Why only F, O and N form Hydrogen Bonds? [duplicate]








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[Is hydrogen bonding generally defined to include only three period two elements?](/questions/21773/is-hydrogen-bonding-generally-defined-to-include-only-three-period-two-elements)

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Why only fluorine, oxygen and nitrogen can form hydrogen-bonds with the hydrogen of another molecule?




No, that's not necessary. Amongst the *strongest* hydrogen bonds are formed by $\ce{ N, O, F}$ because of their high electronegativity. 


There's evidence that the ions $\ce{Cl-, I-, Br- }$ form hydrogen bonds that are much stronger than those of the covalently bonded atoms


$\ce{Cl}$ can form weak hydrogen bonds, but $\ce{Br}$ and $\ce{I}$ form very weak bonds if at all. 


A sulfur atom can also function as weak acceptor., but the $\ce{SH-}$ ion forms much stronger bonds. 


Hydrogen bonding has been directly observed between a negatively charged carbon and an $\ce{-OH}$ group in the same molecule 


Isocyanides' ($\ce{R- N+ #C- }$) carbon atom can also act as an acceptor (forming a rather strong Hydrogen bond). 




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**Reference:** March's Advanced Organic Chemistry, Chapter 3: Bonding weaker than Covalent


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