Atoms in a gas form are farther apart than the atoms in a liquid. In both liquid and gas the atoms are in constant motion, resulting in constant bombardment against each other. In a gaseous form the encounters between atoms cause them to tend to move farther apart, and because of the space available the atoms can move farther away faster. In liquid the atoms, being closer together knock each other around, in closer quarters so they don't move farther apart as quickly.
