Final answer:
In gases, the distance between molecules is about 10 times greater than the size of their molecules, resulting in lower densities.
Step-by-step explanation:
Atoms and molecules are close together in solids and liquids. In gases, they are separated by empty space, resulting in lower densities. The density of a substance is mass per unit volume, and volume is related to the size of the molecules. When the distance between molecules in gases increases by a factor of 10, the volume occupied increases by a factor of 1000, and the density decreases by a factor of 1000.