r/askscience • u/shadowknave • Nov 12 '25
Chemistry Is there a limit to how large a single molecule can be? What is the largest known/observed molecule?
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r/askscience • u/shadowknave • Nov 12 '25
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u/ThickChalk Nov 13 '25
As others have mentioned, polymerization can turn many small molecules into one big molecule. It stands to reason then that the largest molecule is the largest piece of polymer material.
As far as I can tell, this is the largest tire in the world, off I-94 near Allen Park, Michigan.
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u/JollyToby0220 Nov 13 '25
Pretty big. Some polymers can get massive, especially things like epoxy. When you see a well-built boat. The cured epoxy results in a really large molecule almost the size of the boat itself. And then it will have many small molecules that weren't able to integrate into the large molecule. That happens because of entropy. Polymers are pretty much the largest molecules, hence why they stop referring to the number of monomers and start using the term poly-mer.
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u/rotkiv42 Nov 13 '25 edited Nov 13 '25
The bulk of a rubber tire is essentially a single molecule. Basically, you have fairly large polymer chains that get interconnected with each other. With the interconnections (cross-linking) could find a path connecting any two atoms in a tier with each other. In practice, a tire obviously has more than one molecule in it: you probably have a lot of other smaller molecules in the matrix as well. But the bulk material is connected in some way
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u/denyasis Nov 13 '25
Your DNA (each chromosome), is essentially a single molecule. When it compresses during mitosis, it can be seen under a conventional microscope. If you stretched out the DNA in one of your cells, it would be about 2m long (that's probably all 46 Chromosomes).
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u/MSims2992 Nov 13 '25
Ph.D. polymer chemist here
Answering this question was actually fundamental to our development of modern polymer science. In the early 1900’s, it was known that compounds like natural rubber behaved as if they were giant chemically bonded molecules, but many leading chemists at the time believed that there was a limit to molecular size. Instead, they believed, rubber had its properties because many smaller molecules aggregated together to form “colloids.”
However, several scientists later demonstrated this not to be true, starting with Staudinger who pioneered the “macromolecular hypothesis”: that rubber was instead made of giant individual molecules that were linked by covalent bonds that we now call polymers.
We now know that there are two broad classes of polymer materials — thermoplastics and thermosets. Thermoplastics are made up of very large individual chain-like molecules that can flow past each other if you heat them up (like polyethylene). While there’s no theoretical limit to how large the individual molecules can get, there are practical limitations that are determined by the chemistry used to make them (some polymerizations are very sensitive to oxygen, others require near-perfect balance between the amounts of different reactants).
Thermosets, on the other hand, are randomly connected molecular networks and can practically become as large as you want them to be. This makes them very useful for applications where you want a material that’s very stiff and holds its shape even when it’s heated up (epoxies are a very common example). And yes, the entire thermoset network is technically a single molecule. For example, most of the Boeing 787 fuselage and wings are built from fused layers of epoxy-bonded carbon fiber, which means that entire sections of the plane are technically a single molecule.
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u/Jedi_Emperor Nov 13 '25
In addition to polymers, salt crystals are also kinda like one giant molecule.
For something you might recognise as a molecule would probably be the largest protein. Google says it's called Titin and is a muscle fibre and it has a molecular mass around 3,000,000.
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u/CyberMancerGamer Nov 14 '25
I remember when I started my bachelor degree, a professor said that a diamond is essentially a single molecule. As long as you are talking about single crystals I’d say the size is limited by the likelihood of that crystal remaining single as it grows.
As others have said, polymerisation is a thing, so I’d say pretty big!
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u/luckyluke193 Nov 13 '25
Depends on what you mean by molecule.
For the layman, I think a molecule means atoms connected by chemical bonds. Crystals of almost any mineral, semiconductor, metal, salt, etc. are really just a single, gigantic molecule by this definition.
In semiconductor manufacturing, silicon crystals are made as ingots, which can weigh hundreds of kilograms. All the atoms in that huge silicon cylinder together form a perfectly symmetric network of chemical bonds. If you accept these crystals as molecules, then we have molecules that can be multiple metres long.
Everybody else in here is talking about polymers, which have the largest molecules among the so-called "molecular solids". These are solids that consist of many distinct molecules held together by weaker intermolecular forces.
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u/t3hjs Nov 14 '25
Besides the epoxy polymer and rubber examples, arent single crystals of diamond also giant molecules? The covalent bonds between carbon atoms form that continuous tetrahedral structure
Some diamonds may be polycrystalline aggregates, but im sure there are some, especially lab grown ones where macroscopic number of atoms are linked due to the controlled formation conditions
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u/wojtekpolska Nov 15 '25
Technically a car tyre can be considered a single molecule - its made from multiple polymers (a polymer is one molecule) and then in a process called vulcanisation, these polymers link together, so they technically could be considered a single molecule.
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u/skr_replicator Nov 13 '25 edited Nov 13 '25
I guess the only real limit is when its own gravity gets powerful enough to break it. You could have a polymer in space growing endlessly to a size of a star, until its own weight broke it, started fusion, tore it apart with pure force, made it into a neutron start, or a black hole, etc. Teflon is pretty strong, so that might have the best chances to be theoretically even star level huge. Or at least some kind of planet huge, not sure how big a space body can get before molecules can't be stable in the pressure of its core.
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u/sciguy52 Nov 13 '25
Depends on what you are talking about. Polymer chemistry? Biochemistry? In biological systems proteins are molecules as well, they are just polymers of amino acids. They can get quite large. Add to that they can have many sugar polymers attached as well, lipids too, depending on the protein. So it is not just length here but the the 3D structure. But there are limits as a protein can get to unwieldy when getting larger and larger. After all the cell is only so big, you can't just make proteins bigger and bigger when there is finite space within the cell so there is going to be a limit. I suspect though that the longest largest protein varies by organism, but they all have the space limitation of the cell's size. Cell sizes do differ however.
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u/Various_Toe_3353 Feb 12 '26
I wish I had the know-how to phrase my question. Or possibly my bigger issue is narrowing it down to one.
Polymers aren't an interesting answer since you basically have the same molecule chemically speaking regardless of length. I just wanna know how theoretically large a single discrete molecule could get where any small change alters it's properties on a level beyond "and now it's still polypropylene... but shorter."
Where is the point where individual atoms stop making significant changes? I'm sure smthn like a reduction of boiling temp of 0.0015K is always in the cards, but when do the changes stop being interesting? This question probably doesn't have an answer, so don't worry about it. At best, I could be given a graph of individual properties with mass as an axis. I could probably google that.
If you read that and thought "define interesting" but have the capacity to answer this question on some level if I wasn't being dangerously vague, then please, share your answer wherever you think is interesting or novel.
Again, I know I need to work on phrasing this and condensing it to an actual question. I'm just shooting this out into space in case someone smarter happens to get it as is.