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Thread: Valence Bond theory? Molecular Orbital theory?

  1. #1 Valence Bond theory? Molecular Orbital theory? 
    Forum Sophomore oceanwave's Avatar
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    First up, I truly am sorry for flooding the chemistry forum with my replies and all. Next, my question. Yes, we've studied that atoms form nice bonds and all to be stable and how that slowly leads to macro-structures and all. But have anyone ever thought of this:

    Take a piece of diamond (or anything at all) and magnify the very tip of the tip of diamond. What would we see? A LONE carbon atom at the very peak without 3 other carbon atoms to stabilise it (its only stuck to the carbon atom directly below it). Now, please tell me how can this be? (I have posed this question to chemistry geniuses *read: As in chem and study in harvard, etc) and they cant seem to give any answer at all). Perhaps, something I got off wiki might help: Molecular orbital theory. At this point, please do not confuse this theory with that of the hybrid linked orbital theory where electrons are held in orbitals and all. This theory concerns macro-structures and implies (at least that's what I surmise from the article) that all electrons in bonds are delocalised throughout the molecule/compound/structure. Thus, (this is what I thought of) they would then result in a certain 'resonant energy' state which allows the entire structure to share in the many many combined stability which then "spread" this "resonant energy" to the carbon atom at the tip of the tip of a diamond to be stable and thus, not react.

    Part (b) of question. If the above theory (mine and wiki's combined) is true. Could there be a certain amount of energy locked into every macro-structure which ensures their stability? Using chemical energetics, we learn that all the energy is accounted for. But then, usually, if we aren't looking for a particular thing, we won't really find it right? So why don't we test the (mine-wiki) theory by getting 2 pieces of diamonds. One will be exactly 10 times the mass of the other. Check the energy inside both (either by combusting or some other method) and see if there is any unaccounted for energy? It will most probably be fleeting since it is shared by the whole structure and will probably withdraw its influence once the external part of the structure undergoes alot of stress which then makes it more logical for the structure to pull in this "resonant energy" to enforce the bonds in the inner structure thus increasing the strength of the remaining structure and decreasing the speed at which it reacts until finally, the remaining amount of bonds is too little to support this "resonant energy" and the energy cloud "collapses". Some points to include could be the baseline of bond-energy in which such a "resonant energy" can active itself over the whole structure and also the amount of stability given to any "unstable" atom situated at the surface of the structure which then depends on a proposed mathematical theory which links the surface area (hence the number of unstable atoms) with the excess bond energy which can be used to enhanced stability. *proposed formula could be: total excess (unused bond stability energy) energy divided by the number of unstable molecules and the energy required to give them stability (hence surface area in a way) taking into account the fact that although electrons are supposed to be held in fixed orbitals, many molecules (like benzene) have delocalised electrons which allows the electrons to "fly around freely" and allow the whole molecule to be relatively stable then what is expected to be.

    Thanks for taking the time to read through this very long post and please include all comments for they are warmly welcomed.


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  3. #2 Re: Valence Bond theory? Molecular Orbital theory? 
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    Quote Originally Posted by oceanwave
    Take a piece of diamond (or anything at all) and magnify the very tip of the tip of diamond. What would we see? A LONE carbon atom at the very peak without 3 other carbon atoms to stabilise it (its only stuck to the carbon atom directly below it). Now, please tell me how can this be?
    The outer surface of the diamond (including the tip) will usually be covered with carbon-hydrogen and carbon-oxygen bonds. The carbon at the tip will probably be a CH3 or CHO, or something along those lines.
    Part (b) of question. If the above theory (mine and wiki's combined) is true. Could there be a certain amount of energy locked into every macro-structure which ensures their stability? Using chemical energetics, we learn that all the energy is accounted for.
    Delocalization of charge around an extended structure does affect the energy of the structure. It's pretty well understood. And yes, often this means that simply adding up what you expect individual bond energies to be won't give you exactly the correct answer for the total bond energy of the material.


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    erm...other than diamond (which was used just for simplicity's sake), do all other macro-structures have the same theory? meaning that the outermost atom is bonded differently than what is given in the chemical formula?
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  5. #4  
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    one more thing: if the carbon atom at the tip of diamond really was bonded to H or O, then the attributed properties of diamond (namely its unreactivity and hardness) would not be valid since C=O and C-H bonds are not as strong/unreactive as C-C bonds which is why methane/carboxyl compounds react right?
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  6. #5  
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    Quote Originally Posted by oceanwave
    erm...other than diamond (which was used just for simplicity's sake), do all other macro-structures have the same theory? meaning that the outermost atom is bonded differently than what is given in the chemical formula?
    Generally yes. Although it is possible to make surfaces with where some of the outermost layer of atoms is actually "missing" bonds and isn't bonded to anything where the surface ends. These sorts of surfaces are usually very reactive, since the atoms along the surface want to be bonded to things.
    one more thing: if the carbon atom at the tip of diamond really was bonded to H or O, then the attributed properties of diamond (namely its unreactivity and hardness) would not be valid since C=O and C-H bonds are not as strong/unreactive as C-C bonds which is why methane/carboxyl compounds react right?
    Often the outer surface of a material is more reactive than the bulk of the material. As for properties like hardness etc, those are bulk properties. When you cut something with a diamond-tipped saw you generally are destroying the outer few layers of atoms in the material. But it's not something that you would ever actually notice.
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    Generally yes. Although it is possible to make surfaces with where some of the outermost layer of atoms is actually "missing" bonds and isn't bonded to anything where the surface ends. These sorts of surfaces are usually very reactive, since the atoms along the surface want to be bonded to things.
    sorry for the repeated questions which seem to keep bouncing (i still don't quite buy the point ) Somehow, it still sounds a bit fishy (no offense to u) as wouldn't most external surfaces have atoms which are not bonded to anything? Meaning that, either all macrostructures in the universe do not have surfaces with the intended chemical compound (but that of the external atom bonded to an O or something else; which to me would render the whole chemical theory a bit pointless) or if they really don't, then by the theory stated above, these surfaces would be very reactive even when considering relatively unreactive substances and hence, as the other surface reacts, the (formerly) inner layer of atoms would then be the newly formed outerlayer which would then react which would (dear me) continue in a 'chain'/'continuous' reaction which would render any substance extremely reactive. I would rather explain it with theory of why metals like gold and copper are unreactive (which is due to surface energy and the presence of anti-bonding pairs of electrons) which seems more logical.
    **I'm really not out to shoot down your theory, I'm just the kind who stretches a given theory to the extremes and see if it survives. If not, then i believe the theory is flawed in a way. <--- it's similar to Einstein's method of approaching problems.

    Often the outer surface of a material is more reactive than the bulk of the material. As for properties like hardness etc, those are bulk properties. When you cut something with a diamond-tipped saw you generally are destroying the outer few layers of atoms in the material. But it's not something that you would ever actually notice.
    Destroying? the atoms? or u mean the bonds? atoms aren't destroyed per se right? actually, I'm now kinda confused over how one stuff cuts another in atomical terms.
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    Okay, obviously I didn't explain very well. You usually won't find a surface where the atoms have "dangling bonds" that aren't attached to anything. Carbon, for example, wants to have 4 bonds - so if you have a carbon on a surface (like the surface of diamond) where it is only bonded to 3 things, it will still want to form one more bond. But hydrogen only forms one bond, so if you "cap" the last open bonding site on the carbon with a hydrogen, everything is happy.

    As for destroying surfaces, I meant you knock off the first few layers of atoms.
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  9. #8  
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    Quote Originally Posted by Scifor Refugee
    Okay, obviously I didn't explain very well. You usually won't find a surface where the atoms have "dangling bonds" that aren't attached to anything. Carbon, for example, wants to have 4 bonds - so if you have a carbon on a surface (like the surface of diamond) where it is only bonded to 3 things, it will still want to form one more bond. But hydrogen only forms one bond, so if you "cap" the last open bonding site on the carbon with a hydrogen, everything is happy.

    As for destroying surfaces, I meant you knock off the first few layers of atoms.
    oh boy, pardon me for saying so (but i really must clarify): if (for example) the carbon atom at the tip of (let's just use just purely for example) diamond is bonded to say (purely for example) H or O (to ensure that all the carbon atoms have stability), then wouldn't it (i know im repeating myself but something's still not right here) mean that diamond would be more reactive than what it is more well known for (being unreactive)? C=O and C-H bonds are definitely weaker than C-C bonds (which should be the case in the inner part of the diamond structure) and since the external 'part'/'surface' of diamond has 'weaker'/'lousier' bonds, it would by right cause diamond (or any other substance) to be more reactive than what it should be.

    Furthermore, if we look into other macrostructures like it would too face the same problem. Even graphite, etc. I find it unbelievable that such structures need to bond with an element not in its chemical formula just to 'achieve' stability on the surface of its structure as then, we can always throw in this argument for all big scaled structures and then, i think that chemists would have one hell of a big problem since firstly: chemical formulas do not accurately represent a structure's elemental composition. Secondly: mass spec readings would be erm....inaccurate?

    Even if we argue that we can omit the small (may 200 atoms of O or H) in a mass spec reading for (let's assume) diamond, then mass spec readings would also be rendered useless since mass spectrometry (or any other kind of device used to exact a compound/substance's make up) is very accurate (or believed to be so) in determining what elements make up a compound/substance. And if we allow these Os and Hs to be omitted in a final formula of (again, assume) diamond (carbon), we would then be very unsure of other millions of particles that scientists have been measuring and checking their formulas against because we would never know when to throw away readings for H and O (or any other element) and when not to all due to the fact that all structures must have atoms that must have bonds (which i do not think is valid).

    Let's look at the example of benzene. For one, benzene was thought of having alternating double and single bonds between the C atoms because back then, people had this same thinking that all things must have the necessary number of bonds for stability but guess what? The kekule structure of benzene is wrong. It is due to the delocalising of electrons above and below the ring. Thus, the point im trying to make in this thread is this: Could all macro/weird unanswerable structures which does not seem to have the adequate number of bonds to stabilise it have some form of electron delocalisation? and in the case of macrostructures, could it be that since the structure is extremely big, such 'delocalisation' takes on a different form, namely the 'resonance' energy form of which is detailed above.

    **Scifor, thanks for your replies to my post, will be contacting you via PMs (you seem to be the ONLY one replying....... )

    To the REST out there, if you get the gist of what im trying to propose and/or know a way of re-explaining my problem/theory to it, please help me to rephrase the problem.
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  10. #9  
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    Quote Originally Posted by oceanwave
    oh boy, pardon me for saying so (but i really must clarify): if (for example) the carbon atom at the tip of (let's just use just purely for example) diamond is bonded to say (purely for example) H or O (to ensure that all the carbon atoms have stability), then wouldn't it (i know im repeating myself but something's still not right here) mean that diamond would be more reactive than what it is more well known for (being unreactive)? C=O and C-H bonds are definitely weaker than C-C bonds (which should be the case in the inner part of the diamond structure) and since the external 'part'/'surface' of diamond has 'weaker'/'lousier' bonds, it would by right cause diamond (or any other substance) to be more reactive than what it should be.
    Yes, it is true that a C-H bond is weaker than a C-C bond, and it's true that this makes the outer surface of the diamond somewhat more reactive than the internal bulk - but it's still pretty unreactive. C-H bonds are weaker than C-C bonds, but they're still hard to break. Consider, for example, adamantane http://en.wikipedia.org/wiki/Adamantane ; it's the smalles possible "diamondoid" structure, and it's much more stable than any any similar hydrocarbons.

    Furthermore, if we look into other macrostructures like it would too face the same problem. Even graphite, etc. I find it unbelievable that such structures need to bond with an element not in its chemical formula just to 'achieve' stability on the surface of its structure as then, we can always throw in this argument for all big scaled structures and then, i think that chemists would have one hell of a big problem since firstly: chemical formulas do not accurately represent a structure's elemental composition.
    When chemists talk about the bulk chemical formula for a solid material it's understood that the outer surface might be different, especially if you're talking about "extended solids" where the entire structure is basically one big molecule (as opposed to something like a sugar crystal, which is a big collection of individual molecules).
    Secondly: mass spec readings would be erm....inaccurate?
    People use mass spec to analyze individual molecules. You couldn't use a mass spec to analyze a diamond, unless you had a tiny diamond nanocrystal that only had a few dozens or hundreds of carbons in it. And in that case the person doing the mass spec definitely would take the surface hydrogens into account when calculating the mass.

    I think you might be getting confused about the differences between an extended solid and a molecular solid. Again, look at the diamondoids: http://en.wikipedia.org/wiki/Diamondoid Notice that they all have hydrogens along the outer surface, and that these hydrogens are taken into account when calculating the mass.

    # Adamantane (C10H16)
    # Iceane (C12H18)
    # BC-8 (C14H20)
    # Diamantane (C14H20)
    # Triamantane (C18H24)
    # Isotetramantane (C22H28)
    # Cyclohexamantane (C26H30)

    ...and so on. Eventually people stop bothering to name them and just start calling them "diamonds" or "diamond nanocrystals."
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  11. #10  
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    I see I see....hmm...thanks alot for the info...at any rate, can I then safely assume all other big solids are similarly theorised? And oh, mass spec is also used to determine the composition of substances in which their chemical formulas may be still unclear. So counting in the extra H/O/whatevers would really dent any form of experimentalists' attempts to prove that we can deduce a structure's composition mainly by such methods (which do not need to include mass spec per se)..
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  12. #11  
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    Quote Originally Posted by oceanwave
    can I then safely assume all other big solids are similarly theorised?
    Again, it depends on whether you're talking about an extended solid like diamond where the entire structure is basically one big molecules that's so large you can see it and put it in a ring, or a molecular solid like a sugar crystal. A single sugar molecule is complete all by itself. A big sugar crystal isn't a single big molecule, but rather a collection of many individual sugar crystals - so there would be no need to worry about how a sugar crystal's surface terminates. Eventually you would just have one last layer of complete molecules.
    And oh, mass spec is also used to determine the composition of substances in which their chemical formulas may be still unclear. So counting in the extra H/O/whatevers would really dent any form of experimentalists' attempts to prove that we can deduce a structure's composition mainly by such methods (which do not need to include mass spec per se)..
    Mass spec is used to analyze molecules, not extended solids. You could use it to figure out what sugar is, but not what a diamond is (unless you have a very small piece of diamond that was little more than a big molecule, in which case yes, the hydrogens would indeed make things complicated).
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    ok, just had a new thought:

    Since the external surface of diamond/any solid has 'weaker' bonds, wouldn't the hardness of the solid be affected? Say in the case of diamond, the external surface has C=O and C-H bonds. These bonds are weaker compared to C-C bonds so wouldn't diamond be unable to cut something like say, glass? I know u referred to the hardness of a solid which u said is linked to its bulk properties. But isn't it mostly based on the bonds on the outer surface since those are the parts in contact with the subject that we're cutting?

    And if the outersurface of diamond has C-H bonds, if we purposely use reagents and conditions specific to breaking C-H bonds, will we succeed in doing so? if so, then would the entire diamond structure start breaking down? To me, it seems rational that once we know the external surface of a structure and the bonds involved, and use specific methods to break them, the inner atoms would then become the new outersurface of the solid and thus be subjected to the reagents and hence, be broken down (even if it is a slow process).

    And speaking of sugar, how do the small sugar molecules connect to form the huge structure of sugar? are they intramolecular bonds or intermolecular bonds?

    And one last point, in the case of ionic crystals, let's take NaCl for example, the number of + ions must be in the same ratio as the number of - ions right? so then, considering the cubic structure in which it takes, does it mean that the breath, length and width of the structure must only have a specific number of atomic ions? Take for example, a rubic's cube. There is the 3 sq, 4 sq, 5 sq, etc types of cubes. Considering that the number of blocks in the 3 sq cube is 9 (an odd number), a hypothetical NaCl crystal cannot take on this 'length, breath and width' can it? it would take a 4sq, 6sq, etc (even sided) cube in my opinion.

    Next question related to the above would be: how are the structures for salts like Mg(OH)2? there is only 1 cation and 2 anions. so they cant take the structure similar to NaCl can it?
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    Since you seem to have many questions about this sort of thing, I would suggest finding a copy of "Solid State Chemistry" by Smart and Moore. http://www.amazon.co.uk/Solid-State-.../dp/0748775161

    It's relatively cheap, and will answer many of your questions.
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  15. #14  
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    ok, thanks.
    What do you do when the last day of your life is approaching...........?
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    The key concept to remember here is scale. The outer surface is only around 0.5 or so nanometers wide. This would mean the outer surface only occupys (0.5e-9 / 1e-2) =1e-7 of a 1 cm crystal crystals diameter or one 0.000005% of the diameter. The volume occupied by the outer layer is much less (0.00000000006%). So properties that depend on the bulk, like the measured chemical composition and mechanical properties, do not not depend on the outer layer. Hardness is a mechanical property because the inner part of the solid pushes back against any deformation of the surface.

    Molecules react with the outer layer so the surface does matter a great deal in this case. This is why there is such a great difference between regular steel (outer surface is corroded away by water in a self-catalyzing reaction) and stainless steel (outer surface protected from self-catalyzing reaction). This is also why there is such interest in nanoparticles, as the surface to volume ratio is much greater.
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    (i think this should be the last question for this post, *i think*) So supposedly if we have a diamond rod. And the carbon on the external surface is stable due to the bonds of C with H and O, etc. And the internal is mainly/all C-C bonds. IF we break the diamond rod, the point at which it breaks should immediately undergo a reaction with the O2 molecules in the air to stabilise the C atoms now suddenly being exposed.

    Thus the question: does this actually happen and do we/can we record an energy change/chemical reaction for this? Don't think anyone has ever saw a reaction for a broken piece of diamond (if any piece of diamond has actually been broken in the first place).

    **to Scifor: will probably borrow the book from a library if i can find it. No cash to buy new books. Hahahaha
    What do you do when the last day of your life is approaching...........?
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