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Recently I had a question in my chemistry test that said:

Which of the following nonmetals does not have multiple allotropes at room temperature?
A) Oxygen
B) Hydrogen
C) Sulfur
D) Phosphorus

The answer was apparently hydrogen, but can't sulfur also be a correct answer? I am judging this just because in my chemistry book it says that rhombic crystalline form exists below $\pu{96^\circ C}$. Monoclinic exists above $\pu{96^\circ C}$. Are there any allotropes of sulfur at room temperature?

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"Exists" is an awfully tricky word: it can be understood in a thermodynamic sense or in a kinetic (=everyday) sense, and the meanings are sometimes quite different. Ditto for "stable".

Thermodynamically, there exists only one stable allotrope of any element at any given conditions. All other allotropes are not allowed to exist and must eventually transform to that one. This is true, but the problem is that the transformation may take forever.

Now, your book on sulfur was using this meaning, but your test was not. It was meant to be understood in a kinetic sense, where multiple allotropes may coexist all right. In particular, the high-temperature monoclinic sulfur can be cooled down to room temperature and still remain monoclinic. I can attest to that; I used to hold it with my own hands, as did numerous other chemists. That's what people call metastability.

There is also plastic sulfur and other forms, but that's another story. And as you already know, there is oxygen vs ozone, and white phosphorus vs red, so hydrogen is really the only element here that does not have multiple allotropes.

So it goes.

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  • $\begingroup$ "Exists" has a very clear meaning, if you specify what, i.e. the element or rather a specific allotrope. What the OP has is just a sloppily written book. $\endgroup$ – Karl Aug 3 '17 at 21:02

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