This study tested for deep understanding and critical thinking about basic quantum chemical concepts taught at 12th grade (age 17-18). Our aim was to achieve conceptual change in students. A quantitative study was conducted first (n = 125), and following this 23 selected students took part in semi-structured interviews either individually or in small groups that were allowed to interact under the coordination of the investigators. The planetary Bohr model was strongly favoured, while the probabilistic nature of the orbital concept was absent from many students' minds. Other students held a hybrid model. In some cases, students did not accept that the electron cloud provides a picture of the atom. Many students had not understood the fundamental nature of the uncertainty principle. Finally, the mathematical description of the formation of molecular orbitals caused problems in the case of destructive (antibonding) overlap of atomic orbitals. Our approach to conceptual change employed active and cooperative forms of learning, which are consistent with social-cultural constructivism and with Vygotsky's zone of proximal development. It proved effective in a number of cases, and ineffective in others. The variation 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 behaviour in traditional written examinations. (Contains 4 figures, 3 tables, and 1 note.)