This handout guides students through the virtual lab "Building an Atom" simulation found for free at phet.colorado.edu. This activity works well as the "explore" phase of the 5e model for a unit on atomic structure or as a quick review for students who have learned atomic structure before.This is an inquiry-based approach to the simulation, where students are directed to play around and determine which subatomic particles (protons, neutrons, and electrons) affect the mass, charge, and name of a