Final answer:
Gases have lower densities than liquids and solids because the atoms and molecules in gases are further apart. The density of a substance is inversely related to the volume occupied by its particles. As the distance between the particles in a gas increases, the volume occupied by the gas increases, leading to a decrease in density.
Step-by-step explanation:
Atoms and molecules are close together in solids and liquids. In gases they are separated by empty space. Thus gases have lower densities than liquids and solids. Density is mass per unit volume, and volume is related to the size of a body (such as a sphere) cubed. So if the distance between atoms and molecules increases by a factor of 10, then the volume occupied increases by a factor of 1000, and the density decreases by a factor of 1000.