Build Your Model! Chemical Language and ... - ACS Publications

Activity Cite This: J. Chem. Educ. XXXX, XXX, XXX−XXX

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Build Your Model! Chemical Language and Building Molecular Models Using Plastic Drinking Straws Luis F. Moreno,*,†,‡ María Victoria Alzate,† Jesús A. Meneses,§ and Mario L. Marín∥,⊥ †

Grupo Metodología de la Enseñanza de la Química MEQ, Instituto de Química, Universidad de Antioquia, A.A. 1226, Medellín, Antioquia, Colombia ‡ Grupo de QuímicaFísica Teórica, Instituto de Química, Universidad de Antioquia, A.A. 1226, Medellín, Antioquia, Colombia § Departamento de Didácticas Específicas, Universidad de Burgos, C/Villadiego, s/n, 09001 Burgos, Spain ∥ Universidad de Antioquia, A.A. 1226, Medellín, Antioquia, Colombia S Supporting Information *

ABSTRACT: Building and using molecular models puts into action chemical language and concepts, as well as concepts from other disciplines, such as mathematics. It is an important didactic activity for understanding and exchanging meanings about fundamental concepts in basic and advanced chemistry topics. The models described here help students understand molecular representation, which is abstract and difficult to imagine and draw, and then integrate it with chemical situations and knowledge. On the basis of our experience with students, particularly in a class of 44 students, inviting them to build their own molecular models is an activity that promotes their participation in class and the laboratory. We provide details regarding how to use inexpensive plastic drinking straws for assembly; they cost very little and enable analogue models that express connectivity in molecular structures to be built, a strategy that enhances understanding of the spatial organization and characteristics of molecules, as well as that of molecular thought for solving chemical problems. KEYWORDS: High School/Introductory Chemistry, Second-Year Undergraduate, Molecular Modeling, Organic Chemistry, Inorganic Chemistry, Hands-On Learning/Manipulatives, Conformational Analysis, Stereochemistry, Atomic Properties/Structure

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INTRODUCTION As Hoffmann states, 1 it is essential for chemists to communicate three-dimensional structural information, for which there is a variety of conceptual and manual tools. These tools come together to represent one or another molecule, leading to the transformation of the reality of the substance in a symbolic way through universal linguistic codes, which we generally call chemical language. According to Jacob,2 chemical language consists of elementary symbols, molecular formulas, structural formulas, chemical equations, words, propositions, and theories. In the building of models, chemical language intervenes as a mediator that allows the transit of the visible to the invisible, as Laszlo3 would say. Molecular and structural formulas, molecular geometry, connective models, and chemical words and statements operate as mediators in the processes of teaching and learning, and assist communication, representation, and thinking. Therefore, structural molecular models are a fundamental linguistic tool in the communication and acquisition of chemical concepts. It is important to build and use molecular models in order to understand basic concepts taught in chemistry; having students build their own molecular model is an activity that encourages them to participate in basic courses. Construction with © XXXX American Chemical Society and Division of Chemical Education, Inc.

common materials is an alternative to access molecular models for chemistry classes, since in South America, particularly in Colombia, molecular model kits are not sold in stores and must be imported from North America and Europe, which is quite expensive and makes these kits inaccessible to the majority of science students. In this regard, a good number of articles have been published that suggest different methodologies and materials for these models.4−17 For example, some have proposed the use of plastic soda bottles,8 soft drink caps,9,10 and used whiteboard markers.4 In this article, we suggest the use of drinking straws, like Mak et al.,11 although with another construction strategy; it is pertinent to recognize the similarity with the Dreiding models.18 All of these materials are recyclable materials that are easy to obtain. Some of these molecular models have been created to illustrate different topics raised in class, for example, hydrocarbon structures,4 fullerene structures,5 hydrogen bonds,7 and atomic orbitals,8 among others. Received: May 26, 2017 Revised: January 31, 2018

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DOI: 10.1021/acs.jchemed.7b00300 J. Chem. Educ. XXXX, XXX, XXX−XXX

Journal of Chemical Education

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Activity

MATERIALS The list of chemical information and materials for the construction of the models is below: • Table of functional groups • Classification table of polyhedra and polygons • Data table of lengths and types of bond • Periodic table • Ruler • Scissors • Drinking straws or bond straws (Bond-S) Cocktail drink straws or union straws (Union-S) are used as junctions in the molecular model. No glue is required as the pressure between the straws holds the structure together.

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BUILDING PROCESS Once the molecular formula and its corresponding structure to be built are chosen, and the functional groups and geometric forms identified, the next step is to select the scale. For this, a data table of bond lengths is used, and the length of a bond and its equivalent in centimeters is adopted. For example, in the 2chloroethanol molecule, the length of a covalent C−H σ bond equivalent to 110 pm may increase to 5 cm in the molecular model, and this relation then defines the other bond lengths. Thus, the length of the C−C σ bond that is 154 pm corresponds to 7 cm in the molecular model (see Figure 1).

Figure 2. Basic steps of the model construction.

Figure 1. Scale model for 2-chloroethanol.

The bond straws (Bond-S) are then selected; this can be done by color to differentiate the bond types. The plastic straws are cut using a ruler and scissors to the set scale. The union straws (Union-S) function as connectors and do not require a strict scale for their cutting. It is recommended that they be slightly shorter than the bonds where they are to be inserted, usually 0.5−1 cm smaller than the bond length (see Figure 2A). Two Union-S’s are inserted into a Bond-S, pressing them slightly into position. Never insert more than two Union-S’s into a Bond-S: this is a common error when learning how to build models. The resulting structure is shown in Figures 2A and 3A. Next, one of the Union-S’s is inserted into a different BondS, and a different Union-S is inserted into another Bond-S (see Figures 2B and 3B). Through repeating the previous steps twice (see Figures 2C and 3), a square planar structure is created (see Figures 2D and 3D). We can continue inserting new straws as required by the model (see Figure 2E). Then, making a turn of approximately 90° from the center of the 2D structure creates a tetrahedral center, which is then used to continue the construction of the model according to what is required by an open- or closed-chain structure (see Figure 4). Similarly, assembly of double bonds proceeds to complete the structure shown in Figure 3D; subsequently, take two

Figure 3. Basic steps of the model construction, shown with transparency.

Bond-S’s and link their upper parts using two Union-S’s (this is the representation of a double bond). Additionally, a triple bond requires three Bond-S’s, and their upper parts are linked using three Union-S’s. Figure 5 shows how the double and triple bonds are constructed. Using this construction strategy, new bonds are added that represent potentially new functional groups and configure regular and irregular polyhedra, proteins, carbohydrates, metal complexes, or any other structure of interest. It is also possible to construct models for a unit cell and the molecular network of crystal structures. An illustrative video is available in the Supporting Information. B



Activity

Figure 6. Molecular model for 2-chloroethanol, CH2ClCH2OH.

Table 1. Straw Details for Making a Molecular Model of 2Chloroethanol, CH2ClCH2OH Bond Type 1 1 1 1 4 2 8

Bond-S Bond-S Bond-S Bond-S Bond-S Union-S Union-S

Straw Length, cm

Represents

7.0 8.2 6.5 4.4 5.0

1 bond: C−C 1 bond: C−Cl 1 bond: C−O 1 bond: O−H 4 bonds: C−H Red connectors Black connectors

Dodecaborane B12

The plastic drinking straw model for dodecaborane, B12, has 30 Bond-S straws representing 30 edges joined with 60 Union-S yellow connectors. See Figure 7A. Figure 7B shows a section of the structure; Figure 7C shows the finished model.

Figure 4. Tetrahedron built with 4 Bond-S’s and 4 Union-S’s.

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ACTIVITY IN THE CLASSROOM The building of models has been implemented in several courses of the Chemistry Programme at the Universidad de Antioquia: Chemical Structure and Bonding (Level II), Organic Chemistry I (Level III), and Organic Chemistry II (Level IV) and in the Chemical Training outreach program for high school students. Students build models as a complementary activity to the classroom discussions in order to encourage molecular understanding. A particular case is a group of 44 second-level students, organized into groups of four, who participated in a workshop with the purpose of building models of isomers for the molecular formula C4H9NO2, which has 138 commercially available compounds within the different classes of isomers. In the model building process, students face several decisions: first, after drawing a structure using the concepts of molecular formula and chemical valence, with assistance from the lecturer, they identify the chemical name and functional groups, and define the isomer to be modeled. In a second step, they define the link lengths in units of picometers and perform the conversion to centimeters. Scale selection is the first difficulty they face, because smaller link lengths must be in a range that allows them to be manipulated. Once the scale has been determined, they cut the Bond-S and begin to build the model. Here, the most common error consists of inserting more than two Union-S’s in a Bond-S. Once this difficulty has been overcome, they continue to build the sequence, without paying much attention to the geometry they must assemble, achieving a planar square structure. Very few students know how to transform the planar square connective model into the tetrahedral structure; most require support from the lecturer for this modification. The students then put into practice the necessary twist to the planar square structure to transform it into a tetrahedron.

Figure 5. Basic steps of the double and triple bond construction, shown with transparency.

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EXAMPLES

The following text provides details regarding how to make models of some molecules that are of interest in basic level subjects. The information related to the different connectivities of the chemical bonds is presented. 2-Chloroethanol, CH2ClCH2OH

A molecular model for the chemical compound 2-chloroethanol (CH2ClCH2OH, see Figure 6A) can be made using the details found in Table 1. At the ends, Union-S’s of each respective color are inserted according to the culminating nucleus: white for hydrogens and green for chlorine for the purpose of improving visualization (see Figure 6B). C



Activity

Figure 7. Molecular model for dodecaborane B12 (A), which uses 30 Bond-S’s to represent 30 edges with 60 Union-S yellow connectors. Crosssection (B) and finished molecular model (C).

happy with the model building, acknowledge its role in facilitating the comprehension of concepts, and express a capacity to build them independently.

In the third stage, they assemble the different representative fragments of functional groups identified in the isomer. The final stage consists of joining the previous constituent fragments of the structure, and this is easy to do: Students modify the molecular structure, and rectify the flat and dihedral angles. During the entire modeling process, the lecturer uses oral and written explanations to communicate with the students, working on each of the concepts and definitions involved in the development of the assembly activity, using words, equations, and symbols of chemical language. In addition, the lecturer is aware of the appropriate and pertinent use of chemical language, insisting that the students be careful to use this language in order to promote the grasping of concepts and representations of chemistry. The use of the common language hampers the conceptualization of any of these ideas. The exchange of meanings, self-corrections, and concepts by the students regarding the built models, as well as the good results obtained and the support and guidance of the teacher/guidetext, lead to a successful modeling activity. At the end of the activity, the 44 students answer the questions reported in Table 2. In general, most participants are

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CONCLUSION Building molecular models with plastic drinking straws, in addition to enhancing the development of manual skills and focusing attention on molecular characteristics (such as chemical valence, number and class of atoms that they are composed of, identification of functional groups and atom geometry, and bond lengths, angles, and types), can be used to promote the understanding of transformations between different kinds of molecular representations, e.g., manipulation of the different structures corresponding to a group of isomers of a molecular formula. Finally, an important didactic contribution of the construction of molecular models is to put into action chemical language in terms of elementary symbols, molecular name, molecular formula, structural formula, and the field of chemistry’s very own words, concepts, and theories. Through collaborative work, molecular models can be built, and students can explore nomenclature, functional groups, multiple bonds, and molecular geometry. These exchanges and recognitions promote the descriptive and explanatory construction of the molecule, and the molecular relationships in terms of aggregates, as well as encourage thought and dialogue, regarding the substance and its behaviors. As a suggestion, the construction of molecular models with plastic straw can be used to explore the hybridization, the relationship between Fischer projections and line angle structures,19,20 or the Fischer and Haworth projections.21 Furthermore, molecular models built with plastic straws allow rotations through the bonds and can be used to study the conformational analysis of different molecules in organic chemistry (see the Supporting Information section).

Table 2. Comparison of Students’ Opinions about the Molecular Model Construction Questions (Translated into English by the Authors)

Characterization

What degree of difficulty do you give to the construction of molecular models?

High

Is it nice to build models?

Medium Low Yes No Yes

Is it useful to build and manipulate molecular models in order to understand molecular structure concepts? Can you build your own models at home without the help of a teacher or a classmate? Would you consider inviting some of your classmates to build molecular models? Was it satisfying to finish building your models?

Responses, % (N = 44) 14 73 13 95 5 100

No Yes

0 98

No Yes

2 82

No High

18 80

Medium Low

18 2

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available on the ACS Publications website at DOI: 10.1021/acs.jchemed.7b00300. Illustrative video of the construction (AVI) Examples of constructed drinking straw models and teaching guide (PDF, DOCX) D



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Activity

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected], [email protected]. ORCID

Luis F. Moreno: 0000-0002-3775-6261 Notes

The authors declare no competing financial interest. ⊥ Author Mario L. Mariń is deceased.

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ACKNOWLEDGMENTS We acknowledge Prof. Felipe Otálvaro for helpful comments. REFERENCES

́ (1) Hoffmann, R. La Semiótica de la Quimica: En Lo Mismo y No Lo Mismo; Fondo de Cultura Económica: México D.F., 1997. (2) Jacob, C. Analysis and Synthesis, Interdependent Operations in Chemical Language and Practice. HYLE 2001, 7 (1), 31−50. (3) Laszlo, P. La Parole des Choses; Collection Savoir: Sciences, Hermann Éditeurs des Sciences et des Arts: Paris, 1993. (4) Dragojlovic, V. Improving a Lecture-Size Molecular Model Set by Repurposing Used Whiteboard Markers. J. Chem. Educ. 2015, 92, 1412−1414. (5) Chuang, C.; Jin, B. Y.; Tsoo, C.-C.; Tang, N. Y.W.; Cheung, P. S. M.; Cuccia, L. A. Molecular Modeling of Fullerenes with Beads. J. Chem. Educ. 2012, 89, 414−416. (6) Flint, E. B. Teaching Point-Group Symmetry with ThreeDimensional Models. J. Chem. Educ. 2011, 88, 907−909. (7) Schultz, E. Simple Dynamic Models for Hydrogen Bonding Using Velcro-Polarized Molecular Models. J. Chem. Educ. 2005, 82 (3), 401− 405. (8) Samoshin, V. V. Orbital Models Made of Plastic Soda Bottles. J. Chem. Educ. 1998, 75 (8), 985. (9) Siodłak, D. Building Molecular Models Using Screw-On Bottle Caps. J. Chem. Educ. 2013, 90 (9), 1247−1249. (10) Siodłak, D. Building Large Molecular Models with Plastic ScrewOn Bottle Caps and Sturdy Connectors. J. Chem. Educ. 2017, 94, 256− 259. (11) Mak, T. C. W.; Lam, C. N.; Lau, O. W. Drinking-Straw Polyhedral Models in Structural Chemistry. J. Chem. Educ. 1977, 54 (7), 438−439. (12) Hernandez, S. A.; Rodriguez, N. M.; Quinzani, O. An Easily Constructed and Versatile Molecular Model. J. Chem. Educ. 1996, 73 (8), 748. (13) Brickley, M.; Silva, R. A. An Inexpensive Molecular Model. J. Chem. Educ. 1985, 62 (12), 1077−1078. (14) Donaghy, K. J.; Saxton, K. J. Connecting Geometry and Chemistry: A Three-Step Approach to Three-Dimensional Thinking. J. Chem. Educ. 2012, 89 (7), 917−920. (15) Mattson, B. A. Device for Making Classroom Molecular Models. J. Chem. Educ. 1994, 71 (11), 977−980. (16) Birk, J. P.; Foster, J. Molecular Models for the Do-It-Yourselfer. J. Chem. Educ. 1989, 66 (12), 1015−1018. (17) Mattson, B. Cubic Unit Cell Construction Kit. J. Chem. Educ. 2000, 77 (5), 622−623. (18) Dreiding, A. S. Einfache Molekularmodelle. Helv. Chim. Acta 1959, 42, 1339−1344. (19) Starkey, L. The Use of Stick Figures to Visualize Fischer Projections. J. Chem. Educ. 2001, 78, 1486. (20) Moreno, L. Understanding Fischer Projection and Angular Line Representation Conversion. J. Chem. Educ. 2012, 89, 175−176. (21) Zhang, Q. Z.; Zhang, S. S. A New Method To Convert the Fischer Projection of Monosaccharide to the Haworth Projection. J. Chem. Educ. 1999, 76, 799−801.

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Build Your Model! Chemical Language and ... - ACS Publications

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