In which of the following chemical compounds are the building blocks molecules?

(A) $\ce{AgNO3}$
(B) $\ce{HNO3}$
(C) $\ce{KNO3}$
(D) $\ce{NaNO3}$

(B) is the correct answer, but I have no idea why. Could you briefly explain why this is the correct answer?

  • $\begingroup$ Strange question, is this complete? as written in the book? $\endgroup$ – G M Jun 11 '14 at 8:19
  • $\begingroup$ @GM: I agree. Yes, that's all information that's given in the book. $\endgroup$ – cherry8.8vanilla Jun 11 '14 at 8:46
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    $\begingroup$ Can you give some more context, or tell us which book? The question as written does not make sense. $\endgroup$ – thomij Jun 11 '14 at 12:24
  • $\begingroup$ I highly doubt this is correctly copied from the textbook. It isn't even good English AFAIK. $\endgroup$ – Jori Jun 11 '14 at 16:44
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    $\begingroup$ I decided to call my chemistry teacher and he said that possible explanation could be that $HNO_{3}$ is a (nitric) acid, whereas e.g. $KNO_{3}$ is an ionic compound, therefore, it's buildnig blocks are not molecules, but ions. $\endgroup$ – cherry8.8vanilla Jun 14 '14 at 6:57

Answer B is correct because nitric acid is the only compound whose building blocks satisfy the IUPAC Gold Book definition of a molecule, a neutral (uncharged) entity consisting of at least 2 or more atoms. The building blocks of all other given compounds are atomic ($\ce{K+}$, $\ce{Na+}$ and $\ce{Ag+}$) or molecular ions ($\ce{NO3-}$), which by their nature do not fulfill the requirement of being electrically neutral. Per definition, they are molecular entities, i.e. distinct particles which can be identified as separate entities. This term is more general and also includes charged species like ions.


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