This thesis investigates promoting student understanding of the Particulate Nature of Matter (PNM) through Inquiry Based Learning (IBL), visualization and modelling. A Practitioner Action Research methodology was employed and student performance at junior second level school was measured using diagnostic and summative testing. A module on this topic was prepared which included a student workbook and a teaching manual. Student and teacher reflections on the learning issues experienced were obtained through interviews and repertory grid analysis based on Kellyian Personal Construct Psychology (PCP) principles. Summative results show a better comprehension of PNM by the intervention group as opposed to their control group peers. Repertory grid analysis was used to highlight and rate aspects of students’ affective and cognitive learning experiences. Furthermore, in conjunction with PCP, student drawings provided as exam answers when they were asked to think critically within a novel problem scenario were investigated. This exploratory approach enabled the systematic metering of student comprehension of chemistry constructs and served to detect the learning gaps in their construct hierarchy. This work represents a considered approach towards the development of an understanding of student answers including alternative conceptions. It integrates PCP with research on explanations and representations of ideas. It attempts to create an assessment instrument that can be used to visualise and comprehend a student’s chemistry drawings in a more holistic way than a binary judgement of wrong or right. Furthermore, PCP is employed to extend the focus from the interpretation of student responses to include the consideration of key features of conceptual development. These can be related within a proposed model of learning.