Final answer:
Particles in gases are much farther apart than in liquids and solids, allowing gases to expand and fill containers. The smaller density of gases also makes them more compressible. Intermolecular forces are mostly negligible in gases except during particle collisions.
Step-by-step explanation:
The distance between particles in gases is significantly larger compared to the distance between particles in liquids and solids. The large separation means that intermolecular forces can typically be disregarded in gases, with the exception of during collisions. This is one reason why gases are able to expand and fill their containers completely. By contrast, the particles in liquids and solids are closely packed, which means they experience significant forces of attraction that restrict their movement.
Consequently, gases are much less dense and more compressible than liquids or solids, which is evident since their densities are about 1000 times smaller than that of solids or liquids. The larger distances between gas particles imply a significant increase in the volume they occupy as per the laws of density, effectively making them much more expandable and easier to compress.