Mental and physical models are treated in the Australian Science Education Project trial unit prepared for students in a transitional stage between concrete and abstract reasoning. Students are introduced to the particle model of matter through a series of core activities, including a combination game using nuts and bolts, culminating in a "test" of the model. The optional activities from which students are expected to choose are designed as self-paced activities. The options are "A Chemical Reaction" (lead nitrate/potassium iodide); "Chemical Shorthand"; "Paper Clips and Particles" (displacement of copper from solutions of three copper compounds); "Carbon Compounds" (the relation of empirical and structural formulae of organic compounds using model building kits); and "Inside the Atom" (a reading unit outlining the history of the development of models of the atom). The student manual contains instructions for all activities in the core and options, poses questions to be answered in the separate data book where results of laboratory activities are also to be recorded, and contains diagnostic tests after each option. Annotated answers to the test appear at the end of the booklet. The teacher's guide, an "overprinted" version of the student manual, gives objectives for the unit; lists apparatus, supplies and audiovisual aids; suggests references; and indicates possible teaching techniques, including links with other units in the series. (AL)