In common with many other countries, the Iranian science curriculum does not introduce primary children to atoms and molecules but instead leaves the teaching of these concepts until high school. This paper challenges this practice and describes the changes in elementary Iranian children's understanding of atoms and molecules following a 10-h teaching intervention about basic atomic-molecular theory, derived from recently published Australian research. The participants involved in this study are a group of 25 Iranian children aged 9 to 12 years old, who participated in a vacation summer school where they were taught about the structure of atoms and molecules. Thematic and content analysis of children's written responses and drawings before and after the intervention reveal significant changes in their conceptual thinking. The results also show the extent to which the children can generate microscopic representations of the states of matter from their understanding of atoms and molecules.