The Atomic Packing Factor (APF) is a measure of how efficiently atoms are packed in a crystal structure. It is defined as the ratio of the volume occupied by the atoms in a unit cell to the total volume of the unit cell. A higher APF indicates a denser packing of atoms, which can influence the material's properties, such as strength and conductivity. Different crystal structures, like face-centered cubic (FCC) and body-centered cubic (BCC), have varying APFs. For example, the APF for FCC is approximately 0.74, meaning 74% of the unit cell volume is filled with atoms, while BCC has an APF of about 0.68. Understanding APF helps in material science and engineering applications.