The document summarizes key concepts from Chapter 6 of the textbook, including: 1) Modern atomic theory arose from studies of radiation interacting with matter, and electromagnetic radiation has characteristic wavelengths and frequencies. 2) Planck proposed that atoms can only absorb or emit energy in discrete quanta, proportional to frequency via Planck's constant. 3) Einstein assumed light is composed of discrete energy packets called photons, helping explain the photoelectric effect. 4) Bohr incorporated Planck's quantization idea into his atomic model, where electrons orbit in distinct energy levels corresponding to line spectra wavelengths. 5) Later models including de Broglie's matter waves, Heisenberg's uncertainty principle, and