This paper reports on an interview study of 18 Grade 10-12 students' model-based reasoning of a chemical reaction: the reaction of magnesium and oxygen at the submicro level. It has been proposed that chemical reactions can be conceptualised using two models: (i) the "particle model," in which a reaction is regarded as the simple combination and rearrangement of reactant particles and does not involve any change in the identity of the reactants, and (ii) the "atomic model," wherein a reaction involves the transformation of one chemical species into another. This paper suggests that although the "particle model" looks simpler than the "atomic model," it can help to support the learning of some advanced chemical concepts such as energetics and collision theory. Therefore, it is postulated that students who reason using the "particle model" are able to demonstrate some advanced ideas about chemical reactions. The conceptualisation of reactions in terms of the atomic model and the "particle model" allows a flexible understanding of students' learning. Students' representations of the reaction between magnesium and oxygen were analysed with reference to the two models. The models were found to be useful in assessing the students' understanding of the reaction and revealing the novel ways that the students reasoned the chemical reaction. In addition, a student who used the "particle model" to represent the reaction was found to explain the reaction in terms of some energetics and kinetics concepts. The study offers insights for curriculum planners and teachers into the potential of these two models to help students understand chemical reactions.