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Our college is switching to teaching online mid-semester in hopes of slowing the spread of Covid-19. All the molecular model kits are still on campus. How could you build a model of tetrahedral coordination (say methane) from materials found at home?

I'm aware of computer visualizations (and will make those available), but I think having a physical model when first encountering three-dimensional structures adds value. I made a model from lawn toys (see below), but these are not common household items.

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    $\begingroup$ The review bot said "The question you're asking appears subjective and is likely to be closed". We'll see about that ;-) $\endgroup$
    – Karsten ♦
    Commented Mar 16, 2020 at 16:48
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    $\begingroup$ You may also two pairs of compases $\endgroup$
    – Maurice
    Commented Mar 16, 2020 at 17:04
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    $\begingroup$ Just get (or make) a cubical box and put four balls (or whatever) on alternating corners, like here: chemistry.stackexchange.com/a/118202/79678. $\endgroup$
    – Ed V
    Commented Mar 16, 2020 at 17:07
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    $\begingroup$ Not exactly things you have at home, but my research group bought a set of these little magnetic rods and ball bearings and we make tetrahedra/octahedra and lattices all the time with them, they're very useful: image $\endgroup$
    – Kai
    Commented Mar 17, 2020 at 18:37
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    $\begingroup$ Mini-marshmallows, toothpicks and food coloring. $\endgroup$
    – user55119
    Commented Mar 17, 2020 at 20:38

4 Answers 4

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Inflate balloons, and tie them «at their stem» like a bouquet of flowers. If you take four of them, not too much inflated, you well demonstrate a situation close to $sp^3$ hybridization. These models equally work well in larger lecture halls by the way, and intentionally using different colors allows many options.

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(source)

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(screen photo

You need some worked examples? See videos like this or this.

You need a scientific paper? Well, there are as well, e.g., this (open access) expanding the picture to extended $\pi$-systems:

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  • $\begingroup$ This is a great VSEPR demo because the balloons "find the solution on their own", i.e. you are just tying a couple balloons together, and they arrange to maximize bond angles. $\endgroup$
    – Karsten ♦
    Commented Mar 16, 2020 at 22:47
  • $\begingroup$ It get's even better, as youtube.com/watch?v=Kb0mxAMHnfE shows $sp^2-sp^2$ to lead to $\pi$ bonds, too. And his «VESPR» reminds a bit to «Vespa» (smaller motocycles) to .... $\endgroup$
    – Buttonwood
    Commented Mar 16, 2020 at 22:56
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This one is inspired by ideas from Ed V and Todd Minehardt. The cool thing is that the angles are pretty accurate if the chop sticks reach into the opposite corners of the cube:

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As a bonus, you can make an octahedral model if you make holes in the center of each square (or slightly offset so that the chop sticks can pass in the center of the cube).

The next one is just cardboard, but you have to get the 109.5 degrees right. enter image description here

Etcetetra...

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From left to right: Pipe cleaner, pins and glue gun blob, paper model (from Bob Hanson's Molecular Origami book, ISBN-13: 978-0935702309), carrot and toothpicks (inspired by the marchmallow and toothpick activity in Laura Frost's General, Organic and Biochemistry textbook, ISBN 13: 9780805381788).

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    $\begingroup$ @ToddMinehardt - Version 2 of your idea... $\endgroup$
    – Karsten ♦
    Commented Mar 16, 2020 at 17:55
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    $\begingroup$ The cool thing is «is that the angles are pretty accurate if the chop sticks reach into the opposite corner» .AND. if the box is a cube. I recommend to state this second conditon to be met during the lecture. An answer by Todd Minehardt (retracted by him) used a box of probably only $D_{4h}$ symmetry instead of $O_h$; but only boxes qualifying $O_h$ will do this trick where space diagonals coalesce with «space bisectrices» indicated by the chop sticks. $\endgroup$
    – Buttonwood
    Commented Mar 16, 2020 at 18:23
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    $\begingroup$ @Buttonwood I took two flaps of equal with, and used the width of one to measure off the lengths where I had to fold the other. So it is pretty much a cube (and the chop sticks did collide in the center). Thanks for the input (the lecture will have to wait). $\endgroup$
    – Karsten ♦
    Commented Mar 16, 2020 at 18:54
  • $\begingroup$ Yeah, I retracted mine as it was more of an optical illusion than the real deal as @Buttonwood notes. Anyway... $\endgroup$
    – Todd Minehardt
    Commented Mar 16, 2020 at 19:11
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    $\begingroup$ @KarstenTheis The hint to the book (amazon.com/Molecular-Origami-Precision-Scale-Models/dp/…) led to the corresponding «folding program» (stolaf.edu/depts/chemistry/courses/toolkits/123/mo). Maybe interesting as complementary or substitute to the anaglyph googles in crystallography is Sanii's presentation of «Augmented Reality» with Students' Phones in J. Chem. Educ. 2020, 97, 1, 253-257 (doi.org/10.1021/acs.jchemed.9b00577) for 3D objects in general, too. $\endgroup$
    – Buttonwood
    Commented Mar 16, 2020 at 21:06
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With styrofoam balls and toothpicks:

  • Pick a central ball. As an aid for the next step draw three mutually perpendicular great circles on this ball, dividing it into octants.

  • Pick any octant and the three octants catty-corner to it and place toothpicks through the center of each of these octants. This should give a good approximation of the proper angles if your great circles from the first step are reasonably accurate.

  • Attach the coordinated balls on the exposed ends of the toothpicks.

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  • $\begingroup$ Obviously the contents of your home are different to mine. But I imagine the downvote was because this is a different structure with five balls not four $\endgroup$
    – Pete Kirkham
    Commented Mar 17, 2020 at 13:52
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    $\begingroup$ Do we or do we not include the center ball? $\endgroup$
    – Oscar Lanzi
    Commented Mar 17, 2020 at 13:54
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    $\begingroup$ @OscarLanzi In my question, I did not mean to imply specifics about how the model would be used, as model for methane, as model for sp3-hybrids, as VSEPR model etc. Depending what you use it for, a center ball would make sense or not. Instead of the styrofoam ball, lots of different food items will work (I tried an apple and ate it afterwards because while I never want to waste food, this is definitely not the time). $\endgroup$
    – Karsten ♦
    Commented Mar 17, 2020 at 17:37
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You may also use two pairs of compasses, and join them with their tips in opposite directions.

Or you can use your hands wide opened in front of your face. You then turn one of your hands at an angle of 90°, and joined them, without closing them. The thumbs and the two forefingers represent the directions of the four bonds around a carbon atom in $CH_4$.

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