This article describes the development of the Quantization and Probability Representations Inventory (QuPRI) as a measure of student understanding of the electron structure of the atom. The QuPRI was created using a mixed-method sequential design such that the items and distractors were generated on the basis of the analysis of semi-structured interviews in which students were asked to interpret multiple representations of the electron structure of hydrogen, helium, and carbon atoms. The QuPRI was administered to first-semester general chemistry students (N = 655) and physical chemistry/biophysical chemistry students (N = 38). Descriptive statistics and item function are presented for each sample, including evidence for the reliability and validity of the data generated by the QuPRI. Students’ confidence in their responses and reasoning about the electron structure of atoms is discussed.