Carbonyl Sulfide is a molecule with the formula \(\text{COS}\). Its polarity depends directly on how its atoms share electrons and its three-dimensional shape. Carbonyl Sulfide is definitively a polar molecule, possessing a measurable electric dipole moment that influences its behavior and physical properties.
The Molecular Geometry of Carbonyl Sulfide
The molecular geometry of Carbonyl Sulfide is crucial to understanding its polarity, as the spatial arrangement of atoms determines how electrical charges are distributed. The molecule is constructed with a central Carbon (\(\text{C}\)) atom bonded to one Oxygen (\(\text{O}\)) atom and one Sulfur (\(\text{S}\)) atom, represented structurally as \(\text{O}=\text{C}=\text{S}\). This involves the central Carbon atom forming two double bonds.
The \(\text{VSEPR}\) (Valence Shell Electron Pair Repulsion) theory predicts the shape by minimizing the repulsion between electron groups around the central atom. The Carbon atom has two regions of electron density (the two double bonds) and no lone pairs. To achieve the greatest separation, these regions push away from each other to opposite sides.
This results in a linear geometry, with the Oxygen, Carbon, and Sulfur atoms lying in a straight line with a \(180^\circ\) bond angle. This linear shape is similar to Carbon Dioxide (\(\text{CO}_2\)), which is a nonpolar molecule. However, the difference in the two atoms bonded to the central Carbon prevents Carbonyl Sulfide from being nonpolar.
Electronegativity and Polarity of Individual Bonds
Bond polarity is created by electronegativity, which is the measure of an atom’s tendency to attract shared electrons toward itself. The atoms in Carbonyl Sulfide possess different electronegativity values on the Pauling scale, which dictates the distribution of electron density.
Oxygen is the most electronegative atom in the molecule, with a Pauling value of approximately \(3.44\). Sulfur (\(2.58\)) and Carbon (\(2.55\)) have very similar, lower values. This ranking means the electron-attracting power decreases in the order \(\text{O} > \text{S} > \text{C}\).
The unequal sharing of electrons creates two distinct polar bonds. The \(\text{C}=\text{O}\) bond is highly polar because the large electronegativity difference causes electrons to be pulled strongly toward Oxygen. This gives Oxygen a partial negative charge (\(\delta^-\)) and Carbon a partial positive charge (\(\delta^+\)).
The \(\text{C}=\text{S}\) bond is far less polar because the electronegativity values of Sulfur and Carbon are almost identical. The magnitude of its bond dipole moment is dwarfed by the much stronger dipole moment of the \(\text{C}=\text{O}\) bond. This difference in magnitude is the fundamental reason for the overall polarity.
Calculating the Net Dipole Moment
The overall polarity of a molecule is determined by the net dipole moment, which is the vector sum of all individual bond dipole moments. In a linear molecule like Carbonyl Sulfide, the two bond dipoles are opposite to one another, but they do not cancel out completely.
For a molecule to be nonpolar, the individual bond dipoles must be equal in magnitude and arranged symmetrically to perfectly negate each other. This is the case with Carbon Dioxide (\(\text{CO}_2\)), where two identical \(\text{C}=\text{O}\) dipoles pull equally in opposite directions, resulting in a net dipole moment of zero.
In Carbonyl Sulfide, the \(\text{C}=\text{O}\) bond dipole points toward Oxygen, and the \(\text{C}=\text{S}\) bond dipole points toward Sulfur. Since the \(\text{C}=\text{O}\) bond is significantly more polar than the \(\text{C}=\text{S}\) bond, the two opposing vectors are unequal in strength.
The stronger pull toward Oxygen is not balanced by the weaker pull toward Sulfur, leaving a residual, non-zero net dipole moment. This confirms that Carbonyl Sulfide is a polar molecule, with an experimentally measured dipole moment of approximately \(0.71 \text{ D}\).