Polarity
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Topic Summary & Highlights
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Core Concept
Polarity: A measure of how equally electrons are shared between atoms in a bond or distributed in a molecule.
Polar Bond: A bond with an unequal sharing of electrons due to differences in electronegativity between atoms (e.g., H-Cl).
Nonpolar Bond: A bond with equal sharing of electrons, typically between identical atoms or atoms with similar electronegativity (e.g., O₂, H₂).
Practice Tips
Analyze Geometry: Use VSEPR theory to determine molecular shape and symmetry.
Check Electronegativity: Compare values to determine bond polarity.
Draw Dipoles: Represent dipole moments with arrows pointing toward the more electronegative atom.
Polar molecules exhibit stronger intermolecular forces, affecting their physical properties like solubility and boiling points.
Bond Polarity
Electronegativity Difference:
The difference in electronegativity between bonded atoms determines bond polarity.
Nonpolar Bond: Electronegativity difference ≤0.4\leq 0.4≤0.4.
Polar Bond: Electronegativity difference 0.5−1.70.5 - 1.70.5−1.7.
Ionic Bond: Electronegativity difference >1.7> 1.7>1.7.
Partial Charges:
In polar bonds, the more electronegative atom pulls electrons closer, gaining a partial negative charge (δ−\delta^-δ−), while the less electronegative atom becomes partially positive (δ+\delta^+δ+).
Example: H-Cl\text{H-Cl}H-Cl: H(δ+)\text{H} (\delta^+)H(δ+), Cl(δ−)\text{Cl} (\delta^-)Cl(δ−).
Molecular Polarity
Shape and Symmetry:
Even if a molecule has polar bonds, it may be nonpolar if its shape is symmetrical (dipoles cancel out).
Example:
CO2\text{CO}_2CO2: Linear and nonpolar (dipoles cancel).
H2O\text{H}_2\text{O}H2O: Bent and polar (dipoles add up).
Net Dipole Moment:
A molecule is polar if it has a net dipole moment (unequal charge distribution across the molecule).
Nonpolar molecules have no net dipole moment.
Rules for Determining Polarity
Check the electronegativity differences of bonds.
Draw the molecule's Lewis structure.
Consider the molecule's shape using VSEPR theory.
Determine if bond dipoles cancel out or add up to create a net dipole moment.
Effects of Polarity
Intermolecular Forces:
Polar molecules interact through dipole-dipole forces or hydrogen bonding, while nonpolar molecules interact via London dispersion forces.
Solubility:
"Like dissolves like": Polar molecules dissolve in polar solvents (e.g., water), and nonpolar molecules dissolve in nonpolar solvents (e.g., hexane).
Boiling and Melting Points:
Polar molecules typically have higher boiling and melting points due to stronger intermolecular forces.
