Chemical Forums
Chemistry Forums for Students => Undergraduate General Chemistry Forum => Topic started by: OzV on September 27, 2006, 04:06:46 AM
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I'm trying to get my head around how a compound like ammonium fluoride can exist when ammonia and hydrofluoric acid are combined - I think I have it but want to run it by someone to verify it.
As I understand it a coordinate covalnet bond exists when a lewis Acid, in this case the hydrofluoric acid, accepts a pair of electrons from a lewis base (the ammonia) so the hydrogen in the HF is protonated and can therefore attach to nitrogen's lone pair of electrons in ammonia forming NH4+ with a cooridnate covalent bond leaving the fluorine with 8 valence electrons instead of 7 (F-). Because it is an aqueous solution the NH4F forms NH4+ ions and F- ions (or is is F2-), and in the solid state it arranges into a crystal lattice with alternatively arranged +ve and -ve ions as per the classic NaCl model.
Now this is where my confusion arises... I can draw a lewis diagram for NaCl because the electrons can get shared out normally allowing both atoms to achieve noble gas configuration but I can't work out how to draw a lewis diagram for NH4F because there doesn't seem to be anywhere for the fluroine to attach - especially as it already seems to have achieved nobel gas configuration by itself???? Or is it purely a polar attraction? I have searched the net for help on this with very little luck and my old chem texts don't provide much assistance. I can draw the cooridinate covalently bonded hydrogen in the ammonium ion but that's where I get stuck. If anyone can explain this I'd be very grateful - I've been trying to work it out all day :-\
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You usually just draw an arrow for dative bonding from the negative species to the positive species.
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Yep - get that - but what about between the ammonium ion and the fluorine ion? Or is that what you meant?
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Yep - get that - but what about between the ammonium ion and the fluorine ion?
This is an ionic bond
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You usually just draw an arrow for dative bonding from the negative species to the positive species.
Better is to say - from electron pair to the positive species
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I can draw the dative bond in the ammonium. I'm having trouble seeing HOW the fluorine ion bonds with the ammonium ion ionically.
If I had to draw a lewis diagram of NaCl, I could do it easily because the single valence electron of sodium bonds ionically with the 7 valence electrons of chlorine and both species are happy.
(sorry for my crude scans)
(https://www.chemicalforums.com/proxy.php?request=http%3A%2F%2Fwww.school.edu.nf%2Fmiscpics%2FNaCl.gif&hash=86e8ef051a0013486ee1e097f41ede77e2232620)
But the ammonium ion looks like this:
(https://www.chemicalforums.com/proxy.php?request=http%3A%2F%2Fwww.school.edu.nf%2Fmiscpics%2Fammonium.gif&hash=d5ff3f7f4a9e46719220780726a50dbe0cea20ad) (I should have drawn the positive symbol in there - know it's meant to be there...)
There are no unbonded electrons for the fluorine to bond to and the fluorine ion retains the electron from the hydrogen when it forms a proton so it can't gain any more because it already has its octet of valence electrons ???
(https://www.chemicalforums.com/proxy.php?request=http%3A%2F%2Fwww.school.edu.nf%2Fmiscpics%2Ffluorine.gif&hash=b758a2904d0d4afabab38780a0f07738b08daa7e)
This is where I'm having trouble - there's probably something really simple that I am failing to see :(
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Well done. Note - arrow denotes two electrons. This causes positive charge at N atom.
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So how do you draw NH4F (by lewis diagram I mean)?
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To your drawing ad + charge at nitrogen and - charge at fluorine. That's all.
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Ahhhhhhh - I see my error - I was trying to draw the whole thing covalently, and in doing so trying to share the electrons evenly, when I should have been showing the ammonium covalently and the ammonium/fluorine ions ionically to demonstrate electrostatic attraction :-[
So the whole thing should look like this right:
(https://www.chemicalforums.com/proxy.php?request=http%3A%2F%2Fwww.school.edu.nf%2Fmiscpics%2Feqn.gif&hash=0c43843aeb605ad2958270e851ba0049043bb8f3)
All makes sense now. Thanks for the help.
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Ammonium salts are always ionic, nitrogen cannot form 5 bonds