The aim of this study is to determine the pre-service teachers' perceptions of atom and atomic structure through a word association test. It is important to concretize the concepts having a significant influence on the learning of subjects. Through this concretization, students' understanding of the concepts and accordingly of the subjects is facilitated. The word association test used in the current study consists of nine key concepts chosen from the unit "Atom and Atomic Structure". It was given 15 seconds to the participants for each key concept. Content analysis was conducted for the analysis of the data. The frequency table was formed taking into consideration the frequency of given responses. Accordingly, the mind maps were generated to indicate the relations among concepts. With the word association pre-test, it is found that the pre-service teachers could not express many concepts related to the unit correctly. Although the findings of the post-test indicate that there is slight increase in the pre-service teachers' correct expression of the concepts, the results show that the pre-service teachers still have difficulty in perceiving the concepts in the unit "Atom and Atomic Structure".