Plastic Beverage Bottles and More as Orbital Models

by Dean J. Campbell*, Thomas Kahila*

*Bradley University, Peoria, Illinois

It has been noted that beverage bottles can resemble the shapes of some atomic and molecular orbitals. This has led to the development of educational models of bonding in molecules that use plastic bottles to represent orbitals.^1,2 Plastic bottles are plentiful, come in various sizes, and can often be recycled. They can be cut, taped together, and painted. A critical aspect of using these models is finding ways to the connect the bottles together in the correct orientation. Past methods include fastening bottle caps to each other at their narrowest ends (their mouths) by wire or to structures such as shaped wooden blocks or even to other physical models of chemical bonding (e.g., the atoms in a chemical modeling kit).^1,2 The bottles can be cut and connected together with tape, or they can be connected by band-type structures to represent various types of orbital interactions.^1,2 The large-scale models described below build on these concepts, with a few additional ideas along the way.

Models

Figure 1 shows the various bottle cap clusters used to connect to and direct the orientation of the plastic bottles. The leftmost connector is a wooden cube with a cap on each square face, and the bottles that are connected to these caps adopt an octahedral arrangement with 90° angles between them. In the middle of Figure 1 is a wooden triangular prism with a cap on each square and triangular face, and the bottles that are connected to these caps adopt a trigonal bipyramidal arrangement with 90° and 120° angles between them. Cutting a tetrahedron from wood is a little more complex, since there are no 90° angles in the structure, but not impossible. An alternative explored in this work was the Polydron building set.³ This building set consists of plastic plates in the shapes of triangles, squares, and pentagons, that can be assembled along their edges to build three-dimensional shapes. At right in Figure 1 is a tetrahedron with a cap on each triangular face (the structure has been opened to show details about its construction). The bottles that are connected to these caps adopt a tetrahedral arrangement with 109.5° angles between them.

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Figure 1. Bottle cap clusters used to connect to plastic bottle orbitals representing orbitals. (LEFT to RIGHT) Octahedral arrangement of caps, trigonal pyramidal arrangement of caps, and tetrahedral arrangement of caps.

The Polydron building set can be used to build cubes and triangular prisms, which can be modified with bottle caps like the tetrahedron to produce octahedral and triangular pyramidal arrangement of bottles. However, the less expensive wood-based bottle cap clusters were used to build many of the models in this work. It is important to note that when assembling the Polydron polyhedra that all of the plates must be oriented with either the written side facing inward or the written side facing outward, not a mix of the two orientations. It is interesting to note that Polydron tetrahedra can be used to show optical isomerism depending on whether the written sides are oriented inward or outward, Figure 2.

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Figure 2. Polydron tetrahedra optical isomers

The plastic bottles that connect to the bottle cap clusters represent atomic orbitals. The bottle labels were mostly removed with a knife or pair of scissors. Some of the labels and glue that remained on the bottles were removed by wetting the glue with acetone and then carefully scraping with a razor blade. That process worked better with a cap on the end of the bottle to help prevent it from crushing during the scraping process. The plastic bottles can be painted to represent a variety of orbitals, but we opted to leave the bottles with their as-found colors as much as possible. The green bottles are tinted with phthalocyanine compounds; the colorless bottles are not.⁴ The convention we followed for the two-liter bottles was that green bottles represented hybridized orbitals (e.g., sp³, sp², sp). Brown spray-painted parts of bottles represented unhybridized s orbitals. A sigma bond formed between a hybridized orbital and an s orbital was represented by painting the end of a green two-liter bottle with brown spray paint. The correct coverage of the bottle with paint with minimum overspray was obtained by wrapping a strip of two-inch wide packing tape around the bottle, spray-painting the bottle, and then removing the tape with overspray after the paint dried. A sigma bond formed between two hybridized orbitals was represented by cutting the ends from two two-liter bottles and then connecting the ends with two-inch clear packing tape. Colorless bottles represented lobes of unhybridized p orbitals. Pi bonds formed from overlapping p orbitals were represented by using plastic wrap to connect the bottles together. Electrons associated with the orbitals were represented by stickers added to the bottles: yellow for electrons participating in bonding, and blue for nonbonding electrons.

Figure 3 shows plastic bottle orbital models using the tetrahedral bottle cap clusters. At left is a model of methane, CH₄. Each green bottle with a spray-painted end represents a sigma bond formed from the interaction of an sp³ hybridized orbital of the carbon atom and an s orbital from a hydrogen atom. Each bottle also has two yellow stickers representing a pair of bonding electrons. At right is a model of water, H₂O. Each green bottle with a spray-painted end represents a sigma bond formed from the interaction of an sp³ hybridized orbital of the oxygen atom and an s orbital from a hydrogen atom. These bottles each have two yellow stickers representing a pair of bonding electrons. Each green bottle without a spray-painted end represents a nonbonding sp³ hybridized orbital of the oxygen atom. These bottles each have two blue stickers representing a pair of nonbonding electrons.

Figure 3. Plastic bottle orbital models using the tetrahedral bottle cap clusters: (LEFT) methane and (RIGHT) water.

Figure 4 shows the plastic bottle orbital model of carbon dioxide, CO₂, using the octahedral and trigonal pyramidal bottle cap clusters. Each pair of green bottles connected end-to-end with each other represents a sigma bond between an sp orbital of the carbon atom and an sp² hybridized orbital of an oxygen atom. These bottle pairs each have two yellow stickers representing a pair of bonding electrons.  Each green bottle not connected end-to-end with another bottle represents a nonbonding sp² hybridized orbital of an oxygen atom. These bottles each have two blue stickers representing a pair of nonbonding electrons. The colorless bottles representing lobes of p orbitals are connected by plastic wrap to represent pi bonds. These wrap-connected bottle pairs each have two yellow stickers representing a pair of bonding electrons.

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Figure 4. Plastic bottle orbital model of carbon dioxide using the octahedral and trigonal pyramidal bottle cap clusters.

Methane, water vapor, and carbon dioxide are all good examples of greenhouse gases and have rather simple molecular structures.⁵ Models of these molecules might be good props not only in the classroom but also at outreach events where climate change is being discussed. More complex molecules can also be modeled. Figure 5 shows the plastic bottle orbital model of benzene, C₆H₆, using the trigonal pyramidal bottle cap clusters. Each pair of green bottles connected end-to-end with each other represents a sigma bond between two sp² hybridized orbitals of neighboring carbon atoms. Each green bottle with a spray-painted end represents a sigma bond formed from the interaction of an sp² hybridized orbital of a carbon atom and an s orbital from a hydrogen atom. The colorless bottles representing lobes of p orbitals are connected by plastic wrap to represent delocalized pi bonds.

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Figure 5. Plastic bottle orbital model of benzene using the trigonal pyramidal bottle cap clusters and plastic wrap to represent delocalized pi bonds. 

An alternative model of bonding in benzene uses trigonal bipyramidal bottle cap clusters and plastic bottles to represent the sigma bonding framework and inflatable toroids to represent delocalized pi bonding, Figure 6. Inflatable toroids can vary in size and can get quite large (e.g., truck tires), but the toroids used in this model were inflatable pool toys obtained from a discount store. To match the scale of the toroids and the hexagonal sigma bonding framework, a bit of geometry is required. The center of the circular cross-section of the inflatable toroids should be above and below the bottle cap clusters representing carbon atoms at the vertices of the hexagon. Since a regular hexagon can be thought of as six equilateral triangles of the same size all meeting at one point, the length of each side of the hexagon is equal to the radius of a circle that matches all of the vertices of the hexagon. For the model shown in Figure 6, the radii of the circle made by the centers of the cross-sections of the inflatable toroids were 25 cm, so then the length between the centers of the bottle cap clusters needed to be 25 cm. The plastic bottles that scaled best to these toroids were one-liter bottles, which all happened to be colorless for this model. Each pair of colorless bottles connected end-to-end with each other represents a sigma bond between two sp² hybridized orbitals of neighboring carbon atoms. Each colorless bottle with a spray-painted end represents a sigma bond formed from the interaction of an sp² hybridized orbital of a carbon atom and an s orbital from a hydrogen atom. The inflatable toroid swim rings represent delocalized pi bonds.

Figure 6. Plastic bottle orbital model of benzene using the trigonal pyramidal bottle cap clusters and inflatable toroids to represent delocalized pi bonds.

Figure 7 shows two last bottle-based models. At left is a model of water based on a traditional molecular model with a tetrahedral central atom with four spurs. On two of the spurs are stick-like covalent bonds. On the other positions of the tetrahedron are bottle-based shapes representing orbitals containing nonbonding pairs of electrons. The shapes are made from two-liter bottles that have been cut apart and then portions of the bottles taped back together. The bottle shapes are connected to the central atom model by drilling holes in bottle caps, which are slid over the spurs on the atom model.² At the right side of Figure 7 is an octahedral Polydron structure with bottles attached. (In fact, this is really four triangles and their associated bottles resting on a mirror.) This structure is reminiscent of some f orbitals.

Figure 7. (LEFT) Conventional model of water molecule modified with plastic bottles representing nonbonding orbitals. (RIGHT) Octahedral Polydron structure with bottles representing some f orbitals.

Concluding Thoughts

These models are generally easily built from readily available materials. Their low cost might enable them to be built by the students themselves. Their size should make them readily visible as they are manipulated in a classroom or outreach setting. Storing large models might be challenging given the space that they can take up, but perhaps they could be hung from the ceiling of a classroom or lab when not in use.

Safety - Follow all recommended safety precautions when using various cutting tools to build these models. There is some physical resistance to be overcome when cutting the walls of plastic bottles. Only use acetone in a well-ventilated area. Beware of overspray messes and vapors while spray painting objects. Pool toys are not typically designed to be used as serious flotation devices.

Acknowledgements - This work was supported by Bradley University and the Mund-Lagowski Department of Chemistry and Biochemistry with additional support from the Illinois Heartland Section of the American Chemical Society. The material contained in this document is based upon work supported by a National Aeronautics and Space Administration (NASA) grant or cooperative agreement. Any opinions, findings, conclusions, or recommendations expressed in this material are those of the author and do not necessarily reflect the views of NASA. This work was supported through a NASA grant awarded to the Illinois/NASA Space Grant Consortium.

References

1. Samoshin, V. V. “Orbital Models Made of Plastic Soda Bottles.” J. Chem. Educ., 1998, 75, 985.
2. Adcock, L. H. “Soda Bottle Orbital Models.” J. Chem. Educ., 1999, 76, 899.
3. Polydron (UK) Limited. Polydron Platonic Solids Set. https://www.polydron.co.uk/best-sellers/polydron-platonic-solids-set.html (accessed May, 2023).
4. Lippincott, K. A.; Rosengarten, E. A.; Sengupta, A.; Campbell, D. J. “Using Polymers and Pigments to Produce Laser Interference Rings.” J. Chem. Educ., 2019, 96, 2553-2559. 
5. Baird, C.; Cann, M. Environmental Chemistry, 5th ed. W. H. Freeman and Company: New York, 2012.