Wikipedia:Reference desk/Archives/Science/2018 December 14

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December 14

Elephant rescue?

In India, are elephants used for rescue work? If so, what are their usual tasks in this line of work (e.g. removing debris, rerailing train cars, bringing supplies, towing disabled vehicles, or maybe something else)? 2601:646:8A00:A0B3:0:0:0:718C (talk) 06:24, 14 December 2018 (UTC)

See here. --Jayron32 11:48, 14 December 2018 (UTC)

Why is He2 so large?

Someone here above asked "What's the largest diatomic molecule?" and the answer according to Helium dimer is He2.

Why would He2 be so large, larger than O2, Cl2 etc., given than these atoms have much larger nuclei, more electrons etc.?

Apologies, I am not a chemist and thought you could use the atomic number to have an good idea of the relative sizes of atoms, and a double-atom molecule should be roughly double the size of each atom? I must be missing something, if someone could be kind enough to explain. --Lgriot (talk) 12:26, 14 December 2018 (UTC)

Oops, I read further the Helium dimer article and it says it is "due to its extremely long bond length".

So I have another question, does this molecule look like this "O-------O" rather than like this "OO", if we were able to shine light on it? (I know light's wavelength is too large, but if we had some other way to 'see it'). In short, is it very large, but also very thin? --Lgriot (talk) 12:33, 14 December 2018 (UTC)

If you tried to illustrate the appearance of a chemical bond, what would you like it to show? The simplest I can thing of is the electron density. This is what you might observe if you could irradiate the molecule with gamma rays. In the case of this "molecule", the atoms of helium repel each other when close, but when further apart London dispersion forces attract the electrons from one atom slightly to the other. For helium, this is the weakest of any atom. You won't see a long thin stream of electron from one nucleus to another. Instead you will see two balls of electron density, more concentrated around the nuclei, but slightly deformed so that they are slightly lopsided, with a bit more density towards the other atom. So rather than thin, I would call it very distended and pretty wide. Other possibilities to illustrate could be the phase of the matter wave, or the magnetism due to spin and rotation, but I won't guess at that. Don't forget that most molecules are always vibrating and rotating, so they are not just like this: O-O, but in constant motion OO O-O O--O O-O OO ... Graeme Bartlett (talk) 11:24, 17 December 2018 (UTC)

My feeling is that this is more or less a trick. A "Van der Waals molecule" is basically two molecules (or nonbonding atoms) of ultra low pressure gas stuck next to each other (i.e. condensed into a liquid). Surely two adjacent atoms of liquid helium have more right to declare a bond with each other! I am somewhat curious if the Xe2 "Van der Waals molecule" is really smaller (it definitely does exist but I don't know the bond length) but calling this a 'diatomic molecule', as we do in the article, seems like it is meant to mislead those who don't know better. Wnt (talk) 15:28, 14 December 2018 (UTC)

To the contrary, it's not really a trick, it's that our categories for these things are human-created boxes to aid and simplify our own understanding and comprehension of the phenomena involved (i.e. what is called a "model" in science) and all models are wrong. In this sense, the notion we have of things like "molecules" and "covalent bonds" and "intermolecular forces" are useful tools we have to categorize different ways atoms organize themselves, but in reality, all that really exists are electrons and nuclei. Electrons are attracted to various nuclei at a certain strength, and how those electrons and nuclei organize themselves based on the strengths of those attractions is how we classify various structures. Whether or not we call something a "molecule" is an us-problem, not an atom-problem. The reason why He2 (and Van der Waals molecules in general) represent a categorization problem is that they represent an "edge case" where we have clearly stable multi-atom structures that traditional bonding models (valence bond theory or VSEPR or molecular orbital theory or whatever) fail to predict or account for. What we call a molecule that our models predict shouldn't exist, but yet still does, is we qualify it as a "Van der Waals molecule" or call it a trick or a fake, but it isn't any less "real". After all, it is an observable phenomenon. It's the realest thing we have. What's fake is our models. Models are always "fake", they're just "useful", except that there is always some limit outside of which they become not useful. If a model doesn't verify an observable phenomenon, then it isn't the observable phenomenon which is wrong (and the model is not necessarily bad, universally, just bad for this application). --Jayron32 16:32, 14 December 2018 (UTC)

This isn't so much a matter of models as definitions. We know that molecules stick together in larger clumps. The question is, do we call a water dimer a molecule? Do we name it H4O2? Do we say that H2O is chemically unstable because it forms water clusters? A water dimer, or larger water clusters, actually have hydrogen bonds and so are far more stable than He2, I might add. Wnt (talk) 12:20, 15 December 2018 (UTC)

Yes, but the definition you use depends on your context. To an astronomer, oxygen is a heavy metal. When someone says "what is the longest bond between atoms" we need to first establish what we mean by "bond". Are IMFs considered bonds? Like I said, it's all bonds. What particular organization of atoms we define as a "molecule" is all we're asking here, and any limit we set in that definition is arbitrary. We have to set some definitions, so long as we recognize we can always find edge cases that will challenge whatever parameters we set in our definition. Which is fine, but we need to recognize that these definitions, useful as they are, are still not immutable laws of nature, they're just sometimes useful categories we create to simplify the world and give ourselves a common language to talk about it. And language is ALWAYS ambiguous.--Jayron32 22:14, 15 December 2018 (UTC) --Jayron32 22:14, 15 December 2018 (UTC)

Thanks for the background explanation. Reading further, the length of a an He₂ molecule 103.9 pm, versus a single He atom having a radius of 28 pm. This means the molecule is on average 1.855 times longer than 1 single atom. So I imagine/visualise the atoms still "touching", but not being embedded into each other, like O₂ is, as represented here: [1]--Lgriot (talk) 16:53, 17 December 2018 (UTC)

Resolved