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Inter-molecular forces (IMFs) are attractive and repulsive forces between molecules. Types of attractive IMFs include , ion-ion forces, ion-dipole forces, and van der Waals forces, including London dispersion forces.
States of matter are defined by the IMFs present in a substance. Solids and liquids have IMFs, though they are weaker in liquids. During the vaporization of a substance, IMFs are overcome. Because there are no forces holding one gas molecule to another, the gas molecules fill the container they're in (It's also relevant that the gas molecules aren't stationary because their motion - called Brownian motion - is very fast at the temperatures that allow for a substance to be in its gaseous state.)