Teaching Chemistry Without Atomic Structure:

How Far Can One Go?

 

 

Fred Garafalo 

Massachusetts College of Pharmacy and Health Sciences

179 Longwood Ave.,  Boston,  MA  02115

 

 

                 Experience indicates that many freshman science students have difficulty distinguishing between observation and inference, mastering scientific vocabulary, and working comfortably with symbolic representations.  They also confuse descriptions with causes, and fail to recognize the interplay among facts, definitions, hypotheses, and predictions, which is central to the enterprise of experimental science. 

 

In spite of these difficulties, many traditional presentations in chemistry jump around in an unsystematic fashion among three domains:  the world of macroscopic observations, the underlying world of molecules, atoms, and subatomic particles, and  the symbolic representations used to represent substances and their chemical behaviors.  These presentations often pay little or no attention to the intellectual struggle that led to fundamental knowledge in chemistry, such as the relative atomic masses of the elements and the formulas of simple compounds.  Frequently they introduce students to atomic structure and electron configurations early in the course, even though these aspects of the discipline are the most removed from direct experience and were elucidated after the determination of relative atomic masses, chemical formulas, and molecular geometry.  Such approaches miss the opportunity to provide students with some historical perspective on the discipline of chemistry, and a framework within which to develop formal reasoning skills. 

 

This talk will describe some teaching  techniques and topic developments used in a college freshman chemistry curriculum that are designed to help students connect observable phenomena with inferences and hypotheses about the atomic world, and with the associated symbolic representations.  The presenter has been involved in curriculum development at the Massachusetts College of Pharmacy and Health Sciences (MCPHS)  for the past 16 years, primarily in a first-year chemistry sequence.   For the past 11 years, he has been drawing upon research in teaching and learning and performing his own in an effort to provide students with an opportunity to develop reasoning skills, while working their way through the freshman chemistry curriculum.   This work is based on an action research methodology, which consists of planning and implementing specific classroom activities, observing and evaluating the results, and then using conclusions to revise the activities and perform another cycle of the process.  The primary sources of feedback for evaluating this research continue to be observation of students as they engage in active learning, and evaluation of their performances on tests. 

 

The teaching  techniques include the use of instructor-generated handouts to drive discussion-based classes, use of think-aloud problem-solving sessions in various settings including the classroom, extra help sessions, and the laboratory, and use of Socratic lines of questioning to guide students toward constructing concepts.

 

The two-semester chemistry sequence at MCPHS is traditional in the sense that it comprises a survey of a number of important topics.  However, topic development is based on introducing experimental evidence before concepts and theories, and some attempt is made to follow the historical development of ideas.  This talk will focus on some topics presented in the first part of semester one.   In that semester, the behavior of various samples of matter is first used to help students create a library of terms including element, compound and mixture.  The presentation avoids making distinctions between chemical change and physical change, since this often requires more knowledge about the atomic realm than students have at this point.  Evidence suggesting that matter is composed of tiny particles is presented.  Gravitational mass is introduced first as a measure of the attraction of the earth for objects, and then as a way to infer that denser objects contain more matter.   Making hypotheses about observations associated with heating a metal in air encourages students to recognize the importance of making mass measurements.  Conservation of mass is used to infer conservation of particles. 

 

The laws of definite and multiple proportions are “discovered” by interpreting the results of chemical analysis data.  Ideas about pressure and temperature measurement precede discussion of the gas laws.  Avogadro’s law is avoided at this point, since there is no simple experimental evidence for it.  Students work with the competing hypotheses of early 19th century scientists - Dalton’s rule of simplicity and Avogadro’s hypothesis - to deduce possible relative atomic masses of atoms and formulas of compounds.   Students learn how Avogadro’s hypothesis eventually won out, and this leads to Cannizzaro’s method of determining formulas and relative atomic masses.  By eliminating topics like atomic structure, which are typically found in the early part of the chemistry curriculum, the student’s attention is focused on what could be determined based on the limited evidence available at that time in history.  At this point in the course, students do not even have access to a periodic table. 

 

 

Helping first-year chemistry students to develop a useful framework for organizing their knowledge has been a primary objective of curriculum development at MCPHS that goes back to the earliest work in the mid-1980’s.

 

The final part of this presentation will describe how progress in providing opportunities for students to become better formal reasoners has led to a fundamental resequencing of topics in our first-year chemistry curriculum.  In the restructured sequence, atomic structure is not introduced until the end of the first semester, and the beginning of the second. 

 

Since active participation encourages reflection, part of the presentation will invite conference participants to try some of the activities that are used in this curriculum.