and text nor on unmediated personal experience. Instead, they engage in systematic data collection and principled reasoning to construct new understandings both of matter and of the foundations of scientific knowledge.

Thus the strands of scientific proficiency can be used in conjunction with the research to develop understandings in upper elementary school students that build on their learning in grades K-2 and that lay the foundations for reasoning about matter using atomic-molecular models in middle school.

Grades 6-8
Developing an Initial Understanding of the Atomic-Molecular Theory

Children’s macroscopic understandings of matter (now grounded in a well-articulated set of measurable quantities) provide a framework from which they can ask still deeper explanatory questions and, in response to these questions, construct another layer of explanation (i.e., in terms of atoms and molecules). For example, what is the nature of matter and the properties of matter on a very small scale? Is there some fundamental set of materials from which other materials are composed? How can the macroscopically observable properties of objects and materials be explained in terms of these assumptions? These deeper questions arise only as puzzles requiring further explanation if students have a rich, embodied, and sound macroscopic understanding of matter on which to build (Snir, Smith, and Raz, 2003). But given such macroscopic understandings and prior experience with model-based reasoning, students are ready to take on the challenge of investigating, describing, and explaining a host of new phenomena as well as reexplaining and more deeply understanding phenomena with which they are already familiar. In addition, armed with new insights provided by knowledge of the existence of atoms and molecules, they can conceptually distinguish between elements (substances composed of just one kind of atom) and compounds (substances composed of clusters of different atoms bonded together in molecules). They can also begin to imagine more possibilities that need to be considered in tracking the identity of materials over time, including the possibility of chemical change.

One set of puzzling phenomena for students to explain is how the volume of something can change in situations in which its mass or weight has been conserved. Of course, to even describe these situations, students need to not only clearly distinguish the quantities of weight and volume, but also have ways of accurately measuring them to be sure that one has clearly changed without the other. In addition, to be puzzled by this state of affairs,

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