Simple question...why is nitrogen referred to as molecular nitrogen?

Discussion in 'Chemistry' started by Seattle, Mar 21, 2015.

  1. Seattle Valued Senior Member

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    I'm not well versed in chemistry so I'm sure this is a simple question. Why is nitrogen sometimes referred to as "molecular nitrogen"?
     
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  3. exchemist Valued Senior Member

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    It would be helpful to have some context, but I would guess what is probably meant is the diatomic molecule N₂, as opposed to nitrogen combined in more complex compounds (ammonia, urea, organic compounds containing nitrogen, etc), or possibly atomic nitrogen, i.e. atoms floating free in interstellar space or something.
     
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  5. Seattle Valued Senior Member

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    I think the context was molecular nitrogen found in a comet indicating that the solar system started off colder than was previously thought (or something like that).
     
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  7. exchemist Valued Senior Member

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    Hmm. Why the presence of N₂ would indicate cold conditions is not apparent to me, I must admit. I wonder why - does the article give any clues? I suppose a lot of nitrogen might be expected to be in the form of ammonia, given the cosmic abundance of hydrogen, just as a lot of oxygen might be found as water. Buy why would N₂ be favoured by colder conditions? The enthalpy of formation of ammonia is -ve, indicating it is more thermodynamically stable than elemental nitrogen and hydrogen. So naively, I'd have expected ammonia to be favoured, given the very long reaction times available in interstellar space. But I am not an astrochemist…………..
     
  8. TBodillia Registered Senior Member

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    Rosetta Makes First Detection of Molecular Nitrogen at a Comet

    "'Identifying molecular nitrogen places important constraints on the conditions in which the comet formed, because it requires very low temperatures to become trapped in ice,' says Martin Rubin of the University of Bern, lead author of the paper presenting the results published today in the journal Science."
     
  9. exchemist Valued Senior Member

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    Ah, thank you very much. So it is the trapping of N₂ in ice that indicates a low temperature environment for the comet.

    Presumably at higher temperatures a small molecule such as N₂ would be able to diffuse out of the ice. That seems to explain it.
     

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