This is the second part of a classroom-tested learning module on the development of atomic theory. It focuses on the discovery of ions, isotopes, and electron shells. The key idea is that, while the number of protons in an atom remains constant, the number of electrons and neutrons can vary. It includes simple simulations of a hydrogen ion, a hydrogen isotope, energy levels in an “”electron shell”” model of the atom, and animations of electron configurations for the first eleven elements. See Related Materials for a link to Part One of this series. Editor’s Note: This tutorial could be highly useful to teachers seeking additional content knowledge in the basics of atomic structure. The conversational language is appropriate for high school students as well. Don’t miss the “”Questions and Quizzes”” and links to additional resources on the topic.