This work explains the basic conception of matter as composed of atoms, describing electrons and nuclei and showing how spectroscopy, X-rays, and radio-activity reveal atomic structure. It surveys the periodic law, the hydrogen atom and its spectral lines, quantum principles including Bohr’s ideas, electron arrangements in rings, X-ray evidence for inner shells, and theories of nuclear composition and radioactive emission. The text concludes by relating these discoveries to the wave theory of light and to relativity, using clear analogies, qualitative argument, and an appendix with a more detailed mathematical account.