The particle theory of matter or the kinetic molecular theory of matter describes the microscopic properties of atoms (or molecules) and their interactions, which result in observable macroscopic properties (such as pressure, volume, temperature).
The theory can be used to explain why matter exists in different phases (solid, liquid, and gas), as well as how matter can change from one phase to the next.
Postulates of Particle Theory of Matter:
All matter is made up of tiny particles known as atoms.
Particles of matter are constantly in motion.
Particles of matter attract each other.
Particles of matter have spaces between them.
As temperature increases, particles of matter move faster.
Atoms of the same element are essentially identical and atoms of different elements are different.