Egg-lectrons and McLewis Structures

Egg-lectrons and McLewis Structures: More Representations of Electron Arrangements in Atoms and Molecules

Co-Authored with Ali Patel*

*Bradley University, Peoria, Illinois

Many physical models with varying degrees of sophistication are used to illustrate chemical concepts.^1,2 Simple, less expensive models could be more widely used than more expensive models. Egg cartons are inexpensive and have regular arrays of dimples. In addition to holding eggs, these dimples can hold and spatially organize other objects, even going as far as containing chemical reactions.³ In previous works, dimpled trays and small objects such as milk jug caps or plastic eggs were used to illustrate chemical concepts. Dimples in egg crates or mini quiche trays were possible locations for objects representing atoms or molecules within crystal structures with square symmetry.^4,5 Egg trays were also used to represent orbitals in energy diagrams.⁶ The objects distributed among the dimples of the trays were used to represent electrons that can be paired within the dimples. Dimpled trays such as egg crates or mini quiche trays were also cut into shapes to represent atoms, where each dimple represented a location to place the object representing an electron.^6,7 For example, a ten-dimple tray could represent a Period 2 atom that could hold two core electrons and up to eight valence electrons.⁷ A two-dimple by four-dimple tray could focus solely on eight valence electrons of an atom.⁶

In this work, some of the previous ideas are combined to produce a new model of electrons in atoms. Here, the tray dimples represent atomic orbitals within atoms (each with a capacity of two electrons) arranged around a center point. For example, a simple two-dimple by two-dimple tray represents an atom with a capacity for an octet of valence electrons in four orbitals. The objects are labeled with arrows to represent electron spins. As before, trays can be connected to each other to produce flat structures resembling Lewis structures. The net result is a model system that is easy to build with simple, readily-found materials, including those found at fast food restaurants and grocery stores. Many of these materials can also be recycled.

Models

Figures 1 and 2 show how tray-like sections of egg cartons can represent atoms, and objects such as milk caps or plastic egg halves go into the dimples to represent valence electrons. Each object is marked with a half-headed arrow using a permanent marker to represent an electron spin. The egg cartons can be cut into one-dimple trays representing the location of up to two valence electrons in a single 1s orbital. For example, the white one-dimple trays in Figure 1(top), each with a single milk cap, represent hydrogen atoms. The egg cartons can also be cut into two-dimple by two-dimple trays representing the location of up to eight valence electrons in four orbitals. For example, the pink four-dimple trays in Figure 1(top), each with four paired and two unpaired milk caps, represent oxygen atoms. The milk caps are always marked on their edges in the same way with a single half-headed arrow. A single cap placed on edge within a dimple represents an unpaired electron. When two caps are placed on edge top-to-top within a dimple, the arrows point in an antiparallel direction, representing the antiparallel spins associated with paired electrons.

To represent molecules, multiple egg cartons and their milk caps can be combined to represent covalent bonding. To make a molecular model, the trays representing the atoms can be stacked at least partially over one another to create the correct total number of dimples. The number of dimples in the assembled model must match the total number of available orbitals for the valence electrons. The total number of milk caps in the molecular model must match the sum of the caps of the individual trays coming together. This is similar to the way one can draw a Lewis structure by first totaling up the number of valence electrons contributed by each atom. Caps placed into unstacked dimples represent nonbonding electrons; caps placed into stacked dimples represent bonding electrons. For example, models of hydrogen molecules are shown in Figure 1 (middle). Each hydrogen atom that combines into a diatomic molecule contributes one valence electron, giving two electrons that must be distributed throughout the molecule. In the molecular model, a one-dimple tray is placed over another one-dimple tray, and the two caps are placed in the top visible dimple to represent two paired bonding electrons. A model of a diradical triplet oxygen molecule is shown in Figure 1 (middle). Each oxygen atom that combines into the diatomic molecule contributes six valence electrons, giving twelve electrons that must be distributed throughout the molecule. These electrons end up in seven positions, so the two four-dimple trays are overlapped by one dimple to produce a structure with seven visible dimples. Two caps are placed in the top visible overlapping dimple to represent the two paired bonding electrons. In the three dimples in each tray that are not overlapped, two groups of two caps are placed, representing unshared pairs of electrons, and a single unpaired cap, representing a radical electron.

These ideas are extended even further with the models of water molecules shown in Figure 1 (bottom). Each hydrogen atom contributes one valence electron and each oxygen contributes six valence electrons, giving eight electrons that must be distributed in four dimples throughout the molecule. To get four dimples in the molecular model, two one-dimple trays are placed over a four-dimple tray, and the eight caps are placed in pairs in the dimples to represent four bonding electrons and four nonbonding electrons. The molecular models in the middle and bottom panels in Figure 1 represent reactants and products in the important reaction:  

2 H₂ + O₂ → 2 H₂O

For example, this reaction plays a critical role in space applications.^8,9

Figure 1. Egg carton tray and milk cap arrangements representing valence electrons in atoms and molecules. (TOP) Individual oxygen atoms above and individual hydrogen atoms below. (MIDDLE) Oxygen molecule at left and hydrogen molecules at right. (BOTTOM) Water molecules.  

Cardboard egg trays are recyclable and can be painted in various colors, which can be used to represent different elements. Although the polystyrene foam egg trays shown in Figure 1 are not recyclable, they are already available in various colors. Figure 2 shows a model of an ammonia molecule, NH₃, this time using halves of plastic eggs rather than milk caps. The egg halves are still each marked with the half-headed arrow. Each hydrogen atom contributes one valence electron, and the nitrogen contributes five valence electrons, giving eight electrons that must be distributed in four dimples throughout the molecule. To get four dimples in the molecular model, three one-dimple trays are placed over a four-dimple tray, and the eight plastic egg halves are placed in pairs in the visible dimples to represent six bonding electrons and two nonbonding electrons.

Figure 2. Egg carton tray and plastic egg half arrangements representing valence electrons in an ammonia molecule. 

Another simple two-dimple by two-dimple tray is a drink carrier available at many fast-food restaurants such as McDonald’s. The cardboard trays are recyclable and paintable. For objects to be used as electrons, the bottoms can be cut from cups designed to fit the drink carriers. Various cup sizes work, and polypropylene cups are preferred because they are recyclable. The cups are cut so that one or two centimeters of sidewall are still attached to the cup bottom, producing a similar shape to that of the milk caps described above. The cup bottom pieces could even be cut from polypropylene cups that have been cracked at low temperatures from demonstrating glass transitions of polymers.¹⁰ The cup pieces are also marked on their sidewalls with half-headed arrows using a permanent marker to represent electron spins. As with the milk caps, these cup pieces are placed on edge in the dimples in the carrier. For pairing, the cup pieces are placed bottom-to-bottom. Figure 3 (top) shows a model of the molecule methanol, CH₃OH, where the black drink carrier represents carbon, the red drink carrier represents oxygen, and the one-dimple tan pieces of drink carrier represent hydrogen. As above, bonding is represented by overlapping the dimples of the drink carrier. This model resembles the Lewis structure of methanol. Since the model is made from materials that could be acquired from McDonald's, we could call this a McLewis structure! Figure 3 (bottom) shows a model of the molecule ethene, C₂H₄, where the black drink carriers represent carbon and the one-dimple tan pieces of drink carrier represent hydrogen. To represent the carbon-carbon double bond in ethene, the carriers are overlapped at two dimples. This model resembles the Lewis structure of ethene.

Figure 3. Cup holder and cup bottom arrangements representing valence electrons in (TOP) methanol and (BOTTOM) ethene.  

Classroom Use and More Variations

I showed my students one of these models after I had covered atomic orbital diagrams, electron spin pairing, and drawing simple Lewis structures, because these models resemble Lewis structures with the electrons showing their spins. Figure 4 shows models of water molecules using alternatives to the cup pieces to represent electrons. The model in Figure 4 (left) uses stacked cups that are cut in various places so that they stack together and show pairing of the spin arrows. The model in Figure 4 (middle) uses modified toilet paper tubes, where each tube is cut and one half is folded inward. These tubes also stack to show pairing of the spin arrows. I initially used one of these models in my classroom, but I realized that the arrows must be viewed from the side and the dimple arrangement from the top. To show one model to a group, the model must be rocked forward and back to show various orientations, so it was not easily used on a document camera. I later showed the class the model in Figure 4 (right), where the electrons were represented by short segments of a halved pool flotation noodle. Here the cut surfaces of the noodle pieces are marked with arrows. This model was much more easily shown using a document camera.  

Figure 4. Alternative cup holder structure arrangements featuring electrons represented by: (LEFT) stacked modified cups, (MIDDLE) stacked modified toilet paper tubes, and (RIGHT) halved segments of pool noodles. 

There are likely other examples similar to those presented here that can be put forward. Electrons could be made from rings cut from a variety of tubes or disks cut from a variety of rods. A goal in this work was to find easily acquired and preferably recyclable materials.    

Although this dimpled tray system works well to represent simple Lewis structures, it does have some limitations. To show a triple bond, three dimples would have to overlap, which cannot be accomplished by overlapping two two-dimple by two-dimple trays. Atoms where the valence electron count exceeds eight, for example, the phosphorus in phosphorus pentachloride or the sulfur in sulfur hexafluoride, cannot be modeled with a four-dimple tray. An interesting aspect to consider of the overlapping tray approach is the conservation of orbitals. When atoms combine to form covalent bonds, their orbitals are conserved. For example, when two hydrogen atom atomic orbitals are combined, two molecular orbitals (one bonding and the other antibonding) form. If each dimple in a tray represents an atomic orbital, then overlapping trays should still contain the same number of dimples as the separate trays. This is indeed the case, because some of the empty dimples are hidden under dimples containing objects representing electrons. For example, when two hydrogen atom models in Figure 1 (top) combine to form the hydrogen molecule model in Figure 1 (middle), one dimple nests inside the other, but only one dimple contains the pair of milk caps. These empty hidden dimples could be analogous to empty antibonding orbitals. When two hydrogen atoms combine to form a hydrogen molecule, the two atomic orbitals combine to form a bonding molecular orbital containing a pair of electrons, and an empty antibonding molecular orbital. For simple molecule models, orbital conservation can still be represented.

Simplified dimpled tray models without the electrons can also be used in some settings. At a recent outreach event, models of water were built by the participants. Each four-dimple tray, colored red to represent the oxygen, had two tan colored one-dimple trays, representing hydrogen, added onto it without adding the cup pieces representing electrons. These participant-constructed models were then arranged in a simple two-dimensional pattern to represent a solid crystal structure, as shown in Figure 5.

Figure 5. An ordered array of cup holder water molecule models used to represent a crystal structure at an outreach event.  

Another outreach activity using those same trays shown in figure 5 involved a demonstration of condensation polymerization. Volunteers from the audience, each representing a simple sugar such as glucose, were placed side-by-side in a line. In each hand of the volunteers was placed a two-dimple by two-dimple red tray representing oxygen with a one-dimple tan tray representing hydrogen. Therefore, each hand held a model of an alcohol group. Where two volunteers were next to one another, an assistant removed a red tray and tan tray from one hand, and just a tan tray from the other hand. Both tan trays were placed in the red tray to produce a model of a water molecule. Then, one hand from both volunteers was used to hold the remaining red tray to produce a model of an ether linkage between the volunteers. The assistant then moved to the next pair of handheld alcohol models to repeat the process. This way, the volunteers representing glucose monomers were “polymerized” by a condensation reaction to represent a polysaccharide molecule such as starch.

We look forward to using these molecules further in classroom and other settings, and we seek your input as to their utility and possible ways to improve them.

Safety: Wash your hands before and after using models that others might also be using. Consider the hand-eye coordination of individuals being asked to cut egg cartons, cupholders, cups, etc.   

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.

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