Monte Carlo calculations are used to generate contour maps of atomic orbitals in which the contours enclose specified percentages of the total electron density. Maps for the most common atomic orbitals are presented, and recommendations for the presentation of atomic orbitals are made.

The ability of a computer to engage a student in a dialog----can be used in a variety of ways to improve the quality of instruction____ Computer-assisted instruction (CAI) refers to those applications of computers in instruction where a student engages in a dialog with a computer program to achieve a well-defined and measurable understanding or skill. A computer system that can accommodate a large number of such interactive programs, all of which are readily accessible to students who wish to use them, can support a significant part of the instructional load in a given course or curriculum. This article attempts to summarize the current status of CAI in chemistry instruction. The authors have a total of four decades of experience with diverse CAI systems in different environments, and we hope that the opinions and facts collected here will be of value to potential users of CAI.

This article is devoted to descriptions of microcomputer systems that are suitable for computer-aided instruction (CAIDI) in chemistry. Specific instructional applications are described in some detail to illustrate the wide variety of systems available and their adaptability to problems faced by chemistry teachers. Additional systems and applications will be described in a subsequent article, and a summary comparison of features of the various systems will be provided then as well. It is assumed that the reader is familiar with the terminology developed in previous articles in this computer series (I)-All of the authors are excited by the greatly expanded possibilities that microcomputers provide for computer applications in chemical education, and most of us are enthusiastic proponents of the particular system we use. Under these circumstances it would be futile to attempt to achieve consensus on the "best" system. Indeed, which system is best depends strongly on the specific application (or the variety of applications) for which a microcomputer will be used (Id) and on the relative importance of the cost factor. Consequently the editor has asked each contributor to describe several applications of a particular microcomputer system, clearly indicating what hardware is necessary to run the programs and whether programs are available for distribution to others. It is hoped that from these descriptions readers will be able to decide which system or systems might be appropriate to their particular needs.The approach outlined above entails several drawbacks that readers should be aware of. Some systems, such as the stand-alone PLATO terminal, have available a wide range of CAI courseware that has been developed and evaluated over a decade or more. In other cases there may be little more than the software described here, and its instructional effectiveness may not yet have been thoroughly tested. Also, descriptions by ent husiastic programmers tend to accentuate the positive and not mention negative aspects such as how long and hard one must work to develop a program, things students can do inadvertently that will require you to reload a program (or restart the whole system), lack of versatility of a system, and so on. If your primary interest is interactive CAI as defined in the first article of this series (la), then you should compare any system you contemplate buying with the criteria developed earlier for an ideal CAI system (lb). There are wide variations in ability to display lower-case characters, subscripts, and superscripts, ability to embed text in graphics displays, ease of processing verbal as well as numerical inputs, length of lesson that can be accommodated, and many other areas. These caveats notwithstanding, microcomputers are already being used widely and successfully in chemistry in-

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