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 at STP ($\ce{H2}$, $\ce{O2}$, $\ce{N2}$, $\ce{F2}$, $\ce{Cl2}$, $\ce{Br2}$, $\ce{I2}$... though  $\ce{Br2}$ &amp; $\ce{I2}$ 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 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: "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.