Chemistry Question

reminder: H2O can Maximally form 2 H-bond while HF and NH3 can Maximally form 1 H-bond. Maximally! "they're able to form H-bonds" doesn't mean that there must be H-bond.

back to question. what is needed in formation of ONE H-bond?

1. a lone pair of electrons on an electronegative atom; and
2. a hydrogen atom attached onto an electronegative atom.

consider H2O. there're two hydrogen atoms fulfilling [2], as well as two lone pair on oxygen, fulfilling [1].

what about HF? 3 lone pairs on F, but 1 hydrogen only;
NH3? 3 hydrogen but 1 lone pair on N.

that's easy to understand.