Intermolecular Forces & Molecular Polarity

Vocabulary flashcards covering electronegativity trends, bond and molecular polarity, and the major types, origins, and relative strengths of intermolecular forces.

Intermolecular Force (IMF)

An attractive or repulsive force acting between separate molecules, responsible for many physical properties such as boiling point and solubility.

Intramolecular Force

A chemical bond (covalent, ionic, or metallic) that holds atoms together within a single molecule.

Electronegativity (EN)

A measure of an atom’s ability to attract shared electrons to itself in a chemical bond.

Electronegativity Trend

EN increases from left-to-right across a period and decreases from top-to-bottom down a group on the periodic table.

Polar Bond

A covalent bond with an electronegativity difference of about 0.6–1.9, resulting in partial positive (δ+) and partial negative (δ−) ends.

Non-polar Bond

A covalent bond with an electronegativity difference of 0–0.5, producing an even electron distribution and no permanent dipole.

Polar Molecule

A molecule whose overall geometry produces a net dipole moment, resulting in a slightly positive end and a slightly negative end.

Non-polar Molecule

A molecule with no permanent net dipole because bond dipoles cancel or all bonds are non-polar.

Dipole Moment

A quantitative measure of molecular polarity; the vector sum of individual bond dipoles within a molecule.

Dipole–Dipole Interaction

An IMF between two polar molecules where the δ+ end of one aligns with the δ− end of another; stronger than dispersion but weaker than hydrogen bonds.

Ion–Dipole Interaction

An IMF between an ion and the partial charges of a polar molecule; important in dissolving ionic compounds in polar solvents.

London Dispersion Force

The weakest IMF caused by temporary (instantaneous) dipoles arising from momentary uneven electron distributions in all atoms and molecules.

Induced Dipole

A temporary dipole created in an atom or molecule by the presence of a nearby charge or dipole.

Hydrogen Bond

A strong type of dipole-dipole attraction that occurs when H is covalently bonded to highly electronegative N, O, or F and interacts with a lone pair on N, O, or F in another molecule.

van der Waals Forces

A collective term for all relatively weak IMFs, including London dispersion, dipole-dipole, and dipole-induced dipole interactions.

Cation

A positively charged ion produced when an atom or molecule loses one or more electrons.

Anion

A negatively charged ion produced when an atom or molecule gains one or more electrons.

Partial Charge (δ+ / δ−)

A fractional electric charge on an atom in a polar bond due to unequal electron sharing.

Temporary Dipole

A fleeting separation of charge in a non-polar molecule caused by momentary electron movement; basis of dispersion forces.

Johannes Diderik van der Waals

Dutch physicist (1837–1923) who recognized non-ideality in real gases and the existence of intermolecular interactions.

Hydrogen-Bond Donor

The molecule containing the H attached to N, O, or F that participates in hydrogen bonding.

Hydrogen-Bond Acceptor

The molecule with a lone pair on N, O, or F that forms a hydrogen bond with a donor’s hydrogen.

Strength Order of IMFs

Approximate increasing order: London dispersion < dipole-dipole ≈ hydrogen bonding (special dipole) < ion-dipole < ionic bonding.

Bond Polarity Rule

EN difference 0–0.5 → non-polar covalent; 0.6–1.9 → polar covalent; ≥2.0 → ionic (approximate guideline).

Molecular Polarity Rule

A molecule is polar if it contains polar bonds whose dipoles do not cancel due to molecular shape.

Distributed (Symmetrical) Geometry

A molecular shape (e.g., CO2, BF3) where identical polar bonds cancel, yielding a non-polar molecule.

Unsymmetrical Geometry

A molecular shape (e.g., H2O, NH3, SO2) where bond dipoles fail to cancel, producing a polar molecule.

Ion-Induced Dipole

An IMF where the electric field of an ion distorts the electron cloud of a nearby non-polar molecule, inducing a dipole and attracting it.

Dipole-Induced Dipole

An IMF where a polar molecule induces a temporary dipole in a neighboring non-polar molecule, creating attraction.