[Ben Cohen]

Hi guys, I was looking at the electronegativity scale on the periodic table (I think it was Milliken's, the one for which Fluorine is a 4.0). Anyways, I noticed that N and Br are both 3.0, and Cl is actually above N with a 3.2. So then why do they always say that only nitrogen, oxygen and fluorine are involved in hydrogen bonding, when there are two elements that are of equal or higher electronegativies than those three?

[Borek]

1. You are putting the cart before the horse. It is not a rule based on electronegativity that tells us which elements are involved in hydrogen bonding, it is which elements are involved in hydrogen bonding that is used to find the rule. If predictions of the rule are wrong, it is the rule that is wrong, not reality that has to adjust. And please remember that there are half a dozen or so electronegativity scales, and using different scale can make the rule useless.

2. Hydrogen bonding is not something that either exists or not - hydrogen bonds can be stronger or weaker, depending on the elements involved. For the three elements you mentioned hydrogen bonds are strong enough to be an important thermodynamical factor, but I guess they do exist for other elements as well - just are too weak to be important.

[cheese (MSW)]

I looked at this problem some years ago.  See if you come to the same conclusions that I did.
It is not discussed in texts as I recall.   Reasonably strong H bonds X-H...Y are formed only for X and Y = N, O or F (X and Y can, and often are, the same element).  The bonding of F is limited so in essence H bonding only important for O-H and N-H cmpds.  Obviously the electronegativity of X plays a crucial role in order to polarize the X-H bond: δ-X←:Hδ+.  Note the H atom is small and can hence make close approach to the lone pair on :Y.  That cannot be the whole story however because the electronegativity of Cl (χCl = 3.16) is greater than that of N (χN = 3.04) and Cl forms only weak H bonds.  So consider the lone pair on N and Cl: what is the difference?  It is now realized that there is a covalent bonding contribution to H bonding (previously thought to be entirely a strong dipole-dipole interaction) so you might want to consider some overlaps.