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
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.