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In the 1980s when software for drawing 2D chemical structures was in its infancy and most chemists didn't have access to the workstations that could convert 2D to 3D structures, many labs had physical molecular modelling kits that could be used to build 3D space filling models of molecules.

Stick and ball kits were common then and are still common, but the 3D CPK kits seem to have disappeared as soon as most desktop chemical drawing software was able to reproduce pictures of space-filling models.

For those too young to remember this era, most 2D and 3D drawings were done by hand (a tedious task even for 2D structures that used physical templates so your benzene hexagons were not distorted).

But despite the ease of getting high quality 2D or 3D pictures of molecules directly from desktop software, there was something fulfilling about having a physical 3D model you could hold in your hand.

Is there any current source of those 3D CPK kits? Or, is there any accessible way to produce their equivalent physical models by, for example, 3D printing the models generated by desktop software?

PS this almost deserves a "nostalgia" tag.

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Note, CPK (for Corey–Pauling–Koltun) is a colour scheme, i.e., a convention to symbolize atoms of carbon in black, oxygen in red, nitrogen in blue, etc. which took inspiration on the colour scheme in the wired models by Dreiding. Wikipedia cites Chemical News

"Hofmann, at a lecture given at the Royal Institution in April 1865 made use of croquet balls of different colours to represent various kinds of atoms (e.g. carbon black, hydrogen white, chlorine green, 'fiery' oxygen red, nitrogen blue)."

From your description, I assume instead of a kit for ball-stick models, you seek one which is instead is spacefilling. If we assume atoms as spheres, then sections of spheres are called calotte, the same term used in architecture and about the skullcap. On occasion you see them (e.g., ebay), smaller ones still commercialized:

enter image description here

(image credit: Aug. Hedinger GmbH & Co. KG, Stuttgart/Germany)

The set depicted (55 atoms made of a polymer, 30 struts, and a tool to loose the struts) is sold for EUR 32.50 (plus taxes and shipping).

Larger in dimension (intended for demonstration in a lecture) are the ones by VSN:

enter image description here

(image credit VSN).

made of wood (Hofman like), with spheres of 3.5 cm (H), 6 cm (C, N, O), or 8 cm (Cl, S) in diameter. You purchase per atom.


If you don't want to disassemble them, these models may be printed in 3D, too:

enter image description here

(image credit: shapeways.com)

I have no affiliation with either corporation.

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    $\begingroup$ Good clarification that CPK is a colour-scheme (though the kits were often casually called "CPK kits"). And a great link for a source that can create physical models: thank you. $\endgroup$
    – matt_black
    Jan 2, 2023 at 13:42
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    $\begingroup$ @matt_black Today, it depends a bit on the software, too. If you run Jmol, for example and use its terminal (File -> Console) load $glucose; followed by spacefill on; you get the van der Waals model. Yet select all; color rasmol; yields a lighter gray on the carbons than select all; color cpk; (the default color scheme), one may select individual atoms/atom types, etc too. $\endgroup$
    – Buttonwood
    Jan 2, 2023 at 14:01
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    $\begingroup$ By the way, Jmol equally exports the scene with write example.wrl; (VRML 2.0) or write example.stl; a 3D print shop accepts as input format. CCDC's community edition of the Mercury has a similar export function (compared to Jmol, Mercury is more resource hungry for HDD and RAM): a brief how to, and some of the yearly contests (including polychrome models, too). $\endgroup$
    – Buttonwood
    Jan 2, 2023 at 14:04
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    $\begingroup$ Avogadro will also export VRML 2.0 with colors. I've used Shapeways to print the structures in full color. $\endgroup$ Jan 2, 2023 at 17:56
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    $\begingroup$ As the Wikipedia page mentions, Koltun patented an extension of the modeling kits developed by Corey & Pauling -- not just the color scheme, the models themselves. While CPK today is best known as a color scheme, there were actual kits sold as "CPK models" (for example), and that was a common "genericized" name for the kits in the 70s and 80s. (And apparently Harvard Apparatus still sells "CPK" branded kits today.) $\endgroup$
    – R.M.
    Jan 2, 2023 at 19:34
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Is there any current source of those 3D CPK kits?

You have to distinguish space-filling and semi-space filling, see below: enter image description here enter image description here

For semi-space filling, many ball-and-stick models also have pegs instead of sticks to connect atoms without extra space between them, like this Molymod Biochemistry set:

enter image description here

It is more difficult (and more expensive) to find space-filling model kits, but they still exist, e.g. this HGS model kit:

enter image description here

History

Space-filling models are associated with the CPK initials because Corey and Pauling published a home-grown non-commercial space-filling model in 1953, and later Corey, Pauling and Koltun made them available commercially.

enter image description here

They were sold by a company called Ealing, and searching for "Ealing CPK" might lead you to a vintage model such as this one:

enter image description here

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    $\begingroup$ I'm far more interested in the "proper" space filling kits with VdW atomic radii. The semi-space filling kits look like a terrible shonky compromise to me. $\endgroup$
    – matt_black
    Jan 2, 2023 at 14:32
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    $\begingroup$ Re «space-filling vs semi space-filling» in the visualization by Jmol/JSmol: the command to Jmol's command line select all; spacefill on; yields the same result as select all;, followed by spacefill 100%; (100% of the van der Waals-radii). And so offer spacefill off; and spacefill 0% a similar result. Intermediate dials as spacefill 22.2%; are permitted, too (somewhere close to Jmol's default). $\endgroup$
    – Buttonwood
    Jan 2, 2023 at 15:22
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I got a pile of CPK atoms on ebay. Sadly, the bonds had become brittle and useless over the decades. Harvard Instruments was able to supply new bonds, at roughly 100 times the original price of 2 cents a pop. Yep, that's right.

There's a pile for sale on ebay right now: https://www.ebay.com/itm/204536539768. The bonds will almost certainty be of no use though, just so you know.

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Was there ever an answer to the question of whether there is a modern producer of CPK model parts? I've recently obtained a very large set of parts that was sitting in the back of a storage room in our chemistry department, potentially for many years. Unfortunately, the connector pieces are made of a softer plastic that over time degrades and becomes very brittle, so when you try to take these apart the connectors shatter.

Back in the late 1990s when I was a graduate student at UCLA, we had a similar old set, but at that point I was able to order a bag of probably 100 of the connector pieces, so someone was still producing these parts. Unfortunately I don't remember who produced them, and Googling is not helping me.

Bruce

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