Atoms and molecules are fundamental concepts in Chemistry, but they are formally introduced only at the end of middle school or in high school. However, there is increasing evidence that children can grasp abstract notions when exposed to methodologies based on experimentation and modeling.

This study aimed to investigate students' prior knowledge and assess learning gains in 168 students from seven classes in the first four years of primary education (ages 5–9), focusing on the concepts of atoms, molecules, and the structure of matter. Also involved in the study were seven classroom teachers, a Chemistry specialist, and the students' families.

An intervention program was implemented over one academic year, using exploratory and participatory teaching strategies to promote scientific literacy and conceptual understanding. The main activities included:
(i) guided research on atomic and molecular concepts with family support,
(ii) monthly sessions with a Chemistry specialist to deepen students’ knowledge,
(iii) hands-on construction of molecular models, and
(iv) a final exhibition featuring oral presentations by students, showcasing their models, and engaging with the school community.

Data collection included pre- and post-tests to measure knowledge acquisition, field notes recorded during sessions, a focus group with teachers to evaluate the impact and relevance of the program, and family participation in both the final exhibition and the completion of guided research activities.

The analysis of results showed that students initially had very limited knowledge of the topic, with an average pre-test score of only 8%. After the intervention, the average post-test score increased to 58%, suggesting a significant positive impact on learning. Overall, knowledge gains were more pronounced in older students, reinforcing the idea that abstraction skills required to understand atomic and molecular concepts start to develop around ages 7 or 8. During the focus group, teachers highlighted several key outcomes of the program: an expansion of students’ scientific vocabulary, increased accuracy in using specific terminology, and a growing ability to relate learned concepts to real-world applications. Additionally, high levels of engagement and enthusiasm were observed not only among students but also among their families, particularly during the final exhibition. This event, which featured student-created atomic models and oral presentations to the school community, served as an additional motivational factor, encouraging scientific communication, inquiry-based learning, and interdisciplinarity. Students' construction of atomic models demonstrated both scientific accuracy and attention to aesthetic detail, reflecting the integration of multidisciplinary knowledge, particularly within the visual arts. Furthermore, the final presentations revealed a significant development of oral and written communication skills, emphasizing the project's contribution to the consolidation of transversal competencies essential for academic and personal growth.

The results of this pilot study demonstrate that even very young students can develop an initial understanding of the structure of matter, expanding their cognitive and scientific repertoire. Moreover, the active participation of families and the school community suggests that similar programs can have positive impacts beyond academic performance, fostering a greater appreciation for science in society.

Keywords: Chemistry teaching, chemistry learning, atoms and molecules, primary education.