The particle theory states that matter is made up of small indivisible particles that are in constant motion, the degree of freedom of which varies for the different states of matter.
In the case of solids, the particles are very close to each other and fixed in position. They can only vibrate in the structures they are a part of. Solids have the highest mass per unit volume or density.
In liquids, the particles can move relative to each other but their proximity makes the volume almost constant. Liquids have a mass per unit volume or density lying between that of solids and liquids.
In the case of gases, the particles are sufficiently away from each other to allow them to move freely and independent of each other. The volume of gases can change depending on the pressure applied. They have the lowest mass per unit volume or density.