We report the findings of a large-scale (n = 1,337) qualitative descriptive analysis of U.S. high schools students' particulate representations of a chemical reaction, specifically, the combustion of methane. Data were collected as part of an end of course exam. Student representations were coded into 17 distinct subcategories under one of five broad themes: i) Particulate Representations with discrete atoms, ii) Inappropriate Particulate Representations, iii) Quasi-particulate Representations, iv) Non-particulate Representations, or v) Irrelevant Attempts. Only 35.1% of student responses showed representations with discrete atoms that appropriately matched the individual molecular formulas in the combustion reaction. Of student responses, 45% were representations with discrete atoms; however, these representations were not chemically appropriate, displaying either incorrect connections between atoms within individual molecules or inappropriate groupings of atoms into individual molecules. 5.9% of student responses were coded as quasi-particulate, and included representations that displayed some form of particles that did not show discrete atoms. 22.9% of student responses were coded as non-particulate. The remaining 5.5% of responses represented no real attempt to address the question. The various representations are illustrated and described to provide a map of the domain of students' alternative conceptions of chemical reactions. Interestingly, of the 65% of students who were able to balance the equation correctly, more than half were unable to show the appropriate particulate representation. The ability to represent a chemical reaction at the symbolic level does not guarantee the ability to represent the reaction at the particulate level. (Contains 7 tables and 1 figure.)