Hydrogen bond big problems in my mind

[kenny1999]

I have two big problems in my mind about hydrogen bond for many years.

I have checked many textbooks but all of them fails to explain what I don't understand.

1. To form a hydrogen bond (particular strong attraction) Why it is necessary for a hydrogen

atom to attach to a highly electronegative atom and form bond with the lone pair of electrons

of another electronegative atom??

2. For HF (Hydrogen Fluoride), it is said that it can form two hydrogen bond for each molecule, while for H2O (water), it is said that it can form four hydrogen bond for each molecule. Why?? I really don't understand why HF could only form 2 Hydrogen bond after studying the problem for a very long time.

I think there are three lone pairs of electrons in F atom, so it can possibly form three hydrogen bond with neighboring HF molecule, while H atom in HF can form another hydrogen bond so it results in a total of four hydrogen bonds formed for each molecule instead of two which is said by most textbooks. But I know I should be wrong, but I really can't understand.

Please explain it to me. Thanks very much!

 

[mishrashubham]

1. To form a hydrogen bond (particular strong attraction) Why it is necessary for a hydrogen

atom to attach to a highly electronegative atom and form bond with the lone pair of electrons

of another electronegative atom??

Electronegativity of an element is the tendency of an atom of the element to attract a shared pair of electrons towards itself in a a combined state. So when hydrogen forms a bond with a highly electronegative atom such as oxygen, fluorine etc, it tends to slightly let go of its shared electrons and develop a partial positive charge. The acceptor in turn gains a partial negative charge. This results in the slightly positive hydrogen being attracted to the negative electrons in the lone pair of another molecule. This is in fact a special case of dipole-dipole interaction. Note that electronegativty is not the only factor involved in H-bonding. Chlorine despite being as electronegative as nitrogen (as per the Pauling scale) rarely forms H-bonds due to larger size.

2. For HF (Hydrogen Fluoride), it is said that it can form two hydrogen bond for each molecule, while for H2O (water), it is said that it can form four hydrogen bond for each molecule. Why?? I really don't understand why HF could only form 2 Hydrogen bond after studying the problem for a very long time.

I think there are three lone pairs of electrons in F atom, so it can possibly form three hydrogen bond with neighboring HF molecule, while H atom in HF can form another hydrogen bond so it results in a total of four hydrogen bonds formed for each molecule instead of two which is said by most textbooks. But I know I should be wrong, but I really can't understand.

This has been discussed before.
www.physicsforums.com/showthread.php?t=514328

 

[ampakine]

When a H atom bonds to a highly electronegative atom it pretty much becomes a bare proton which is highly electropositive. Electropositive things attract electronegative things so this proton will be attracted to electronegative atoms like O or N.