California Science Test (CAST) Practice Exam 2026 - Free CAST Practice Questions and Study Guide

Carbon nanotubes are carbon arranged into long hollow cylinders by rolling a sheet of carbon atoms arranged in a hexagonal lattice into a tube. The hollow interior is a defining feature, giving the structure its name and many of its unique properties. The tube can be single-walled or multiple-walled, and its tiny diameter paired with great length leads to exceptional strength and interesting electrical and thermal behaviors.

Other forms don’t form hollow cylinders: diamond is a rigid three-dimensional network of carbon atoms, graphite consists of stacked planar sheets of hexagonal rings, and fullerenes are closed cages (like spheres or ellipsoids) of carbon.