Three ways molecules attract

A water molecule has strong covalent bonds inside it. Separate water molecules also attract each other; these are intermolecular forces.

Cambridge uses van der Waals' forces as the umbrella name for all intermolecular forces. Some books use the term more narrowly, so remember the syllabus convention.

• Instantaneous dipole–induced dipole (id–id) forces, also called London dispersion forces, begin when a brief uneven spread of electrons in one molecule nudges electrons in a neighbour. Their opposite ends attract.
• Permanent dipole–permanent dipole (pd–pd) forces act between molecules that already have a slightly positive end and a slightly negative end.
• Hydrogen bonding is a particularly strong, special case of pd–pd attraction. We will zoom in on it next.

London forces can act between any molecules. Permanent dipole attractions need polar molecules.