Final answer:
The three types of holes in closest-packed structures are tetrahedral holes, octahedral holes, and cubic holes. Tetrahedral holes touch four closest-packed spheres, octahedral holes touch six, and cubic holes touch eight.
Step-by-step explanation:
The three types of holes in closest-packed structures, in order of smallest to largest, are tetrahedral holes, octahedral holes, and cubic holes.
Tetrahedral holes are the smallest and they touch four closest-packed spheres. Octahedral holes are larger and they touch six closest-packed spheres. Cubic holes are the largest and they touch eight closest-packed spheres.
Learn more about types of holes in closest-packed structures