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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.

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    $\begingroup$ ;/ I've a feeling you wouldn't make more ambiguous title even if you tried. As far as your q. is concerned, it's just that specific form of sulfur is largely irrelevant for reaction. $\endgroup$
    – Mithoron
    Mar 19, 2020 at 21:04
  • $\begingroup$ @Mithoron Yes, I reckon that the title is not good. I would appreciate any suggestion to improve it $\endgroup$
    – cesaruliana
    Mar 20, 2020 at 2:40
  • $\begingroup$ Kinda better now. $\endgroup$
    – Mithoron
    Mar 20, 2020 at 23:16

2 Answers 2

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Generally, other than the seven diatomic gases at STP ($\ce{H2}$, $\ce{O2}$, $\ce{N2}$, $\ce{F2}$, $\ce{Cl2}$, $\ce{Br2}$, $\ce{I2}$... though $\ce{Br2}$ & $\ce{I2}$ 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.

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    $\begingroup$ This is indeed a clear statement of the rule, but do you have an argument for why it doesn't affect stoichometric calculations to just ignore the molecular nature of the substance and treat it as monoatomic? $\endgroup$
    – cesaruliana
    Mar 20, 2020 at 2:43
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    $\begingroup$ As for stoichometric calculations, for solids and liquids, they're based on the mass based on the chemical formula for solids and liquids. Since gases are traditionally measured volumetrically, knowledge of the number of particles is required. $\endgroup$
    – DrMoishe Pippik
    Mar 22, 2020 at 3:26
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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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    $\begingroup$ Thanks, this contains relevant information. But is the presence of various allotropes in a mixture what allows one to treat it as monoatomic or either way it will act as if it were monoatomic? $\endgroup$
    – cesaruliana
    Mar 20, 2020 at 2:53
  • $\begingroup$ Different allotropes of sulfur will react in the same way with oxygen, chlorine or any other reagent, producing the same products, with the same rate law. $\endgroup$
    – Maurice
    Mar 20, 2020 at 8:51
  • $\begingroup$ @Maurice, the rate of reactions do depend on allotropic form, e.g white phosphorous reacts readily in air, with luminescence, though black and red forms are relatively inert. Ozone reacts readily with saturated hydrocarbons at STP, but O2 does not. $\endgroup$
    – DrMoishe Pippik
    Mar 20, 2020 at 19:45

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