The purpose of this study was to develop pre-service chemistry teachers' pedagogical content knowledge (PCK) regarding atoms and the periodic table via mentoring. Two pre-service chemistry teachers with different levels of academic achievement participated in the study. The development of the participants' PCK was investigated in terms of the following components: orientations toward science teaching, knowledge of curriculum, knowledge of instructional strategies, knowledge of learners, and knowledge of assessment. The data of this study were collected over four months using multiple data collection tools, including semi-structured interviews, observations, content representations, card-sorting activities, and field notes. The results of this study provided evidence for the relationship between the development of PCK and mentoring. However, as a result of the mentoring, it was determined that the improvements in the PCK components did not occur at the same levels for these participants. One of the participants had an advanced level of improvement in all PCK components through mentoring, while the other participant had varying levels of improvement in PCK components. It was found that the more academically successful participant had more advanced development in all components of PCK as a result of mentoring. For the other participant with lower academic achievement, there was no improvement in some respects of PCK components. According to the results of this study, it can be said that the development of PCK can be fostered through mentoring in pre-service chemistry teacher education. In light of the results of this study, implications are stated for in-service chemistry teacher education, pre-service chemistry teacher education, and chemistry education researchers.