This chapter focuses on matter and its interactions and discusses the importance of how a rich understanding of atomic theory leads to a deeper understanding of why there are so many different substances in the world. A full understanding of matter and interactions is linked to the other physical sciences core ideas of forces, energy, and waves. Building an integrated understanding of these disciplinary core ideas (DCIs) will allow learners to explain a host of phenomena they experience in their everyday lives. Throughout the development of the core idea of matter and its interactions, it is important that students always connect the ideas they are developing to concrete examples, observations, and experiences. A list of materials, teacher background information, assessment, standards, safety, and references are provided. This book selection also includes the Table of Contents, Foreword, Acknowledgments, About the Editors, Contributors, Appendix, Image credits, and Index.