Why is Gaseous Water Lighter than Liquid Water?

Clouds are lighter than the water they are made of. When they become gaseous (e.g. by heating), the volume increases by a factor of 1672. This makes the gas lighter than the air. The question has been occupying scientists for over 200 years, when water rises from a large surface as fog to form clouds. A publication by the Italian Avogadro (1811) had prompted the French scholar Ampère to write a letter to Count Berthollet and to publish it immediately (1814) in the Annals of Chemistry. In it, he argues that a gas particle must consist of at least 8 particles. In doing so, he laid the foundation for the quantum number n = 2, for the smallest common gas volume.