Styrofoam-and-Velcro: An Alternative to Ball-and-Stick Models.

INTRODUCTION

For students learning biology at an introductory level, one of the most significant instructional barriers is their lack of preparation in chemistry. In upper-division college chemistry courses, students employ ball-and-stick models in order to visualize molecular structures, but at the introductory biology level, models are inconsistently used and, at the secondary level, they are avoided altogether. Traditional ball-and-stick models perform poorly at all levels because they only show bonds, never valence electrons. This poses a problem for students who are visual or kinesthetic learners, as modeling electrons in the bonding process may be critical to understanding the mechanisms behind the biochemical reactions that serve as a foundation for biological concepts. Our molecular modeling kits show the action of valence electrons and correctly deal with the issue of polarity and partial charge, while still illustrating structure and function similar to ball-and-stick models, allowing students to model nearly every reaction or molecule they may need to learn. Additionally, this kit will foster model-building exercises required as part of the Next Generation Science Standards.

PROCEDURE

In these kits, atoms are modeled using STYROFOAM balls that have been coated for durability (see Appendix 1 for detail on building kits, as well as video demonstration of kit usage at http://www.jht.com/icuf/Science-Math%20Master/Science-Math_Master_Molecules.html). Each period within the table of elements is differentiated by size, starting at 5-cm-diameter balls for the first period, then increasing by roughly 2 cm in diameter for each subsequent period. Soft (loop-side), VELCRO patches are used to represent each valence orbital in the period (usually 1 or 4 patches), spaced evenly to allow for demonstrating tetrahedral structure. A single soft VELCRO patch of a distinct color, the “ionic” patch, holds what signifies a positive or negative charge.

Electrons are represented by duct-tape flags with hooks-side VELCRO for attachment. Charges are represented by small squares of duct tape, also with VELCRO for attachment. To assemble a specific element, valence electrons are placed in the same way as they are placed in an orbital diagram following the Pauli Exclusion Principle: one electron in each orbital until all have one, then moving to two on each patch. Only two electrons will fit in an orbital and they fit together to form a rectangular shape.

With these models, covalent, ionic, and hydrogen bonds can be simulated through various means. Covalent bonds are formed by placing one orbital patch on another using two electrons to bond, simulating the sharing of electrons. Double and triple bonds are made by using bond extenders (illustrated in Appendices 1 and 2), each end of which has a soft VELCRO patch (1 × 2 cm) on the orbital patches. On each atom, one electron is present and the bond extender is used to connect the two. This is repeated for the other bonds involved.

Ionic bonds are made using the charge patches to connect between the ionic patches. For example, sodium gives up one electron to chlorine, filling each of their valence shells, but giving them each a charge. So, one positive (red) charge is placed on sodium, and one negative (blue) charge is placed on chlorine (Fig. 1A). The two can then be connected by sticking the two charges together.

FIGURE 1.

Panel A illustrates Na and Cl, Na+ and Cl−, and the ionic bond between them. Panel B represents 2 H atoms and 1 O atom, and then H2O.

Hydrogen bonds can be formed in a way similar to ionic bonds. When hydrogen is bonded to a highly electronegative atom, a positive charge can be added, signifying its partial positive charge. This can then be attached to any un-bonded electron pair on another polar molecule, simulating places where partial negative charges form, as in water (Fig. 1B).

From these modeled elements and bonds, nearly any reaction can be simulated. During a demonstration at a teaching workshop for high school biology teachers, we modeled nucleophilic substitution in a condensation reaction between amino acids (Appendix 2). We demonstrated the un-bonded electron pair in the amino group attacking the carbon in the carboxyl group, the intermediate state, and then the leaving of water. These kits can be used to model many other reactions as the students are expected, by the design, to keep track of electrons and their movement during a reaction. This is something that ball-and-stick models fail to do, but is vital to the understanding of organic reactions. Simple exercises appropriate for introductory college courses are illustrated in Appendix 3, with more advanced case studies also supplied to foster interdisciplinary concept coverage (1).

Because these kits excel at illustrating atomic structure and reactions, large processes like cellular respiration can be modeled by forming the initial molecule and performing each reaction to form intermediates, although it takes longer to accomplish. This would allow students to understand in a very visual and kinesthetic manner a wide array of life processes that simply cannot be simulated with traditional kits, as students can track electrons throughout the process and “handle” the compounds.

CONCLUSION

These kits represent a new way for students to learn chemistry, from 6th grade science all the way up to biochemistry. They take the traditional ball-and-stick models down to the level of valence electrons in order to see what truly happens in reactions and in bonding, concepts that many students fail to grasp. This kit has been tested with several groups of high school biology teachers as part of a grant from the Florida Department of Education, and has been well received, with many of these teachers planning to incorporate the kits to teach chemistry concepts in biology classes. Further testing of the efficacy of these kits occurred in a 60-student general biology class at the University of Tampa, and the student response was overwhelmingly positive. As a result of using models in class, students requested that modeling kits be made available for study outside of class, demonstrating the need for kinesthetic tools to teach chemical concepts integral to understanding basic biochemical principles.

Appendix 1: Molecular modeling kit instructions

Appendix 2: Periodic table key

Appendix 3: Example lesson for using kits for simple molecular modeling