In this paper we report on the process of argument construction in whole class discussions and how this was used by participants as a tool for joint meaning-making. The study draws on Vygotsky's view of learning as the active construction of knowledge and on theories of argumentation. Most previous research in South African science classrooms has reported on argumentation by teachers and by learners in small group discussions, not in whole class science discussions. We analysed episodes of teacher-learner discussions in a Grade 11 chemistry lesson in a township school in Gauteng, South Africa. The lesson happened two years into a school-based intervention programme to introduce science teachers to argumentation as one of the pedagogic approaches to stimulate meaningful learner participation and engagement in science lessons. Data was collected in the form of video recordings of a whole class discussion on changes in potential energy during covalent bonding. We identified components of an argument as defined in literature in order to determine the pattern of arguments jointly constructed by the teacher and her learners. In the lesson analysis we found evidence that with guidance, learners could argue and eventually provide rebuttals for their peers' arguments. While for the main part the teacher's focus was on guiding the discussion towards a specific goal of the lesson, we provide evidence that the teacher and her students used argumentation as a tool for joint sense-making. This is an unusual use of argumentation and we consider the potential for this form of interaction in South African science classrooms.