Molecular Polarity - Chemistry

1. Fundamental Concepts

Definition: Molecular polarity describes the distribution of electrical charge within a molecule. A molecule is considered polar when the individual bond dipoles do not cancel out. In contrast, a molecule is nonpolar when bond dipoles cancel due to molecular symmetry, even if the bonds themselves are polar.
Electronegativity: The relative ability of an atom in a chemical compound to attract shared electrons to itself (dimensionless, relative value).
Dipole Moment: A measure of the separation of positive and negative charges in a molecule, represented by the symbol $$ \mu $$ . It is a vector quantity with both magnitude and direction. (magnitude = charge × distance, direction points from δ⁺ to δ⁻)

2. Key Concepts

Bond Polarity vs. Molecular Polarity: Bond polarity comes from electronegativity difference (ΔEN) between bonded atoms. Molecular polarity depends on both bond polarities and molecular geometry (3D shape); polar bonds can sum to a nonpolar molecule if symmetric.

Physical Properties Linked to Polarity
Solubility: like dissolves like — polar dissolves in polar solvents (H₂O); nonpolar dissolves in nonpolar solvents (hexane).
Boiling/Melting Points: Polar molecules have stronger dipole‑dipole forces, generally higher bp/mp than nonpolar molecules of similar molar mass.
Conductivity: Polar liquids can support solvation of ions; nonpolar liquids cannot dissolve ionic compounds.

3. Examples

Example 1 (Easy)

Problem: Classify CO₂ and H₂O as polar or nonpolar, and explain briefly.

Solution:

CO₂: Nonpolar
Although each C=O bond is polar, CO₂ has a linear geometry, so the bond dipoles are equal in magnitude and opposite in direction, causing them to cancel out.
H₂O: Polar
H₂O has polar O–H bonds and a bent geometry, so the bond dipoles do not cancel. This results in a net dipole moment, making H₂O a polar molecule.

Example 2 (Medium)

Problem: Explain why CH₄ is nonpolar but CH₃Cl is polar, using geometry and bond dipoles.

Solution:

CH₄ is nonpolar because it has a tetrahedral geometry with four identical C–H bonds. The bond dipoles are equal in magnitude and symmetrically arranged, so they cancel out, resulting in no net dipole moment.
CH₃Cl is polar because, although it also has a tetrahedral geometry, one of the bonds is a C–Cl bond, which is more polar than the C–H bonds. This breaks the symmetry of the molecule, so the bond dipoles do not cancel. The strong dipole of the C–Cl bond produces a net dipole moment, making CH₃Cl a polar molecule.

Example 3 (Hard)

Problem: Compare and explain the polarity of BF₃ and NH₃. Include electron geometry, molecular geometry, and dipole cancellation.

Solution:

BF₃: Electron geometry: Trigonal planar. Molecular geometry: Trigonal planar. Polarity: Nonpolar.
Although each B–F bond is polar, BF₃ has a symmetrical trigonal planar shape. The three identical bond dipoles are evenly spaced at 120° and cancel each other out, resulting in no net dipole moment.
NH₃: Electron geometry: Tetrahedral. Molecular geometry: Trigonal pyramidal. Polarity: Polar.
NH₃ has three N–H bonds and one lone pair on nitrogen. The lone pair changes the molecular geometry to trigonal pyramidal, making the molecule asymmetrical. As a result, the bond dipoles do not cancel, producing a net dipole moment.
Conclusion: BF₃ is nonpolar due to dipole cancellation in a symmetrical geometry, whereas NH₃ is polar because its asymmetrical shape prevents dipole cancellation.

4. Problem-Solving Techniques

Electronegativity Table: Use a periodic table with electronegativity values to quickly find the electronegativity of elements.
Use a Two-Step Approach: 1. Are the bonds polar? 2. Does the molecular geometry cancel the dipoles?
Always Analyze the Shape: Lewis structure → VSEPR → polarity.
Vector Addition: For molecules with more than one polar bond, use vector addition to determine the net dipole moment.
Bond Polarity ≠ Molecular Polarity: Memorize common symmetric nonpolar molecules with polar bonds, e.g., CO₂, BF₃, CH₄, CCl₄.