This study tested for deep understanding and critical thinking about basic quantum chemical concepts taught at twelfth grade. Our aim was to achieve conceptual change in students. A quantitative study was conducted first (n = 125), from which 23 selected students took part in semi-structured interviews in person or in small groups that were allowed to interact under the coordination of the investigators. In this paper, we confine our results to just one question that was about the possibility of the electron of the ground-state hydrogen atom to be found outside the space that is defined as 1s orbital. It was found that the probabilistic nature of the orbital concept was absent from many students? minds. Our approach to conceptual change used constructivist pedagogy, employing active and co-operative forms of learning, and proved effective in a number of cases. The variety in students? approaches was explained on the basis of Ausubel?s theory about meaningful and rote learning and of the ability to employ higher-order cognitive skills. Nevertheless, the methodology used can be useful for all students, irrespective of their behavior in traditional written exams.