Can hydrogen fluoride form three or more hydrogen bonds? [duplicate]

As far as I know, each $\ce$ molecule has two hydrogen bonds, one formed by its hydrogen atom and one which the $\ce$ -atom forms with hydrogen atom of a third $\ce$ molecule. In other molecules, like $\ce$ , where both of oxygen's lone pairs are used in hydrogen bonding each water molecule forms as many as four hydrogen bonds. Why does fluorine, which has three lone pairs, not use all its lone pairs up to form three hydrogen bonds instead? 38.4k 14 14 gold badges 133 133 silver badges 223 223 bronze badges asked Jul 3, 2022 at 2:59 user1039203 user1039203 165 7 7 bronze badges

$\begingroup$ Then which part of these very electronegative atoms are the hydrogen atoms attracted to? $\endgroup$

Commented Jul 3, 2022 at 5:08 $\begingroup$ They're just attracted to the atom, not any part as such $\endgroup$ Commented Jul 3, 2022 at 6:01 $\begingroup$ There are not enough H atoms to use all lone pairs. $\endgroup$ Commented Jul 3, 2022 at 6:44

$\begingroup$ @Infinite (cc ananta) Thank you for doing editorial work, this has been really helpful. One nuance: we prefer to not use MathJax in the title field due to issues it gives rise to; see here for details. Also, it's a good practice to avoid chemical formulas and abbreviations in titles altogether whenever possible. $\endgroup$

Commented Jul 3, 2022 at 14:01 $\begingroup$ chemistry.stackexchange.com/questions/20149/… $\endgroup$ Commented Jul 3, 2022 at 17:06

3 Answers 3

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Hydrogen fluoride has only one hydrogen atom per fluorine atom, making a branched hydrogen-bond network difficult to form. (This is one reason hydrogen fluoride also forms hydrogen-bonded structures in the gas phase, unlike water where hydrogen-bond branching is more likely to occur and drive condensation.)

Ammonium fluoride, $\ce$ , is not so constrained. In this compound each fluorine can use all four of electron pairs to form bonds that are apparently both hydrogen bonds and ionic bonds:

Ammonium fluoride adopts the wurtzite crystal structure, in which both the ammonium cations and the fluoride anions are stacked in ABABAB. layers, each being tetrahedrally surrounded by four of the other. There are N−H···F hydrogen bonds between the anions and cations.[1] This structure is very similar to ice, and ammonium fluoride is the only substance which can form mixed crystals with water.[2]

Cited References

  1. A. F. Wells, Structural Inorganic Chemistry, 5th ed., Oxford University Press, Oxford, UK, 1984.
  2. Brill, R.; Zaromb, S. "Mixed Crystals of Ice and Ammonium Fluoride". Nature.173 (4398): 316–317. https://doi.org/10.1038/173316a0.