as a nonpolar molecule. This is due to the molecule’s perfectly linear and symmetrical shape. Each polar bond creates a small charge separation, or dipole moment, which acts as a vector pointing toward the more negative sulfur atom. Since the carbon atom is centered, the two sulfur atoms are pulling on the electrons with equal strength in exactly opposite directions. Because the opposing dipole moments are equal in magnitude, they directly cancel each other out, resulting in a net dipole moment of zero.
How Nonpolarity Affects Carbon Disulfide’s Properties
The nonpolar nature of \(\text{CS}_2\) dictates many of its physical and chemical behaviors, particularly its ability to dissolve other substances. The rule “like dissolves like” means nonpolar solvents dissolve nonpolar solutes. Because carbon disulfide is nonpolar, it is an excellent solvent for other nonpolar compounds. It is highly effective at dissolving materials that resist water, such as elemental sulfur, phosphorus, iodine, fats, oils, resins, and waxes.
Conversely, carbon disulfide is immiscible with polar solvents like water. This characteristic makes it valuable in industrial processes, including the manufacture of rayon fiber and cellophane, where it acts as a selective solvent. The liquid is also highly volatile, meaning it readily evaporates, a property linked to the weak intermolecular forces typical of nonpolar compounds.