Particle density refers to the number of particles present in a given volume of a substance. It is typically expressed in units like particles per cubic centimeter (particles/cm³). Higher particle density indicates that more particles are packed into a specific space, which can affect the properties of the material, such as its mass and behavior. In different states of matter, particle density varies significantly. For example, solids generally have a higher particle density than liquids and gases because their particles are closely packed together. Understanding particle density is essential in fields like chemistry, physics, and materials science for analyzing and predicting material behavior.