[Frank Ose]

Dear teachers, I would be delighted if you can help me with my doubts.

Chlorine atom and nitrogen atom have the same electronegativity which is 3.0.
As a result , both are supposed to be able to form hydrogen bond with the electronegative/electropositive atom of other molecules.

However, only NH3 can form hydrogen bond and HCl cannot form it although they share the same electronegativity.

Anyone mind explaining this to me?

[Astrokel]

hey Frank Ose,

im not a teacher. you have to consider the effect of the relative size of the atom. A smaller atom would have higher electron density because the electrons occupied a smaller volume. Therefore what consequences does it have on the requirement of hydrogen bonding?

[AWK]

In most cases HCl does not exist in solution as a molecule. But a weak hydrogen bonds can be found in crystal between covalently bonded chlorine and donors of hydrogen.
But for example, HF forms extremely strong hydrogen bonds even in water solution.

[Frank Ose]

hey Frank Ose,

im not a teacher. you have to consider the effect of the relative size of the atom. A smaller atom would have higher electron density because the electrons occupied a smaller volume. Therefore what consequences does it have on the requirement of hydrogen bonding?

I know the size of the electronegative atom has to be considered.But I do not know why it has to be considered and I can't think of the consequences. Any logical explanation on how the size of electronegative atom affects the formation of hydrogen bond?

This is my understanding at the moment, correct me if this is erroneous.
A smaller atom would have higher electron density, thus attracting the barely shielded atom more strongly.In this case, N atom in NH3 is smaller and so the barely shielded H atom from other NH3 molecule can approach it closer and form hydrogen bond with it.And for HCl,electronegative Cl is bigger in size, hence the electron density is scattered and thus less strong than that in NH3. Therefore, the barely shielded H atom in HCl could hardly approach the electronegative Cl of other similar molecules because the attraction is not strong to hold them together.

Thanks in advance.

[Frank Ose]

In most cases HCl does not exist in solution as a molecule. But a weak hydrogen bonds can be found in crystal between covalently bonded chlorine and donors of hydrogen.
But for example, HF forms extremely strong hydrogen bonds even in water solution.

Do you mean when HCl is poured into solution, the partial positively charged hydrogen atom forms hydrogen bond with the chlorine atom? But why some websites assert that hydrogen bond can only be formed when H is attached to either F , O or N?

Thanks in advance.

[AWK]

[
Do you mean when HCl is poured into solution, the partial positively charged hydrogen atom forms hydrogen bond with the chlorine atom? But why some websites assert that hydrogen bond can only be formed when H is attached to either F , O or N?

Thanks in advance.

HCl in water solution does not exist as molecule because of practically complete protolysis (dissociation).

[Frank Ose]

[
Do you mean when HCl is poured into solution, the partial positively charged hydrogen atom forms hydrogen bond with the chlorine atom? But why some websites assert that hydrogen bond can only be formed when H is attached to either F , O or N?

Thanks in advance.

HCl in water solution does not exist as molecule because of practically complete protolysis (dissociation).

I misunderstood your previous post.
Thanks, I have fully comprehended now.

[Astrokel]

hey, you are quite right on the idea.

Nitrogen atom has a smaller volume relative to chlorine, therefore its electron is concentrated in a smaller volume thus has a greater 'attraction' towards the electron of hydrogen in the covalent bond, making the hydrogen atom even bare or 'naked' relative to the case in chlorine. Therefore the 'bare' hydrogen in NH₃ able to form a greater hydrogen bonding. Furthermore the small concentrated N atom able to attract the hydrogen atom in the neighbouring molecule of NH₃ than that of Cl in HCl.

so does the point mentioned by AWK

[Frank Ose]

I would be delighted if you can help me with my doubts.

What are your doubts? 

[Frank Ose]

hey, you are quite right on the idea.

Nitrogen atom has a smaller volume relative to chlorine, therefore its electron is concentrated in a smaller volume thus has a greater 'attraction' towards the electron of hydrogen in the covalent bond, making the hydrogen atom even bare or 'naked' relative to the case in chlorine. Therefore the 'bare' hydrogen in NH₃ able to form a greater hydrogen bonding. Furthermore the small concentrated N atom able to attract the hydrogen atom in the neighbouring molecule of NH₃ than that of Cl in HCl.

so does the point mentioned by AWK

Indeed, thanks for all your opinions. 

[Astrokel]

hey,

i have found this link http://www.sciencebyjones.com/extra_bonding_notes.htm, in the 9th paragraph it talks about your question, good luck!

[Frank Ose]

hey,

i have found this link http://www.sciencebyjones.com/extra_bonding_notes.htm, in the 9th paragraph it talks about your question, good luck!

Thanks for the link, it pretty much summarises what we have been saying.