Talk:Valence bond theory

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History[edit]

Sadi, please join in a discussion here before you add anymore history. In your edit comment you say "the theory of valence and bonding goes a lot farther back than Lewis". That is true, but this article is not about "valence" or "bonding" in general. These are dealt with in Valence (chemistry) and chemical bond. Valence bond theory does not even go back to Lewis. It goes back to the paper by Heitler and London in 1927. It is a particular way of solving the Schrodinger equation in an approximate way for molecules. There and more significantly under Pauling a method was developed that maps Lewis structures to approximate wave functions. It has nothing to do directly with geometric constraints (see VSEPR for that) or the octet. I think all your recent edits should be removed along with the section on Lewis and the article should start with a simple introductory paragraph and then talk about Heitler and London. --Bduke 00:19, 21 December 2006 (UTC)

Let's go through this piece by piece, I added parts on valency, bonding, molecular orbital ideas, etc. to many different articles lately. So I'm not quite sure what additions you have objections to? To start with, the following is sourced word-for-word (it's not my statement):
In 1789, William Higgins published views on what he called combinations of "ultimate" particles, which foreshadowed the concept of valency bonds.[1]
So are you saying this statement should be removed from this article? Thus, I know there is an extreme amount of overlap between wave mechanics, valency, valence bond theory, affinity, molecular orbitals, molecular orbital theory, electron pair bonds, etc., which has been developed over the last 150 years of so. Comments? --Sadi Carnot 00:35, 21 December 2006 (UTC)
Also, do you have a link to the copy of the 1927 paper by Heitler and London (so I can read it)? --Sadi Carnot 00:38, 21 December 2006 (UTC)

Sadi, this is quite difficult and quite complex. I am up to my eyes in preparing our house for an influx of family from the other side of the world and I can not spend a long time on it. Specifically, I am objecting to your edits on this article, valence bond theory. Valence bond theory is quite specific and it is not "anything about valence bonds". It is not even "the theory of valence bonds". That could include molecular orbital theory as well as valence bond theory. So the ideas of Higgins should be somewhere else - probably valence (chemistry). Much of the overlap you mention is due to imprecise thinking and we need to work a lot of the overlap out of these articles.

The best description I know of for "valence bond theory" is "The VB theory is a direct translation of the Lewis theory into the language of quantum mechanics" from the earlier book (than the one referenced in the article) by Murrell, Kettle and Tedder called "Valence theory". Note the title, and note that they discuss molecular orbital theory, ligand field theory and so on, as well as valence bond theory. It starts with Heitler and London on H2, but then Pauling in particular showed how we could use approximate wave functions by a direct mapping from Lewis structures to VB structures which are actually many electron functions. Resonance arises when a single Lewis structure is not sufficient as in benzene or when one wants to add ionic structures and so writes down ionic Lewis structures. Hybridisation arises to get a single atomic orbital at the end of each bond to overlap each other and appear in the wave function just like the two 1s orbitals in H2. Thus under Pauling a whole series of quantitative ideas came to have a life of their own and spread in chemistry, but they come from valence bond theory. The high point was the publication in 1939 of Pauling's "Nature of the Chemical Bond". After the war, molecular orbital theory grew in strength because it was easier to program to get quantitative results and the qualitative ideas grew along side. Recently VB theory has become more computationally easier so its popularity is growing again.

The Heitler-London paper is in German and I have only read a translation. However it is discussed all over the place in chemistry texts and is often the main thing discussed in physics texts under "molecules". Let me know if you can not find a good discussion or if you want me to comment whether a source is a good discussion. --Bduke 01:25, 21 December 2006 (UTC)

Yes, no rush! I'll probably be back working on this topic next Mon-Wed and then the Mon-Wed after that. As to the above comments, feel free to paste whatever you don't agree with into the link mentioned below; As to the origin of modern theory of valence, according to Pauling, it is Gilbert N. Lewis's 1916 article The Atom and the Molecule. I've read this article as well as about four or five other related articles and I have Pauling's textbook On the Nature of the Chemical Bond, which I am reading. --Sadi Carnot 01:53, 21 December 2006 (UTC)
Of course Lewis is the origin of the modern theory of valence, but that, as I keep pointing out, is not the same as valence bond theory. As you indicate below, you do not understand the difference between MOT and VBT. They are different. There was quite a war between them in the 1930s, until people realised that both are approximations and that both can be extended (MO by adding excited configurations and VB by adding ionic terms) and then they become equivalent. Note however that those extensions can only be done for fairly small systems. What edition of Pauling's book do you have? The 1939 edition is much better than the later 1959? --Bduke 02:31, 21 December 2006 (UTC)
I have the 1960 3rd edition (the other editions are too expensive for me). I am researching both of the theories these coming weeks. What modern book would you recomend in regards to both VBT and MOT? --Sadi Carnot 02:47, 21 December 2006 (UTC)
Pity about the edition of Pauling. He seems to willfully ignore molecular orbital theory even when its explanations are so much simpler as for boron hydrides. The 1st edition is a classic. The 1960 edition is an embarrassment. I did have a copy but sold it long ago. I recently got a copy of the 2nd edition, 1945, which is still very much like the 1st edition. I like Levine's "Quantum Chemistry". Lowe's book of the same name is good. Coulson's Valency is good even though it is old. --Bduke 03:05, 21 December 2006 (UTC)

Chemical bond article[edit]

Also, something about the presentation in the chemical bond article, in relation to valence bond theory and molecular orbital theory, seems out of place or out of historical order. I'm still researching and reading into this so it may take some time. I'm also thinking about starting a time-line table in the molecular orbital theory page (which is currently a redirect). I already have the table, in my computer file, up to about 17 key people. Any comments?

Yes, the chemical bond page is a bit of a mess. I have been trying to improve it, but others have ideas. It also gets a lot of vandalism. Let me look at your table. You could e-mail it to me if you like. I have e-mail enabled from my user page. MO theory should talk about simple quantitative calculations and then point to Hartree-Fock theory. We also have MO diagram and much else. --Bduke 01:25, 21 December 2006 (UTC)
O.K. I'll send you the table, but don't make fun of it, I am still a little blurry as to the difference between VBT and MOT; to me they seem about the same? I'll continue to be reading about these theories, with focus on modern molecular orbital theory, as to how it came into inception, over the next few weeks (this I will likely fiddling with related article in this area). If you want to remove any of my contribs from whatever related article, just paste them here: Talk:Valence bond theory/notes so that we can discuss the issue. Seeing that you made a VBT computer program, you are probably more of an expert on the topic than I am. --Sadi Carnot 01:41, 21 December 2006 (UTC)
I do know about VB theory and have for 40 years, but whether I can write simply about it is another matter. I'll not make fun of your table and will discuss it only by e-mail. I'm probably going to leave any major edit in this area until after the time my kids are here. --Bduke 02:31, 21 December 2006 (UTC)
I couldn't figure out how to attach the file in Wiki-mail, so I typed it up here: History of molecular orbital theory. Use the talk page there if you want to help me (or correct me) build the table; I'm guessing that we can do one table for both valence and orbital theories? --Sadi Carnot 02:40, 21 December 2006 (UTC)

Other comments[edit]

Another note, is the Heitler-London theory is not in WP, from what I know? The main page, for example, lists this: Heitler-London theory (1927), which just goes to bios? Another point I have, is that valence bond theory did not just "pop" into existence in 1927; it is built on the shoulders of hundreds of others. --Sadi Carnot 00:56, 21 December 2006 (UTC)

Heitler-London theory should not have its own article but be central here along with the various improvements that followed. Very little just pops into existance but that does not mean that the shoulders of others have to be discussed everywhere. VB theory starts with Heitler and London. It builds on Lewis and other idea of valence on the one hand and on Schrodinger and his equation and the solutions for the H atom, plus Pauli and so on, on the other hand. We can reference back all these "shoulders". Any text starts the discussion of VB theory with Heitler and London. I must get on to other things. --Bduke 01:25, 21 December 2006 (UTC)
I'll have to find and read the Heitler-London paper before I can comment on this. --Sadi Carnot 01:55, 21 December 2006 (UTC)
Please, read accounts in modern texts and you will then understand the modern terminology. The original paper is not that readable I think. Try pg 114 on in "Coulson's Valence", 3rd Ed, by Roy McWeeny, or section 13.10 of "Quantum Chemistry" by Ira Levine, 4th Ed. Also, it very important to understand several papers that extend it by varying the orbital exponent, adding ionic terms, etc. Historically the HL paper was very important but we have to put it into the larger picture now. --Bduke 02:31, 21 December 2006 (UTC)
Agreed. I did yesterday order a copy of Coulson's Valence (1961 ed.). Presently, however, I am digging my way around the historical roots of both theories before digging into the modern versions. Running out of time for the day (I'll talk more next week). Adios: --Sadi Carnot 02:51, 21 December 2006 (UTC)

Question[edit]

According to VBT, when the two atoms A and B approach each other, then there are two types of attractive forces that develop between them( the repulsive forces also develop, but Iam not discussing about it here): 1. Between nucleus of Atom A and electron of atom A. 2. Between Nucleus of Atom A and electrons of atom B and vice versa. Reference : NCERT class XI (INDIA)

Can anyone explain how the first case is possible? Nshul2007 (talk) 23:06, 7 January 2009 (UTC)

What is the problem. Electrons of atoms A are attracted to the nucleus of atom A at all distances. Perhaps "develop" is the wrong word, but both types of force alter as A and B approach each other. --Bduke (Discussion) 00:14, 8 January 2009 (UTC)

Drawbacks of Valence Bond Theory[edit]

Please consider adding a section on the drawbacks of Valence Bond Theory-

(i) The origin of their absorption spectra could not be explained.

(ii) Different complexes of the same metal show different colours.

(iii) Relative stabilities of different complexes could not be explained.

Ramakrishnan.nikhil (talk) 05:55, 8 September 2013 (UTC)

What is your source that says that these are drawbacks? --Bduke (Discussion) 07:37, 8 September 2013 (UTC)
    • ^ Partington, J.R. (1989). A Short History of Chemistry. Dover Publications, Inc. ISBN 0-486-65977-1.