This study investigates the students’ ideas about an atom by asking them to describe an atom on a paper and pencil questionnaire. Students’ understanding of the structure of an atom, its constituents and their approximate locations, the size of an atom, and energy released by an atom are investigated. Analysis of responses was based on the phenomenographic method. The study does not attempt to develop a catalog of students' "misconceptions" of atoms. It explores how students describe atoms when they are presented with an open-ended question. We can then learn what ideas are foremost in students' thoughts when they think of atoms.