Both sulfur and oxygen have 6 electrons in their outermost shell. Since both only need two electrons to stabilize, why doesn't just one atom of oxygen react with one sulfur atom, forming sulfur monoxide SO instead of sulfur dioxide $\ce{SO2}$?


One atom of oxygen is a fictional species outside of a plasma. Oxygen is naturally diatomic, and the process by which it oxidizes sulfur is radical-based, leading to both oxygen atoms reacting. The fact that SO2 forms instead of SO is a result of the [redundant answer] thermodynamics favor SO2 over SO.

Given that simple fact here is a quantum mechanical rationalization: Sulfur can expand its octet, which means in SO2 it has 12 electrons 'around' it. This allows resonances forms where O- has a significant contribution to resonance, while S still has at least 8 electrons around it. In SO no such resonance forms exist, meaning the highly electronegative oxygen must share its electrons with with sulfuer to avoid exposing a six-eletron sulfur species. Hence SO2 should be a better structure than SO overall.


Its just like answering formation of $\ce{CCl4}$ against $\ce{CCl2 }$ for $\ce{SO}$ and $\ce{SO2}$

  • It is highly unstable
  • It becomes more stable after forming a double bond with O ( maybe resonance )


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It has an electronic configuration of $\ce{[Ne]} \: 3s^2\: 3p^4$

It has 4 bonds due to uncoupling of p electrons (a little energy is spent), but due to the formation of four bonds, a large amount of energy is released.

  • $\begingroup$ in most reactions I've come across, SO2 is evolved. I've rarely seen SO anywhere. So, in what kinds of reactions is sulphur monoxide formed? $\endgroup$ – Ghost Dec 27 '12 at 15:45
  • $\begingroup$ FROM WIKIPEDIA Production and reactions Production of SO as a reagent in organic syntheses has centred on using compounds that "extrude" SO. Examples include the decomposition of the relatively simple molecule thiirane 1-oxide: as well as more complex examples, such as a trisulfide oxide, C2H4OS → C2H4 + SO The SO molecule is thermodynamically unstable, converting initially to S2O2. SO inserts into alkenes, alkynes and dienes producing molecules with three membered rings containing sulfur. $\endgroup$ – Siddhartha Sinha Dec 27 '12 at 15:49
  • $\begingroup$ Generation under extreme conditions In the laboratory sulfur monoxide can be produced by treating sulfur dioxide with sulfur vapour in a glow discharge. It has been detected in single bubble sonoluminescence of concentrated sulfuric acid containing some dissolved noble gas $\endgroup$ – Siddhartha Sinha Dec 27 '12 at 15:50
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    $\begingroup$ It would be better if you explained why $\ce{SO}$ is unstable compared to $\ce{SO2}$. Currently the answer doesn't have much conceptual content. $\endgroup$ – ManishEarth Dec 27 '12 at 15:58
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    $\begingroup$ I don't think this is a good analogy. The two phenomena are completely different. See answer below. $\endgroup$ – Lighthart Feb 25 '13 at 4:59

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