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Sulphur has 6 electrons in its outermost shell, while even oxygen has 6 electrons in its outer shell. Since both need just 2 electrons to stabilize, why doesn't just one atom of oxygen react with one sulphur atom, forming Sulfur monoxide(SO) instead of sulfur dioxide(SO2)?

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2 Answers 2

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.

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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 )

Structures:

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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.

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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? –  Ghost Dec 27 '12 at 15:45
    
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. –  Siddhartha Sinha Dec 27 '12 at 15:49
    
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 –  Siddhartha Sinha Dec 27 '12 at 15:50
    
Uses/occurence -Ligand for transition metals -Astrochemistry -Biological chemistry –  Siddhartha Sinha Dec 27 '12 at 15:51
    
If SO is unstable like you said, why doesn't SO2 get formed in such cases? sorry I'm quite a novice here –  Ghost Dec 27 '12 at 15:55

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