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Question About Bonding


Fromage
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I do not fully agree with encipher. Metals with such high charge, usually are not forming fully ionic compounds anymore. For example, I certainly would call FeCl3 not purely ionic, in fact, it is quite covalent (e.g. it dissolves in acetone without problem). FeCl3 is a black fairly volatile solid, which dissolves in many organic solvents. The compound PbCl4 really is covalent. AlCl3 also is a covalent compound.

 

The higher the oxidation state, the more covalent the compounds.

 

Purely ionic compounds only exist of +1 ions (e.g. alkali metals), +2 ions (the heavier earth alkali metals, combined with very electronegative elements) and some more complicated systems with ions, which are consisting of multple atoms (such as ammonium, nitrate, sulfate, etc.).

 

Even in the latter class, many compounds are partly covalent. So, NiSO4 is an insoluble mostly covalent compound, while NiSO4.6H2O is an ionic compound, easily soluble in water.

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Woelen,

 

Pure ionic bonding does not exist. All bonds have some degree of covalent or metallic bonding. It is the same with a lot of covalent bonding. Due to differences in electronegativaty, so called 'covalent' bonds aren't COMPLETELY covalent.

 

But for the purpose of his question, I think that is an reasonable response.

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Encipher, it doesn't really matter. Consider the extreme lack of orbital overlap between Cs and F as well as Li and I. Who cares that there is a finite positive degree of overlap? The point is that CsF and LiI are extremely hygroscopic to the point of deliquesence.

 

As for "covalent" bonds not being "completely" covalent, yes, this is true with many compounds that inept high school chem teachers call "covalent." However, we only really notice the "polarization" of the C-H bond in methane when running NMR with molecules that only show the inductive effect. Yes, the protons are waay upfield. Yes, the carbon is more shielded than it would be if the alkyl chain were more substituted, but what difference does it make in the chemistry of the molecule? It is at all nucleophilic? No. Electrophilic? No.

 

Concerning the previous response you gave that such atoms as aluminum are commonly found in +3 ionic or "semi-ionic" or "very polar covalent" states, I must respectfully disagree. Trivalent (NOT ionic) aluminum is a very useful Lewis acid and such reactions as the Weinreb amidation would not proceed without this property. As well, the reduction of amides, esters, and carboxylates would not proceed if not for the fact that Al is not at all inclined to be in a cationic state. Moreover, it generally takes on a negative forma charge and in fact, its acidity can be observed by its reaction with water:

2AlCl3+3H2O-->6HCl+Al2O3

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budullewraagh,

 

I get your point, you and woelen are correct. The point to what I was saying, was basically if someone was asking a simple question of that sort, factors such as those you and woelen just mentioned are probably not what is being looked for. Although I see how, if that wasn't mentioned a person could have a wrong interpretation of this.

 

Thanks

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I agree with encipher here. There is a VERY valid reason WHY they simplify things in high school chemistry. If they started out by teaching advanced level chemistry to high schoolers or beginners in college, NOBODY would understand it. NOBODY!!!!! An analogy to this is cooking. You can't just throw someone in a kitchen and tell them to make a fancy creme-broule. They need to be taught how to cook, how to use the ingredients, how to use their tools.

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I only agree partially with jdurg. I have no objections against the use of simplifications, but one should also be told that simplifications are used.

Nowadays, pupils think that this is the whole story, simply because no one tells them there is more to say about this.

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I strongly agree with Woelen on this one. I remember when high school AP chem students used to try to call me out on bonding rules after I had taken organic I and II and read Pauling's Nature of the Chemical Bond. They believed in their arguments very strongly and I couldn't help but explain to them how wrong they were.

 

Personally I think that, for example, the concepts of orbital overlap applied to simple molecules could be a very useful supplement to resonance and the inductive effect that polarizes molecules.

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The problem is that if you go too much too soon you'll confuse the living hell out of a LOT of people and cause many to hate the subject forever. As it is right now, who cares if some random bloke who will NEVER have a career or interest in chemistry is told just the simplifications. Those who truly need to know the reality will learn it, but they'll learn it at a pace that will allow them to fully grasp the concepts. Trying to teach people all about every aspect of bonding right from the get go is like teaching someone calculus at the same time you are teaching basic algebra. I just don't agree with it at all. It is far more dangerous for someone to be taught the complexities of many chemical concepts than it is to simplify it for them and ensure that they grasp them and use them properly.

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Yes, I agree with you jdurg, but as I told before, the people must also be told that they are studying simplifications, without actually going into detail about the more complicated and intricate things. That is the thing which I miss in your story.

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