This classroom-tested learning module gives a condensed, easily-understood view of the development of atomic theory from the late 19th through early 20th century. The key idea was the discovery that the atom is not an “”indivisible”” particle, but consists of smaller constituents: the proton, neutron, and electron. It discusses the contributions of John Dalton, J.J. Thomson, Ernest Rutherford, and James Chadwick, whose experiments revolutionized the world view of atomic structure. See Related Materials for a link to Part 2 of this series. Editor’s Note: This tutorial could be highly useful to teachers seeking additional content knowledge in the basics of atomic physics. 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.