The document discusses the structure of atoms. It describes the three main parts of an atom as protons, neutrons, and electrons. Protons are positively charged particles found in the atomic nucleus, neutrons are uncharged particles in the nucleus, and electrons are negatively charged particles that surround the nucleus. The document also defines related terms like nucleus, quarks, isotopes, and the electron cloud model, which depicts electrons as moving in a cloud-like region surrounding the nucleus.