[cliverlong]

Why are only H-F, H-O and H-N bonds said to exhibit hydrogen bonding?
Why is H-CL bond said only to be “polar”?

From: http://en.wikipedia.org/wiki/Electronegativity

Get following electronegativity values

H: 2.20
N: 3.04
O: 3.44
F: 3.98
Cl: 3.16

H-F has largest electroneg difference therefore seems OK to say molecule with this bond will exhibit hydrogen bonding.

However, H-CL gives 3.16 – 2.20 = 0.96
whereas H-N give 3.04-2.20 = 0.84 (a smaller difference)

So why is H-N bond said to exhibit hydrogen bonding whereas the larger difference on the H-Cl bond does not exhibit hydrogen bonding between molecules?

Clive

[Schrödinger]

Clearly, electronegativity is not the only factor. The only other thing I can think of is size. Chlorine is bigger than N and that probably would make the H-Cl---H-Cl bond weaker compared to N---H bond. 

H---X represents Hydogen bond

[cliverlong]

Clearly, electronegativity is not the only factor. The only other thing I can think of is size.

OK, seems a plausible suggestion

Chlorine is bigger than N and that probably would make the H-Cl---H-Cl bond weaker compared to N---H bond (H---X represents Hydogen bond). 

You are probably correct, but I don't see why size causes such a dramatic difference in the inter-molecular force when you compare Cl-H---Cl--H to ...N-H---N-.

Thanks
Clive

[savy2020]

What's a hydrogen bond? - It is just a electrostatic force between hetero-atom and hydrogen. It is just a kind of vanderwaal's force,. A dipole-dipole interaction, but what puts it apart is it's high bond energy compared to other intermolecular forces.
Both N-H bond and Cl-H bond are almost equally polar because the electro-negativity difference is almost the same, So it wont be a factor in explaining the weak intermolecular force.
How size affects.. Due to polar nature of the bond, partial charges δ+ on H and δ- on Cl/N develop. As Cl size is more, the charge density is less, the electrostatic force depends on charge density and not exclusively on charge. So that's why even though the two bonds are equally polar, the intermolecular forces are high in case of N-H than Cl-H.