The Next Generation Science Standards call for changes in not only what is taught in elementary science but also how students engage in the learning experience to develop understanding of core disciplinary ideas. In this study we examined 5th-grade teachers' pedagogical content knowledge (PCK) for 1 particular core idea: the small particle model (SPM) of matter. We assessed teachers' initial PCK through a lesson plan task, the Content Representation tool, and interviews and then adapted and tested a scoring rubric to facilitate comparison of teachers' PCK. Although teachers' lessons included the SPM, they had difficulty identifying where this topic fit into the traditional matter unit sequence and why the SPM was important to understanding scientific phenomena. When teachers introduced the SPM, they did so by introducing and explaining the model to students (i.e., as a teaching model) rather than engaging students in developing, using, and critiquing models. Although PCK scores overall were quite similar, we observed that they tended to be higher for teachers with greater teaching experience at grade level (as opposed to teaching experience overall). In light of this, we give critical consideration to the re-novicing of elementary teachers by grade-level reassignment and the impacts this has on their development of PCK.