The mental models of both novice and advanced chemistry students were observed while the students performed a periodic table activity. The mental model framework seems to be an effective way of analyzing student behavior during learning activities. The analysis suggests that students do not recognize periodic trends through the examination of elemental data. Even simple relationships (e.g., the sequence of atomic mass values) proved important to novice understanding of the trends in the periodic table. The use of common heuristics for decision making, and well-known common errors associated with those heuristics independent of the students' chemistry background, were also observed. (Contains 3 tables and 3 figures.)