Why does NH3 turn into NH4+? Also question about bond EN.

[Hammad Shahid]

Hi guys.
So I know that atoms bond to obtain stability / full octet.
But my question is that NH3 is already stable and each N and H has a full octet. So why does ammonium form? I know that an H+ is attached so no e- are added, but why is there a need for this change?
Same question for PH3 -> PH4+, and other such cations.

Another question I have is that where do we draw a line between an ionic or covalent bond?
Past what electronegativity difference is a bond considered ionic and why?

Thanks you. I only have basic level understanding in chemistry so sorry if these questions are too simple.

 

[rootone]

Consider ammonia dissolved in water, it exists and is a cleaning product available in your local supermarket or similar.
Technically it is ammonium hydroxide in that state, but still it is NH3

 

[Hammad Shahid]

Consider ammonia dissolved in water, it exists and is a product available in your local supermarket or similar.
Technically it is ammonium hydroxide in that state, but still it is NH3

Hm ok so NH3 (aq) is NH4OH? Okay, so then NH4+ can’t exist as free ions though right? It will be NH3 and H+ in water technically?

 

[Hammad Shahid]

What I mean is that ammonium can form an ionic compound with other elements as long as it’s in water and then the solute precipitates, is that correct? Otherwise in water it’s as NH3 and H+?

 

[Cutter Ketch]

N with 5 valence electrons bonds with three H to complete the shell. However NH3 has a geometry problem. 3 of the 5 electrons are in covalent bonds with Hydrogen and two aren’t, there is no symmetric way to arrange these. The electrons in the bonds and the remaining two valence electrons all repel as much as possible. As a result the three bonds bend out of plane making something like a three legged stool and the other two electrons hang out more on top of the stool. It is this broken symmetry that allows a fourth bond. Despite the canonical 8 electrons the screening still can’t be symmetric. There is too much electron probability on top and that attracts another proton.

Now with 4 H bonds the molecule can arrange in a perfectly symmetric tetrahedron. There is no concentration of electron probability anywhere,

 

[Hammad Shahid]

@Cutter Ketch Oh ok that makes a lot of sense. The proton stabilizes the polarity then. Also, does that mean that the trigonal pyramidal shape is less stable than tetrahedral in general?

 

[Cutter Ketch]

@Cutter Ketch Oh ok that makes a lot of sense. The proton stabilizes the polarity then. Also, does that mean that the trigonal pyramidal shape is less stable than tetrahedral in general?

Yes.

 

[Hammad Shahid]

Yes.

Thank you.