[OzV]

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 F^2-), 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 

[Mitch]

You usually just draw an arrow for dative bonding from the negative species to the positive species.

[OzV]

Yep - get that - but what about between the ammonium ion and the fluorine ion? Or is that what you meant?

[AWK]

Yep - get that - but what about between the ammonium ion and the fluorine ion?

This is an ionic bond

[AWK]

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

[OzV]

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)



But the ammonium ion looks like this:

(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 



This is where I'm having trouble - there's probably something really simple that I am failing to see 

[AWK]

Well done. Note - arrow denotes two electrons. This causes positive charge at N atom.

[OzV]

So how do you draw NH₄F (by lewis diagram I mean)?

[AWK]

To your drawing ad + charge at nitrogen and - charge at fluorine. That's all.

[OzV]

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:



All makes sense now. Thanks for the help.

[AWK]

Ammonium salts are always ionic, nitrogen cannot form 5 bonds