Which atoms hydrogen bond? Sulfur?

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Hi @wormboge, you are right in that fluorine, nitrogen and oxygen can hydrogen bond and for the MCAT, this is sufficient. It is important to note, however, that for a hydrogen bond to be present, you need both a hydrogen bond donor and an acceptor. A hydrogen bond donor is simply one of those 3 elements bonded to a hydrogen. A hydrogen bond acceptor is one of those 3 elements with at least one lone pair to "accept" the hydrogen bond. This is why water can technically make four hydrogen bonds as it has two hydrogen bond acceptors (the two lone pairs) and two hydrogen bond donors (two O-H bonds). Hope this clarifies the overall concept.
 
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