Water molecules and hydrogen peroxide molecules each consist of only hydrogen and oxygen atoms. A water molecule consists of two hydrogen atoms and one oxygen atom. A hydrogen peroxide molecule consists of two hydrogen atoms and two oxygen atoms. In this unit, students develop models to describe the atomic composition of simple molecules and extended structures and develop and use a model to describe how the total number of atoms does not change in a chemical reaction and thus mass is conserved. A table is provided that shows molecular models for hydrogen, oxygen, and water. Teacher background information, standards, assessments, and references are provided. This book selection also includes the Table of Contents, Foreword, Preface, About the Author, Introduction, Appendix D, and Index.