Topic 4 of 6

Attractions between particles

Instantaneous and permanent dipoles, hydrogen bonds, ice and water.

A-Level 9476 (2026-2027)

Attractions between molecules have an electrostatic origin

All particles can form instantaneous dipoles; only suitable molecules form hydrogen-bond networks.

At any instant an electron cloud may be uneven, creating an instantaneous dipole. It induces a dipole in a neighbour, and opposite partial charges attract. These instantaneous dipole-induced dipole attractions occur in all atoms and molecules, including non-polar Br₂ and the monatomic liquid noble gases. A larger, more easily distorted electron cloud is generally more polarizable and gives stronger attractions.

Permanent dipole-dipole attractions also occur between polar molecules, such as CHCl₃. Favourable orientations bring a δ+ region near a δ- region. CHCl₃ still has instantaneous-dipole attractions as well; do not choose exactly one force for every substance.

Hydrogen bonding is a particularly strong, directional interaction involving H covalently bonded to N, O or F and a suitable lone pair on N, O or F of another molecule (or another part of the same molecule). In water, O-H groups are donors and oxygen lone pairs are acceptors. In ammonia, N-H groups donate and the nitrogen lone pair accepts.

Identify the force before explaining a boiling point

Find possible hydrogen-bond donors and acceptors
An O atom alone is not enough to make a pure substance donate hydrogen bonds; it needs an appropriate O-H, N-H or F-H group.
Check molecular polarity
Polar molecules also have permanent dipole attractions.
Compare polarizability and shape
Electron-cloud size and contact between molecules affect instantaneous-dipole attractions.
State what separates on boiling
Molecules separate; ordinary covalent bonds inside them remain intact.

Check your understandingWhy can Br₂ and argon liquefy although neither has a permanent molecular dipole?Think it through, then reveal the answer

Their electron clouds fluctuate and induce dipoles in neighbouring particles. The resulting attractions can hold particles close at sufficiently low temperature. Argon consists of atoms, not Ar molecules.

Hydrogen bonding makes ice an unusually open solid

An open network explains floating ice; persistent attractions help explain water's thermal properties.

In ordinary ice, water molecules form an open hydrogen-bonded network. Each water molecule can donate two hydrogen bonds through its O-H groups and accept two through oxygen lone pairs. The roughly tetrahedral arrangement leaves more empty space than in liquid water.

On melting, enough of the network is disrupted for molecules to pack more closely. Ice is therefore less dense than liquid water and floats. Liquid water still contains hydrogen bonds: melting does not remove every attraction, and it does not break water into H and O atoms.

Structure explains the observation
Observation	Molecular explanation
Water boils unusually high for a small molecule.	Substantial energy is required to overcome its extensive intermolecular hydrogen bonding.
Ice floats.	The open solid network has a larger volume per molecule and lower density.
Water has a high heat capacity and enthalpy of vaporisation.	Energy changes involve an extensive network of intermolecular attractions as well as molecular motion.
Small alcohols mix readily with water.	Their O-H groups can hydrogen-bond with water; larger non-polar portions can reduce solubility.

Ammonia also hydrogen-bonds, but its available lone-pair sites and the strength and extent of the network differ from water. Do not explain every boiling-point comparison by molar mass alone, or claim that any molecule containing hydrogen can hydrogen-bond.