Hello.
Firstoff, I see no General Chemistry board, so while this is not strictly organic chemistry, I placed this post here as this is the most populous of the fields of chemistry. Relocation to the correct board will be welcomed.
Ask any physicist who has accomplished anything noteworthy as to what the greatest unanswered questions are in physics, and you will get the same set of answers every time. Grand Unification, does the Higgs Boson exist? (recently answered, I know) etc. etc.
Thusfar, neither the chemists I know nor from my fellow chemical engineers have been able to furnish me with a coherent answer when asked a similar question concerning the field of chemistry.
So, to all the professional chemists, chemical engineers and all others with insight who populate this forum, what are the five greatest, unanswered questions in the field of chemistry?