3.1Properties of Substances and Mixtures

Intermolecular and Interparticle Forces — AP CHEMISTRY Revision Notes & Topical Questions

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3.1 Intermolecular and Interparticle Forces — Notes — AP CHEMISTRY notes: Compare the types of intermolecular force and relate them to physical properties.Intermolecular forces (IMFs) are attractions between molecules and are much weaker than covalent bonds within molecules.Boili…

Dipole-dipole forces
Hydrogen bonding
Ion-dipole forces
Dipole-induced dipole forces
London dispersion force
Polarisability

Section 1 of 9Intermolecular forces

Intermolecular forces

Intermolecular forces (IMFs) are attractions between molecules and are much weaker than covalent bonds within molecules.

Boiling and melting molecular substances overcome IMFs, not covalent bonds.

Stronger IMFs give higher boiling point, higher melting point, higher viscosity, higher surface tension and lower vapour pressure.

Section 2 of 9London dispersion forces

LDFs arise from Coulombic interactions between temporary, fluctuating dipoles and are present in all molecules.

They are often the strongest net force between large molecules.

Strength increases with polarisability, which increases with the number of electrons (molar mass) and contact surface area.

n-Pentane has a higher boiling point than neopentane because of greater surface contact.

In large nonpolar molecules LDFs can outweigh the dipole-dipole forces of small polar molecules, so I₂ is a solid while HCl is a gas.

Section 3 of 9Forces involving dipoles

Forces involving dipoles

Dipole-dipole forces

Dipole-dipole forces the δ+ end of one polar molecule attracts the δ− end of another.

Hydrogen bonding

Hydrogen bonding a strong dipole-dipole attraction between H bonded to N, O or F and a lone pair on N, O or F of another molecule.

Ion-dipole forces

Ion-dipole forces an ion attracts the oppositely charged end of a polar molecule,
- such as Na⁺ and the oxygen of water,
- and are important in dissolving ionic compounds.

Dipole-induced dipole forces

Dipole-induced dipole forces a polar molecule induces a dipole in a nonpolar one.

Relative strength and orientation can be understood from the signs of the partial charges.

Section 4 of 9Hydrogen bonding and water

Hydrogen bonding explains water's unusually high boiling point compared with H₂S.

Section 5 of 9Comparison of forces

Force	Present in	Depends on	Example
London dispersion	All molecules	Number of electrons, surface area	I₂, CH₄
Dipole-dipole	Polar molecules	Size of dipole	HCl
Hydrogen bond	H bonded to N, O or F	Number and strength of bonds	H₂O, NH₃
Ion-dipole	Ions with polar solvents	Charge and dipole	NaCl in water

Section 6 of 9Large biomolecules

Noncovalent interactions occur between different molecules or between regions of the same large biomolecule.

Section 7 of 9More from the specification

1Hydrogen bonding between electronegative atoms

Hydrogen bonding between electronegative atoms electrostatic attraction binds hydrogen atoms covalently attached to nitrogen, oxygen,
- or fluorine to lone pairs on adjacent atoms.

2Dipole moment

A measure of the polarity of a bond, shown by an arrow pointing to the δ− end.

Section 8 of 9Key terms

Term	Meaning
Intermolecular force	Attraction between molecules.
London dispersion force	Attraction between instantaneous and induced dipoles.
Polarisability	Ease of distorting an electron cloud.
Hydrogen bonding	Attraction between H bonded to N, O or F and a lone pair on another N, O or F.
Ion-dipole force	Attraction between an ion and a polar molecule.
Dipole-induced dipole force	Attraction between a polar and a nonpolar molecule.

Section 9 of 9Exam technique

Saying boiling breaks covalent bonds in molecular substances

Using the term London dispersion forces only for large molecules — it applies to all

Saying hydrogen bonds form with any hydrogen atom

Ranking forces by molar mass alone when polar interactions are present

Last updated: 15 October 2026