Electron configuration provides insight into the chemical behaviors of elements and is an important concept for students to master in introductory chemistry. To better strengthen undergraduate students' mastery of electron configurations of atoms and ions, we designed a novel, interactive chemistry game called ChemisTree that uses active-learning techniques (e.g., physically building electron configurations onto a game board, small group work, and whole class discussion) and incorporates the three rules for electrons filling orbitals (Aufbau principle, Pauli exclusion principle, and Hund's rule), using LEGO tiles and plates to represent electrons and orbitals. Students watch an introductory video explaining the rules and components of the game prior to applying their knowledge of electron configuration by working in small groups to build the orbital diagrams of atoms and ions using the materials provided in their ChemisTree game kits. Student understanding is assessed using multiple-choice pre/post-test questions, scoring sheets with peer discussion, and self-reported postactivity evaluations. Students who played the game scored significantly higher in the post-test (62.7%) compared to the pre-test (51.2%), representing an 11.5% increase in average score. Here we provide details about the activity and present data suggesting that student understanding of electron configuration is improved.