Re: why there is only one H-bond in HF?

     My first inclination was to complicate the response with high-
sounding theory, but on reflection thought that the reason might be as 
follows:  Water contains two hydrogen atoms and forms two hydrogen bonds; 
hydrogen fluoride contains one hydrogen atom and therefore forms only one 
hydrogen bond.
     I suggest the following texts for reference:
(1) CHEMISTRY OF THE ELEMENTS, 2nd Ed., 1998 by N. N. Greenwood and A. 
Earnshaw, Butterworth-Heinemann Publishers, ISBN 0-7506-3365-4.  There is 
a good section on pp 52+ on hydrogen bonding, its influence on properties, 
etc.  On pp. 812-13 hydrogen bonding in HF is discussed.
(2) CHEMISTRY, 5th Ed., 2000 by S. S. and S. A. Zumdahl, Houghton Mifflin 
Co., ISBN 0-395-98581-1.  On page 439 bonding in heteronuclear diatomic 
molecules such as HF is discussed.

     The best to you in finding the "real" answer!

Sincerely,
Charles Riner
email:  Motaatom@aol.com