Question

Which molecule is most polar? a. \(S=C=S\) b. \(\mathrm{O}=\mathrm{C}=\mathrm{O}\) c. \(\mathrm{O}=\mathrm{C}=\mathrm{S}\)

Short answer

The most polar molecule is \\(O=C=S\\).

Step-by-step solution

Determine electronegativity of each atom

First, identify the electronegativity values of the atoms involved: - Sulfur (S) has an electronegativity of 2.58. - Carbon (C) has an electronegativity of 2.55. - Oxygen (O) has an electronegativity of 3.44. The difference in electronegativity between the atoms in a molecule will help us determine the polarity of the bonds and overall molecule.

Analyze the molecular structure and bond polarity

Let's analyze the bonds: - In option (a), - the bond S=C has an electronegativity difference of |2.58 - 2.55| = 0.03, and - C=S has |2.55 - 2.58| = 0.03. - In option (b), - the bond O=C has |3.44 - 2.55| = 0.89 and - C=O has |2.55 - 3.44| = 0.89. - In option (c), - the bond O=C has |3.44 - 2.55| = 0.89 and - C=S has |2.55 - 2.58| = 0.03.

Assess molecular symmetry and net dipole moment

A molecule is non-polar if it is symmetric because the dipole moments cancel each other out.- Option (a) \(S=C=S\) is linear and symmetric, so the dipoles cancel making it non-polar.- Option (b) \(O=C=O\) is also linear and symmetric, so it is non-polar.- Option (c) \(O=C=S\) is not symmetric because the bond strengths (O-C vs. C-S) differ, causing a net dipole moment. Hence, it is polar.

Key concepts

Electronegativity

Electronegativity is a fundamental concept in chemistry that refers to the ability of an atom to attract electrons towards itself when it is bonded to another atom. It plays a critical role in determining the polarity of a molecule. The more electronegative an atom is, the stronger its pull on the bonding electrons. This can lead to an unequal sharing of electrons and contribute to the formation of polar bonds.

In the provided exercise, we see different electronegativities for sulfur, carbon, and oxygen:

• Sulfur (S) has an electronegativity of 2.58.
• Carbon (C) has an electronegativity of 2.55.
• Oxygen (O) has an electronegativity of 3.44.

A high electronegativity difference between atoms usually means the molecule will have regions of partial positive and negative charges. In essence, a high difference may lead to a more polar bond, such as the O=C bond with a difference of 0.89, which indicates a stronger polar character compared to others in the problem.

Understanding the electronegativity differences helps predict which molecules will have polar bonds contributing to their overall polarity. The greater the difference, the more polar the bond tends to be.

Molecular Symmetry

Molecular symmetry refers to the symmetrical arrangement of atoms in a molecule. Symmetry plays an essential role in determining whether a molecule is polar or non-polar. If the structure of a molecule is symmetrical, the dipole moments of the bonds can cancel each other out, resulting in no net dipole moment and making the molecule non-polar.

For example, in the problem, carbon dioxide (\(\mathrm{O} = \mathrm{C} = \mathrm{O}\)) is a linear and symmetrical molecule. Both O=C bonds have similar electronegativity differences, causing their dipole moments to cancel out, rendering the molecule non-polar.

In contrast, carbonyl sulfide (\(\mathrm{O} = \mathrm{C} = \mathrm{S}\)) lacks symmetry due to the differing atoms on each side of carbon. This lack of symmetry results in dipole moments that do not cancel completely, making the molecule polar. Recognizing symmetry in molecules helps understand their net dipolar behavior, influencing molecular interactions and properties significantly.

Dipole Moments

Dipole moments arise when there is an uneven distribution of electron density in a molecule. This generates a positive end and a negative end, leading to a dipole. The magnitude of the dipole moment can indicate the polarity of a molecule—the greater the dipole moment, the more polar the molecule.

The problem shows us examples of linear molecules where analyzing dipole moments becomes pertinent to understanding polarity:

• In a symmetrical molecule like sulfur dioxide \(\mathrm{S} = \mathrm{C} = \mathrm{S}\), the dipole moments from each S=C bond cancel out, due to identical electronegativity differences on either side, resulting in a non-polar molecule.
• Similarly, in carbon dioxide \(\mathrm{O} = \mathrm{C} = \mathrm{O}\), the dipole moments cancel, thanks to symmetrical alignment.
• In contrast, in carbonyl sulfide \(\mathrm{O} = \mathrm{C} = \mathrm{S}\), the bond dipoles differ in magnitude due to the different atoms involved, causing a net dipole moment and hence, endowing the molecule with polarity.

This concept is vital because molecules with net dipole moments interact strongly with other polar entities, influencing solubility, boiling points, and reactivity.