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). This measurement helps scientists understand how closely packed the particles are within a material, which can influence its properties, such as strength and conductivity. 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 more tightly packed. Understanding particle density is essential in fields like materials science and chemistry, where it affects reactions and material behavior.