Talk:Van der Waals radius

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I'm not a chemist (still in high-school), but shouldn't this read PV = nRT? :-\ /splintax

You are right and wrong! I think the original author intended V to equal the molar volume but due to the limitations of the font used here, the "~" couldn't be placed over the V. In practice you try to use equimolar volumes in your calculations. I'll try and place a link to the actual equation.

Also, I don't want to confuse my high-school friend but, there should be something said about polarizability volumes and van der Waals. I can remember right now what the connection is (there may not be any) but I'll look over my notes from college.

afn

Yes, there should be something said about that! I'm working on it, slowly... Physchim62 (talk) 15:31, 27 April 2007 (UTC)[reply]

When a word is quoted in a definition, it is a signal, first, that the reader should not take the meaning of the quoted word literally, and second, that the author already has or soon will clarify what is meant by the quoted term.

Unfortunately, the author never did explain why he was not satisfied with the word "occupied" when referring to the volumetric space taken up by a molecule at a given position. If the word is not "occupied", then what word is it?

This may appear to be a minor quibble, but it is far from it: this entire article is ABOUT measuring the space taken up by a molecule -- and that figures quite fundamentally in our understanding of the natural world. The author may believe he is in possession of an inexpressible subtlety of molecular mechanics, and is using the quoted word to signal such priestly knowledge. If so, forget it! Wikipedia/Chemistry is all about elegantly expressing the what is known and unknown about our material world, so that the common man may better appreciate its majesty and order.

Inquiring minds, including but not limited to my own, want to know: does a molecule occupy space or not? There is a suggestion here to the contrary, and I find that profoundly disturbing. Contrablue (talk) 02:15, 10 February 2011 (UTC)[reply]

How does one find this radius, is there an equation to find it? The equation on this page doesn't seem to have the "radius" as a variable. Fresheneesz 03:19, 1 June 2006 (UTC)[reply]

I'm not a chemical scientist, in fact, I am far from being one, so correct me if I am wrong. I noticed the values for some of the chemicals listed on the right on the top of the page seem a little wrong. In one of the papers I am using, most of the VDW radii are identical, except Fluorine (F), Phosphorus (P), Sulfur (S), and Chlorine (Cl), and I don't have the value for copper. I then checked up on the individual pages for all the listed chemicals, and realised the radii listed on those pages are the same as mine (except copper, which is missing from my records). On the individual pages, F has a radius of 1.47 A compared to the listed 1.35, 1.80 compared to 1.9 for P, 1.80 compared to 1.85 for S, and 1.75 compared to 1.8 for Cl. Can the maintainer of the page please correct them. Thanks! Wpliao (talk) 04:21, 31 July 2008 (UTC)[reply]

There is no "maintainer" to the page - it's a wiki and anyone can edit!. The ideal person to update this would be you, as you already looked up those values in textbooks ans so can also cite where the values are coming from! (that is better than just copying them from the other pages as you can put a citation in and the data becomes verifyable) Iridos (talk) 23:26, 23 September 2008 (UTC)[reply]

I changed them - you can still double-check my edit and perhaps put your source in (I just compared to the articles and to the web-page given in this article) - if you looked the values up in a textbook and they are correct, please put <cite> your textbook citation</cite> on top of the table (if your literature covers all values) Iridos (talk) 23:38, 23 September 2008 (UTC)[reply]

I've checked the original source of these values (the Bondi paper now cited in the article) and the table is OK. As the article now points out, there are other values out there, although I think Bondi's are the most widely used. Physchim62 (talk) 20:27, 24 September 2008 (UTC)[reply]

What's all the stuff about an ideal gas doing in the stub about van-der-waals radii? Just because it includes the name of van der Waals, doesn't mean it's related - I'm removing that part for now, if you think there is a justification to keep it in _this_ article, then rewrite it such that the connection becomes obvious Iridos (talk) 23:26, 23 September 2008 (UTC)[reply]

Actually, it is related! Fundamentally so! I have replaced it and tried to explain it better, including an example of the calculation of the van der Waals radius of helium based on gas data. Physchim62 (talk) 10:05, 24 September 2008 (UTC)[reply]

Of course it is "fundamentally" related - but it was not, in the article, related to the van-der-Waals radius at all - and with that I mean something like connected/linked by text and grammar. As such useless in explaining the lemma (I might have phrased that better in my 1st comment). Moreover, there are separate articles for Van_der_Waals_equation and Van_der_Waals_force - overlaps with those should be kept to a minimum... and specially so when they are just thrown in without explanation as to how they relate to this lemma.

I think you did a good job explaining things further in the introduction. As for the article - I'm still not content with the structure, as a lot is explained before your helium-example without the reader knowing where you want to go... I rearranged things quite a bit to get everything into a more logical order, please check for consistency, it is so easy to inadvertently mis-arrange something in the process. argh, got logged out while fixing the article Iridos (talk) 20:28, 25 September 2008 (UTC)[reply]

If you still have several of the more commonly used sources for van-der-Waals radii at hand, perhaps the table could be extended to show several ("both"?) commonly used sets? Or would that go beyond the scope of the article? If the most commonly used sets were listed, people looking up the lemma because they read about some radius/radii used in some text/program/paper would also learn if an older or more recent version of them was used... Iridos (talk) 02:52, 26 September 2008 (UTC)[reply]

Guess from your lack of answer that you at least don't dislike my last restructuring, then and don't think any further changes are necessary at the present time (or of course you're just too busy with something else). I'm rather content with the change of the rearrangement (although there are probably still many details that could be improved) putting the VDW-equation-part under the heading 'Methods of Determination' immediately makes clear where this is going, your example neatly demonstrates the application and this also got rid of the "other methods of determination" heading. (Perhaps I'm a bit too focused on document structure sometimes [I suspect that might come from writing in latex] but for me things are expressed much clearer now) Iridos (talk) 08:39, 29 September 2008 (UTC)[reply]

Both assumptions are correct! I've been busy on other things and I think the changes are OK! I think we need to expand the lead a little, so as to introduce the article structure and to point out that there is no single definition of VDW radius. Also, the section headings shouldn't have full capitalization, only the first word and proper nouns (such as "Waals", but not "van der"). Physchim62 (talk) 08:48, 29 September 2008 (UTC)[reply]

I also think we should have a "Physical interpretation" section and an "Applications" section.This seems like a good source for the latter. Physchim62 (talk) 09:58, 29 September 2008 (UTC)[reply]

There doesn't appear to be a formal definition of the van der Waal's radius here. The introduction states that they can be used to model physical properties of atoms and molecules, but then goes on to talk about the van der Waal's volume and never gives a formal definition. —Preceding unsigned comment added by Joshdkatz (talk • contribs) 23:22, 16 September 2009 (UTC)[reply]

http://www.ncbi.nlm.nih.gov/pubmed/12968888

Abstract

The van der Waals volume is a widely used descriptor in modeling physicochemical properties. However, the calculation of the van der Waals volume (V(vdW)) is rather time-consuming, from Bondi group contributions, for a large data set. A new method for calculating van der Waals volume has been developed, based on Bondi radii. The method, termed Atomic and Bond Contributions of van der Waals volume (VABC), is very simple and fast. The only information needed for calculating VABC is atomic contributions and the number of atoms, bonds, and rings. Then, the van der Waals volume (A(3)/molecule) can be calculated from the following formula: V(vdW) = summation operator all atom contributions - 5.92N(B) - 14.7R(A) - 3.8R(NR) (N(B) is the number of bonds, R(A) is the number of aromatic rings, and R(NA) is the number of nonaromatic rings). The number of bonds present (N(B)) can be simply calculated by N(B) = N - 1 + R(A) + R(NA) (where N is the total number of atoms). A simple Excel spread sheet has been made to calculate van der Waals volumes for a wide range of 677 organic compounds, including 237 drug compounds. The results show that the van der Waals volumes calculated from VABC are equivalent to the computer-calculated van der Waals volumes for organic compounds.

Just granpa (talk) 14:24, 20 November 2010 (UTC)[reply]

I read in the present article: "For helium, b = 23.7 cm3/mol. Helium is a monatomic gas, and each mole of helium contains 6.022×1023 atoms... Therefore the van der Waals volume of a single atom Vw = 39.36 Å3, which corresponds to rw = 2.11 Å."

A 4x factor seems to be missing from the equation: another article (https://en.wikipedia.org/wiki/Van_der_Waals_equation#Conventional_derivation) states that "The excluded volume b is not just equal to the volume occupied by the solid, finite-sized particles, but actually four times that volume" and goes on explaining why.

After dividing b by four, I find rw = 1.33 Å, which is closer to the Van der Waals radius found in other sources (1.40 Å), for example http://www.webelements.com/helium/atom_sizes.html

Can somebody a bit more experienced than me look into that? Cvereb (talk) 23:54, 13 December 2014 (UTC)[reply]

Template:Atomic radii

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Just granpa (talk) 10:15, 10 August 2017 (UTC)[reply]

Space

Pushpa R Waghmare (talk) 16:54, 14 November 2017 (UTC)[reply]