This paper reports a cross-sectional study of Taiwanese physics students' understanding of subatomic phenomena that are explained by quantum mechanics. The study uses students' explanations of their answers to items in a questionnaire as a proxy for students' thinking. The variation in students' explanations is discussed as is the development in the way in which students link different concepts. A discussion of the source of students' ideas turns to the way schema contain mental models that derive from sensori-experiences. The principal recommendation for teaching is the need to include practical activities on a range of precursor phenomena so as to extend the students' repertoires of mental models. This advice is different from that given in previous studies.