Wikipedia:Peer review/Metallic bond/archive1

From Wikipedia, the free encyclopedia
Jump to: navigation, search

Metallic bond

FORMULASSSSS

This peer review discussion has been closed.
I've listed this article for peer review because the main contribuitor asked for feedback. Nergaal (talk) 10:33, 3 August 2008 (UTC)

My comments I am going to add a few more important comments and then add more as these get solved:

  • the first sentence of the intro isn't doing a very good job at introducing the reader to the topic of the article. change it, or move it at the end of the paragraph.
  • the 2D section does not talk at all about metallic bonds. I suggest putting in in a format that is better linked with the topic of the article.
  • there are very few references. Try to have at leat one per paragraph.
  • specific terms, such as karat should be wikilinked as in [[karat]].
  • the paragraph:

    Clearly, the electron deficiency is an important point in distinguishing metallic from more conventional covalent bonding. Thus, we should amend the expression given above into: Metallic bonding is an extremely delocalized communal form of electron deficient covalent bonding.

    is very unusual for a wiki article. It might be ok for a academic journal article, but think of wikipedia uses less personal language.
  • I don't like the sentence: "Of course, electron deficiency is a relative term: it means fewer than half of the electrons needed to complete the next noble gas configuration. E.g. lithium is electron deficient with respect to neon, but electron rich with respect to the previous noble gas, helium." It confuses the reader.
  • I would add another section about conducticity and superconductivity.
  • I would add more about strength, malleability, and ductility of metals at macroscopic scale.
  • What about having a comparrison b/w covalent and metallic bonds?

The rest of the article looks great! Thanks for all the great work! Nergaal (talk) 10:40, 3 August 2008 (UTC)

Comments from Axiosaurus
A lot of content has been added and a lot of work put in. To date there are a lot of wiki articles around this important subject area (e.g. Nearly free electron model, Electronic band structure, Tight binding, Particle in a one-dimensional lattice (periodic potential), Bloch wave, Fermi surface) and these tend to be mathematical, difficult to follow and are not very helpful for the general reader- so this revision is a good attempt to make the topic accessible. My problem is that it makes no mention of the conventional physicist/metallurgist theories which are different from the MO approach. Anyway that said - some specifics:-

  • The opening statement "A (single) metallic bond does not exist, but metallic bonding does. " Well I know what you mean but without specifying what is different about metals and some discussion the statement can simply be challenged as for example metal-metal bonds are well known e.g. chromium(II) acetate
  • the opening line "the electrostatic attraction between delocalized electrons, called conduction electrons, and the metallic nuclei within metals." - this classical description of metals and not very helpful IMO and doesn't sit very well with the non-classical delocalised description given later.
  • "Electron deficient metals- if carbon has precisely the right number of electrons why is Pb a metal? Also the phrase electron deficient always seems to exercise some folk and while it is arguable for group 1 and group 2 metals its a bit harder to argue for the group 9,10,11 elements in particular. The original phrase used was valence electron count (VEC), another tricky one IMO - how do you decide how many in some transition metals? - but it is less likely to offend the electron deficient allergic reader.
  • The phrase "extremely delocalized communal form of covalent bonding" - is potentially confusing- covalent bonding is often taken to mean localised electron pair bonds.
  • there are some properties of metals that are better related to lattice structures e.g. ductility differences between hcp and ccp metals
  • ther is no mention of the differences beween metals and how these can be explained even at the simplest level- e.g why are group 6 metals so high melting.
  • metal properties : e.g. temperature dependant electrical conductivity and Pauli paramagnetism (an early triumph for theoreticians in the 1920's)should be mentioned
  • the diamond argument re electrical conductivity- there are a number of schools of thought- e.g. some say that individual electrons are delocalised but as one moves another has to move in to take its place - therefore no current flow- and the other view is that what is inherently unobservable should not be considered. The issue with diamond is why it is different from silicon. Carbon whilst beloved of organic chemists is the odd man out in the periodic table, the metallic elements are in the majority. Personally I wouldn't give C much of a mention- it makes it sound like the standard element rather than the exception.
  • a section comparing metallic bonding, covalent(1 centre 2 electron) and ionic would be useful. In this way delocalised bonding (aromatic- and even 3 centre/2 electron) could be viewed as being between extremes of covalent (localised) and metallic (infinite 3D delocalised).--Axiosaurus (talk) 16:34, 3 August 2008 (UTC)

reply by Jcwf[edit]

Whoa, that's perhaps more comments than I bargained for, but let me try and answer. But before I do: let me thank both Nergaal and you for your prompt answer. First of all I am an Inorganic solid state / Physical chemist, rather disgruntled at how poorly this whole topic is taught, particularly in gen chem texts. My reason for writing the story is mostly that I am to teach Chem 101 this fall and got frustrated with all the superficial and often downright erroneous stuff you see in textbooks. Hopefully this article will help some clean up of the topic, but as it is admittedly somewhat creative and original if only in its juxtaposition of things, peer review seems essential: thank you for taking the trouble once again. I hope you also understand that references of a general nature will be hard to find. For individual tidbits that's a different story, I'll do my best.
  • The opening statement "A (single) metallic bond does not exist, but metallic bonding does. " Well I know what you mean but without specifying what is different about metals and some discussion the statement can simply be challenged as for example metal-metal bonds are well known e.g. chromium(II) acetate
    My point would be that metal-metal individual bonds are something different from metallic bonding. The former can often be described as single, double or triple individual covalent bonds, the latter is inherently collective in nature. No balls or sticks please! The latter is very difficult to tell people who identify balls and sticks with chemistry (like most chemists do, I'm afraid).
Try something like: metallic bonding is a form of chemical bonding that is present in a metallic crystal. It should not be confused with metal-metal bonds, which are an example of covalent bonding. Aso....
  • the opening line "the electrostatic attraction between delocalized electrons, called conduction electrons, and the metallic nuclei within metals." - this classical description of metals and not very helpful IMO and doesn't sit very well with the non-classical delocalised description given later.
    Even the quantum mechanical view is still based on the electromagnetic attraction between electrons and nuclei, although admittedly there are different terms like Coulomb and exchange etc, but the fundamental force is still the electromagnetic attraction and repulsion. This is why I left the sentence in, but I did hesitate I admit.
  • "Electron deficient metals- if carbon has precisely the right number of electrons why is Pb a metal? Also the phrase electron deficient always seems to exercise some folk and while it is arguable for group 1 and group 2 metals its a bit harder to argue for the group 9,10,11 elements in particular. The original phrase used was valence electron count (VEC), another tricky one IMO - how do you decide how many in some transition metals? - but it is less likely to offend the electron deficient allergic reader.
    Ah! Good question. The problem already starts with carbon in the graphite form. Why is it not a semiconductor like hex-BN? I suppose one could say that the overlap of conduction and valence bands makes the element electron deficient (holes in valence band) and electron rich (electrons in conduction band) simultaneously: this is what a semimetal is. For the hexagonal structure this kind of zwitter-deficiency already happens at carbon, for the cubic diamond structure you need to descend in the column of the periodic table a bit more: germanium is still semiconductive, tin becomes a semimetal: the bands become broader than the bandgap and it goes zwitter-metallic. (Translation: semimetallic).
  • The phrase "extremely delocalized communal form of covalent bonding" - is potentially confusing- covalent bonding is often taken to mean localised electron pair bonds.
    If all you have is two balls and a stick, everything looks like an organic molecule, I know! It is exactly why metallic bonding is such a stepchild of chemistry.... But even organic chemists know about benzene, don't they? In fact it is exactly for that reason that I insist upon saying a few things about graphite and diamond to stick a stick between those ..., eh, legs. Covalency is often erroneously taken to mean localized electron pair bonds. (If you insist upon me speaking such heresy, well there you have it. Set the stake ablaze if you must.)
  • there are some properties of metals that are better related to lattice structures e.g. ductility differences between hcp and ccp metals
    Certainly, but that is not really the topic of the article: I am trying to talk about metallic bonding in general, the specific structures it manifests itself in are secundary. I do not agree with Nergaal that the story should talk more about ductility and malleability, because I think they deserve their own story. Maybe I am prejudiced, because I am a (solid state) chemist, not a metallurgist. In any case, if it does need more detailed discussion that is probably better done by someone with a background in metallurgy
  • ther is no mention of the differences beween metals and how these can be explained even at the simplest level- e.g why are group 6 metals so high melting.
    No, because I much prefer to discuss the actual breaking of the bonding. That is at the boiling point, not the melting point! Imho there is a very unfortunate confusion on that point, which is why I try to be loudly silent anout melting points. For the latter I repeat: I am not as much interested in the precise structures and the melting point refers to the choice between one structure and the other. Yes it is true that I did not try to explain why tungsten is so sturdy (even in bp), mostly because I did not want to get lost in the specifics of how many d-electrons there are.
  • metal properties : e.g. temperature dependant electrical conductivity and Pauli paramagnetism (an early triumph for theoreticians in the 1920's)should be mentioned
    Why? The article is not about metals and their properties, but about bonding. Yes of course it is an important topic, but I would rather see it on a page about metals and their properties
  • the diamond argument re electrical conductivity- there are a number of schools of thought- e.g. some say that individual electrons are delocalised but as one moves another has to move in to take its place - therefore no current flow- and the other view is that what is inherently unobservable should not be considered. The issue with diamond is why it is different from silicon. Carbon whilst beloved of organic chemists is the odd man out in the periodic table, the metallic elements are in the majority. Personally I wouldn't give C much of a mention- it makes it sound like the standard element rather than the exception.
    I do not intend to make it standard, but I do want to dispell the erroneous idea that it is somehow an exception. The issue with diamond is 'not: why it is different from silicon. The issue is that it is not different, apart from the size of its band gap. And no that does not localize everything into an "insulator". (The latter does not depend on the width of the band gap, but on the width of the band in relation to any correlation potentials like Hubbards U and that is a different issue.) Yes, that puts all these (organic) chemists with their balls and their sticks on the wrong footing. But I think it is very healthy to challenge this ball-and-stick view of things, if only because as soon as your stick is broken, like in an organic molecule with a radical unpaired electron, that unpaired electron is not a localized phenomenon either. In doped diamond the mobility is pretty high, which means that the idea that 'localized electrons have to make place' is refuted by the facts: correlation between the electrons (and phonons), (i.e. Hubbards U) has to be small compared to the band width (delocalization). Yes, there are many people that cannot fit that fact into their balls and sticks
  • a section comparing metallic bonding, covalent(1 centre 2 electron) and ionic would be useful. In this way delocalised bonding (aromatic- and even 3 centre/2 electron) could be viewed as being between extremes of covalent (localised) and metallic (infinite 3D delocalised)
    I agree, but shouldn't that be part of a lemma on chemical bonding in general?

hmmm[edit]

This article is developing more and more into an essay, and looks less and less like an article. The difference b/w these two is that the latter should have a neutral point of view. I suggest creating txet based on referrences and listing those references with <ref>....</ref>. Nergaal (talk) 23:52, 5 August 2008 (UTC)

Comments from headbomb[edit]

I gave a (very) quick glance at this and I think there should some form of mention of the Drude and Sommerfeld theory of metals, as for history, and of band theory for current understanding. I may or may not review this article in greater details later. Headbomb {ταλκWP Physics: PotW} 19:57, 10 August 2008 (UTC)