The purpose of this assessment probe is to elicit students’ ideas about the size (in diameter) of atoms. It is designed to reveal how students use the crosscutting concept of scale to compare relative sizes by estimating size in general and using orders of magnitude. To visualize extremely small sizes such as the size of an atom, it helps to use a conceptual model such as an analogy to compare relative scale sizes. This probe can be used with students in grades 6–12 and supports three-dimensional assessment and learning. The probe is provided in English and Spanish. A materials list, teacher notes, standards, assessment, and references are provided. This book selection also includes the Table of Contents, Preface, Acknowledgments, Dedication, About the Authors, Introduction, and Index.