To provide a solid understanding of chemistry at early stages of learning, any introductory course strives to link macro phenomena and their micro models in students' thinking through some practical work with substances and related modeling at micro level; however, the transition to elements and atoms does not get adequate consideration. Our goal is to design a teaching strategy that reveals the essence of the notion of chemical element at both levels of its representation. We assume that (1)

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... nnections between the macro and the micro can be realized if we allow students to plan the experiments, and (2) the necessary context in which the concept of chemical element really works is a problem of preparation of a certain substance from other chemicals. Upon these principles, we have developed a local instructional theory that has then been tested and revised during three long-term macro-cycles of our design research experiment in 6-7 grades. We build on the Guided Discovery Approach to teach students how to distinguish and interpret simple physical and chemical changes through the substance-based particle model. In our course, students compose and explore copper and iron cycles of transformations, model composition of particles of the substances involved, revise interpretations of the experiments, investigate displacement reactions, acids, bases, etc. without explicit references to the concept of atoms. Along this way, students realize and master the idea that elements are conserved in reactions and every element gives rise to a cycle. Atoms then emerge as units of elements. The principal advantage of this approach is the appearance of meaningful questions that the subsequent chemistry course will answer. Students acknowledge that models are revisable as needed by the experiments. Such an introduction makes the subject of chemistry much more meaningful for students and eliminates many obstacles to successful learning.