The present work explores students' repertoire of explicit knowledge fragments relevant to the atomic structure, seeking to answer which of those and through which routes are activated by students when constructing their explanations for some everyday situations. For this purpose, six tasks describing different situations were given to 225 students of the 10th and 11th grades of secondary schools from Northern Greece. Students' responses were qualitatively analysed through a five-step scheme, where any discrete explicit knowledge fragments included in students' explanations was extracted and categorised in relation to the atomic or subatomic characteristics. Extracted fragments of knowledge were also analysed according to their complexity and the ways that are organised when being activated by students. Implications for science education are analytically discussed.