There is growing research interest in the challenges and opportunities learners face in representing scientific understandings, processes and reasoning. These challenges include integrating verbal, visual and mathematical modes in science discourse to make strong conceptual links between representations and classroom experiences. Our paper reports on a project that aimed to identify practical and theoretical issues entailed in a representation-intensive approach to guiding students' conceptual learning in science. We focus here on a teacher developing students' understanding of the formation of ions and molecules. We argue that the representations produced by students in this process met the criteria for representational competence proposed by diSessa ("Cognition and Instruction," 22, 293-331, 2004) and Kozma & Russell (2005). The students understood that an effective representation needed to show relevant information, focus on pertinent points, be self-sufficient in its claims about the topic and provide coherent links between different parts of the representation. The final activity showed that their representations reached Kozma & Russell's (2005) highest level of competence, where the students were able to use specific features of their representations to critique their suitability for explaining bonding and were able to show how their representation linked to the periodic table as a representation. We conclude by considering the implications of these findings.