Question

Which of the following molecules would you expect to be polar: (a) \(\mathrm{HCN} ;\) (b) \(\mathrm{SO}_{3} ;\) (c) \(\mathrm{CS}_{2} ;\) (d) OCS; (e) \(\operatorname{SOCl}_{2} ;\) (f) \(\operatorname{SiF}_{4} ;\) (g) \(\operatorname{POF}_{3}\) ? Give reasons for your conclusions.

Short answer

(a) HCN is Polar (b) SO3 is Nonpolar (c) CS2 is Nonpolar (d) OCS is Polar (e) SOCl2 is Polar (f) SiF4 is Nonpolar (g) POF3 is Polar

Step-by-step solution

Analyze Molecular Structure - HCN

Hydrogen cyanide (HCN) has a linear structure, which means the hydrogen atom, carbon atom, and nitrogen atom are aligned in a straight line. Nitrogen is more electronegative than carbon, resulting in a dipole moment directed towards nitrogen. The carbon and hydrogen atoms, however, do not create a significant dipole due to the hydrogen's low electronegativity. So, HCN is a polar molecule as there is a net dipole.

Analyze Molecular Structure - SO3

Sulfur trioxide (SO3) is a planar molecule consisting of a sulfur atom bonded to three oxygen atoms, with no lone pairs of electrons on the sulfur. Here, the symmetry of the molecule cancels out any effects of individual bond dipoles, resulting in a nonpolar molecule.

Analyze Molecular Structure - CS2

Carbon disulfide (CS2) is a linear molecule consisting of a carbon atom with two sulfur atoms. The molecule is symmetrical, causing any polar bonds to cancel out; as such, CS2 is nonpolar.

Analyze Molecular Structure - OCS

OCS (carbonyl sulfide) has a linear molecular structure. Oxygen is more electronegative than sulfur, so the net dipole is from sulfur to oxygen, making it a polar molecule.

Analyze Molecular Structure - SOCl2

Sulfuryl chloride (SOCl2) is a polar compound because its structure is not symmetrical, and the individual bond dipoles do not cancel out. There are two chlorine atoms and an oxygen atom around the sulfur atom, and the oxygen atom has unpaired electrons that contribute to the net dipole moment of the molecule.

Analyze Molecular Structure - SiF4

Silicon tetrafluoride (SiF4) is a symmetrical molecule with silicon at the center and fluorine atoms at the corners of a regular tetrahedron. Each Si-F bond is polar, but the dipole moments cancel out due to the symmetry of the molecule. So, SiF4 is nonpolar.

Analyze Molecular Structure - POF3

Phosphoryl fluoride (POF3) has a trigonal pyramidal molecular structure. This molecule lacks symmetry, and hence POF3 is a polar molecule.

Key concepts

Electronegativity

Electronegativity refers to the ability of an atom to attract shared electrons within a chemical bond.
Atoms with higher electronegativities draw electrons closer, creating a greater difference in charge within a molecule.
This can lead to bond polarity. For instance, in HCN:

• Nitrogen is more electronegative than carbon, pulling electrons toward itself.
• Hydrogen being less electronegative creates less impact on the overall polarity.

A large difference in electronegativity, such as between oxygen and sulfur in OCS, causes a shift in electron density, resulting in a dipole moment and making molecules like OCS polar.

Dipole Moment

The dipole moment is a measure of the separation of positive and negative charges in a molecule.
It indicates the polarity of a molecule — the higher the dipole moment, the more polar the molecule.
Dipole moments arise when electrons are unevenly distributed in a bond, creating partial positive and negative charges.

• HCN has a significant dipole moment due to the nitrogen at one end pulling electrons.
• In a symmetrical molecule like SO3, individual dipoles cancel out, leading to no net dipole moment.

The net dipole moment can be visualized as a vector sum of all the individual bond dipoles in the molecule.

Molecular Geometry

Molecular geometry defines the spatial arrangement of atoms around a central atom in a molecule.
This structure plays a vital role in determining the molecule's properties, including polarity.
Knowing the shape helps in predicting whether the dipoles will cancel out or not.

• For instance, CS2 is linear and symmetrical, leading to nonpolarity due to cancellation of dipoles.
• In contrast, the trigonal pyramidal shape of POF3 lacks symmetry, leading to a net dipole and thus, polarity.

Recognizing shapes like linear, tetrahedral, and trigonal planar is crucial for assessing a molecule's behavior.

Symmetry

Symmetry in a molecule affects the overall dipole moment and hence determines its polarity.
A symmetrical molecule has evenly distributed charges, which often makes it nonpolar.

• SiF4 is an example of a symmetrical structure where the fluorine atoms spread out evenly around silicon, leading to no net dipole.
• Conversely, asymmetry as seen in SOCl2, where the arrangement of atoms prevents dipole cancellation, typically results in a polar molecule.

Accurate visualization of a molecule’s symmetry allows for predicting if its dipoles will negate each other.

Polar and Nonpolar Molecules

Molecules can be characterized as polar or nonpolar based on their dipole moments and symmetry.
Polar molecules have a net dipole moment, while nonpolar molecules' dipoles cancel out.

• In polar molecules like HCN and OCS, differing electronegativities across the molecule create a dipole moment.
• Nonpolar molecules such as SO3 and CS2 have balanced electron distribution due to symmetrical shapes, eliminating net dipole moments.

This concept helps in understanding molecular interactions, such as solubility and reactivity, important in both natural and industrial processes.