The atoms are able to diffuse into each others solid matrix thus causing them to adhere to one another but only after several years being in contact with each other's surface. On the other hand, liquid substances are composed of particles that are less packed and indeed can move more freely. Having more space between the liquid particles makes diffusion more likely to occurs at a faster rate. For example, if one placed a drop of liquid colored ink into a glass of liquid water the ink drop would slowly but noticeably diffuse out throughout the water. Since the ink particles are farther away from each other, and the water molecules are spread out, the molecules of ink can diffuse fairly rapidly so that the water is colored homogeneously throughout with the color of the ink in a matter of minutes.
Gaseous substances are in a league all to themselves.
The molecular particles of gas are much more distant from one another than either liquid or solid particles are to each other. That is why gases are transparent. Light can pass through the relatively large spaces between the molecules of gas without any distortion. Liquids distort light more so because the particles are closer ,and the light passing through a liquid can be deflected more easily. If a gas possessing an odor such as Ammonia were allowed to escape an open bottle, it would not take but a few seconds for the detectable odor of Ammonia to be detected throughout the room.