Particle theory, also referred to as the kinetic theory of matter, is a theory in science that describes all forms of matter as being composed of tiny particles that are always in a constant state of motion. The theory also states that the degree to which these particles move around depends on the amount of energy that is present in the matter and what the nature of the relationship is between the various particles.
Particles can refer to various forms of matter, such as atoms, molecules and ions. When scientists use the word "particle" in particle theory, they specifically refer to unspecified units of matter. In other words, it doesn't matter if they are referring to atoms, molecules or ions; all are particles.
Particle theory is a very important field of science because it helps scientists explain why matter behaves the way it does, while providing a reliable method of visualizing the processes that occur to matter.
Within the context of particle theory, certain rules are established concerning the various states of matter, including solids, liquids and gaseous substances. The particles in solids are packed together very tightly and their fixed positions prevent them from vibrating.The particles in liquids are also very attracted to each other but they can move around much more freely. The particles can't go anywhere they please, though, because their movement is still based upon their attraction to each other. At the opposite end of the spectrum, however, the particles found in gaseous substances have very little attraction between them. They can move around as they please and are widely spaced out.