Why Molecular Solids can be treated as Monoatomic in Reactions

I'm trying to figure out what is the precise rule for the formula of elemental substances in reactions. I'm afraid I can't word the question properly, so I'll try to give some examples.


1)Oxidation of some metal, e.g. Calcium


$2Ca+O\_2 \rightarrow 2CaO$


Since Calcium is a metal I understand that each atom reacts independently and the formula is plain $Ca$


2)Oxidation of some molecular gas, e.g. Hydrogen


$2H\_2+O\_2 \rightarrow 2H\_2O$


Now atomic hydrogen in gas form is not what one usually expects, but rather the diatomic gas, so the formula must be $H\_2$.


3)Oxidation of Sulfur


$S+O\_2\rightarrow SO\_2$


Now, here I get confused. I just copied the reaction from a textbook, but everywhere I looked in the internet says that this is the correct formula. On the other hand the textbook says that the most common allotrope of sulfur has formula $S\_8$. And now I wonder why the reaction should not involve molecular sulfur and be


$S\_8 +8O\_2\rightarrow 8SO\_2$.


I do understand that either way one would get the correct mass proportions, but I'm sot sure if that wold be the case in other reactions or with other substances.


So, I get that for metals the formula for the elemental substance that appears in reactions is just the atom. I get that for carbon and other network covalent materials the same applies. And I get that for molecular gases you need the diatomic formula, because each molecule participates as a whole. What puzzles me is what is the rationale beyond plain atoms appearing in molecular solids like sulfur, phosphorous, etc.


I apologize for not being able to state the problem in a more succinct fashion, feel free to edit the question or ask for further clarifications.




Generally, other than the [seven diatomic gases](https://www.thoughtco.com/what-are-the-seven-diatomic-elements-606623) at STP ($\ce{H2}$, $\ce{O2}$, $\ce{N2}$, $\ce{F2}$, $\ce{Cl2}$, $\ce{Br2}$, $\ce{I2}$... though $\ce{Br2}$ & $\ce{I2}$ have fairly low vapor pressure at room temperature), for convenience, other elements are considered as if they reacted as individual atoms, though, as you state, that is *not* actually the case. Phosphorus, for example, can be in [allotropes](https://chem.libretexts.org/Bookshelves/Inorganic_Chemistry/Modules_and_Websites_(Inorganic_Chemistry)/Descriptive_Chemistry/Elements_Organized_by_Block/2_p-Block_Elements/Group_15%3A_The_Nitrogen_Family/Z%3D015_Chemistry_of_Phosphorous) with formulas of $\ce{P2}$, $\ce{P4}$, or even a graphitic form, but in a reaction, it's usually considered to be monatomic (unless one must take into account the energy to break the bonds of a specific form).


BTW, iodine vapor is diatomic only at moderate temperatures. From [Wikipedia](https://en.wikipedia.org/wiki/Iodine): "1% of a sample of gaseous iodine at atmospheric pressure is dissociated into iodine atoms at 575 °C." Furthermore, at STP, iodine *solid* behaves as a semimetal, so perhaps it shouldn't be considered diatomic... but it's easier to just say "all the halogens are diatomic", ignoring iodine and fleeting astatine.


In brief, then, this is the conventional list.


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Sulfur is made of $S\_8$ molecules when cooled down from gaseous or liquid state. But when it is obtained by a chemical reaction at room temperature, it is a mixture of many allotropes, like $S\_n$, where *n* is not well defined and may vary. 
Also, by cooling abruptly liquid sulfur in carbon disulfide and extracting the soluble part, the dissolved sulfur is $S\_4$ The residue $S\_6$. 


Anyway, whatever its formula, it will react the same way with $O\_2$ or any other reagent. 
That is why usually sulfur is described simply by the formula S. 


Reference (in French) : Clement Duval, Soufre, Presses Universitaires de France, Paris 1967.


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