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From "Smarter than a 5th Grader":

What is the simplest structural unit of an element or compound?

  • molecule,
  • atom,
  • neutron,
  • proton

For covalent compounds, the answer would definitely be molecule. But I'm unclear as to how to show that the answer for ionic compounds or elements is "molecule".

For example, https://sites.google.com/site/rccchem2a/chapter-4-chemical-bonding-the-i states that ionic compounds: "Do not have molecules as basic structural unit. Instead extended array of positively and negatively charged particles called ions." Is the term 'structural unit' defined across Science as only for covalent compounds?

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    $\begingroup$ It is a bit unclear what you mean by structural unit. Of course, most compounds have fragments, repeating throughout the compound structure. However, in condensed phase these fragments are linked into infinite structure. Of course, there is no 'smallest' finite unit, that can be used to model chemical properties of condensed phase adequately, only finite elements and their linkage. This is a permanent source of troubles in chemical modeling. In case this does not satisfy, please, expand your question. $\endgroup$
    – permeakra
    Commented Jul 7, 2014 at 9:02
  • $\begingroup$ "What is the simplest structural unit of an element or compound? molecule, atom, neutron, proton" To clarify, this is the exact question from "Smarter than a 5th Grader" game. I think that the question is too brief so does not have an answer, and that the accepted answer "molecule" should be incorrect. But how can I prove it? Or prove them correct? $\endgroup$
    – Tom Anderson
    Commented Jul 7, 2014 at 11:56
  • $\begingroup$ Further comment is that the definition was written by a lexicographer in Wordnet 3.0 for molecule. It seems incorrect to me and Wordnet is not a dictionary, but it's widely used in academia. Can anyone answer definitively yes or no, this is a real definition? $\endgroup$
    – Tom Anderson
    Commented Jul 7, 2014 at 12:13
  • $\begingroup$ I would think a "simplest structural unit" would be also the simplest possible molecular entity - and may therefore be almost anything. However, the answer molecule does not seem fit here, e.g. carbon in its common form - graphite - does not really consist of molecules but rather two dimensional macromolecules. For phosphorous it might be even more confusing. $\endgroup$
    – Martin - マーチン ♦
    Commented Jul 7, 2014 at 12:26
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    $\begingroup$ @TomAnderson Well, the smallest structural element considered in chemistry is Atom. Protons and neutrons are more domain of physics. Atoms may assemble directly into lattice or assemble into molecules that assembles into lattice or molecules can be included into a lattice/molecule. So, in case I forced to answer the question as is, I'd stick with molecule. But the question in too dependent on wording, I would insist on expanding it. Moreover, it has at least one incorrect concept: chemical elements are sets and do not have structural units. $\endgroup$
    – permeakra
    Commented Jul 7, 2014 at 14:49

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For a solid lattice (metal, ionic solid, crystal) I would say the answer is unit cell, for a non-charged compound that isn't a lattice (including solution/gas phase) I would say molecule, and for a charged one I would say ion-pair / ternary ion-associates / quaternary-ion associates for two / three / four ion clusters. For polymers, I would refer to the basic unit(s) being the component monomer(s).

There is, as has been identified in the comments, an issue with allotropes, Carbon (Diamond, Graphite etc) can be encompassed within 'unit cell', but for elements like phosphorus and sulfur, it's a little more complicated. There I would contend that the basic unit of e.g. P4 (white Phosphorus) is the molecule, and for the rest, I would return to referring to the unit cell, or 'monomer' in the case of amorphous networks.

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    $\begingroup$ To add to the complexity, some elements are monoatomic, notably the noble gases. $\endgroup$
    – Ben Norris
    Commented Jul 7, 2014 at 20:48
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    $\begingroup$ Further down the rabbit hole we go! $\endgroup$
    – Sam
    Commented Jul 7, 2014 at 23:12

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