Greek philosophers first hypothesized that matter was composed of atoms, but the theory would not resurface again until the late 17th century. The idea that that atoms joined to form structures called molecules first appeared in the 19th century and helped explain why gases, liquids, and solids behave differently from one another. In the 20th century subatomic particles were discovered and atomic structure was finally understood. These breakthroughs led to the development of quantum theory and quantum mechanics. This book details the inspiring and heroic discovery, delving deeply into intriguing stories, reviewing major scientific landmarks, and introducing readers to the women and men who helped discover and map the microscopic universe that is the atom. Supplemental content includes an activity spread, a substantial and highly detailed timeline, and a list of key people with mini-biographies.