may wonder which of the definitions of transition elements he ought to offer to his students. I suggest that the approaches and implications to teaching might be stressed, in connection with that answer, in the following way. The definition of transition elements, or any other definition, for that matter,may be solely operational or solely conceptual or a combination of both. I n any event, the definition depends upon a model and thus carries the strengths and limitations of that model. This point in particular, it seems to me, should be presented to students in the classroom. Further, for example, when in any single instance such as when a discussion of the definitions of transition metals is presented, parallel comparisons to other multiple definitions and their appropriate models ought to enhance the learning process. I n my teaching of high school chemistry, I have found it convenient to list all the elements between Groups I1 and 111 as transition elements. This makes a neat package as far as the Periodic Table is concerned-a kind of operational approach. Limiting most of my teaching to the scandium row, I point out the properties common to the ten members of the row. These properties include metallic nature, possession of multiple oxidation states (except zinc), formation of colored ions (generally) and ability to form complex ions. This completes the operational approach, enhanced, where possible, by suitable experimental work. The conceptual approach then follows to tell why this behavior is characteristic of the transition elements. Here we stress the filling of the next-to-the outermost energy level, that is, in the scandium row, the use of 3d orbitals for bonding. Since zinc is somewhat atypical, I point out that it may or may not be considered a transition element. Some of its properties (complexing) resemble the general properties of transition elements but our model of this behavior, like all models, is limited. This, I feel, is a sufficient picture for both terminal and science-oriented students.
course, i.e., mechanisms of reactions involving carbonyl group cont.aining compounds requiring the postulation of t,ransition states in which carhaniori charge dclocalization is required, even though the inclusion of heteroatom effects was not explicitly dealt with. Student interest in the mat,erial was evidenced hy several indicators: participation in class and individual library research in particular. A set of notes based on my lecture notes wa,q handed out near the end of the series of lectures. The student.^ were teste! hy the assignment of a take home examination a t the conclusion of the Iecturc series. They were asked to calculate the h20 energy level diagram for the cation, free radical, and anion of methylenecyclopropene and to discuss the relative stahilitics of the three species in terms of resonance energy. They were also required t,o calculate the electron density, charge density, bond order, and free valence for the molecule. The student response on the t,al
