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Electronegativity and stability of ions?

  1. Aug 29, 2012 #1
    I've just started a course on organic chemistry and the professor said some things which did'nt exactly make sense to me:

    1) In a period, atoms are more electronegative as we go to the right. The more electronegative an atom is, the more stable it becomes.

    2) In a group, atoms increase in size as we go downwards. The greater an atom's size, the more stable it is.


    Now I know the periodic trends but what I don't get is how an atom is more stable if it is more electronegative?

    Here's what I think (feel free to point out any mistakes). By stability we mean an atom's inertness or the tendency of a certain atom to not react. For example suppose S is more reactive then Q, so we could say that Q is more stable than S.

    But how does an increase in electronegativity increase stability. The way I see it, if a certain atom is more electronegative it should attract a neighbouring atom with more force and thus be more reactive and less stable.
     
  2. jcsd
  3. Aug 29, 2012 #2

    chemisttree

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    It is best you forget you ever heard this. It will never be of use to you because it's not true. You probably misunderstood what your teacher said or meant by this.

    Forget you ever heard this as well. Just as wrong as the electronegativity/stability relationship in 1).

    That sounds good to me. Stop there and forget all that other stuff.
     
  4. Aug 29, 2012 #3
    Thanks Chemistree.

    Now I'm sure I heard my professor correctly on those two points. I don't know how exactly the first point was possible but I do know how the second point is possible.

    It's got to do with the dispersion of electrons. The more dispersed an electron cloud around the nucleus of an atom is the more inert/stable it is. So it's logical to assume that as we go down a group, as the atomic size increases the inertness or stability of successive atoms down a group increase as well.


    Is this all right, wrong or does this at least make sense (but some other factor is at play here) ?
     
  5. Aug 29, 2012 #4
    both points are BS. I don't even know where you got the "dispersion" idea from. All electrons are dispersed. By the Heisenberg Uncertainty Principle you cannot even point to where an electron is.

    Want some counter examples? Go down group 1. Lithium in water fizzles. Sodium in water lights on fire. His statement only works for certain groups like oxygen group and halogens.

    For his first point that's even easier to disprove. Fluorine is #1 in electronegativity. It is #1 in reactivity.

    Oxygen is #2 in electronegativity. It is not #2 in reactivity normally but that's due to its double bond. Oxygen atoms (not molecular oxygen) are *EXTREMELY* reactive. You see the damage on re entering spacecraft? That's when the hypersonic flow across the metal surface has so much energy, the metal actually catalyzes molecular oxygen breaking up, and then the atomic oxygen starts eating away at the ship.
     
  6. Aug 30, 2012 #5
    Sorry about that confusion. You were right. It was'nt atoms but ions. So along the period with increasing E.N the anions become more stable and cations become less so. But down the group, anions become more stable and cations become less so. Now I know these to be correct (again if I'm wrong like last time do point it out) but WHY? I want to know the reasons behind these tendencies.
     
  7. Aug 30, 2012 #6
    By the way could you give examples along the period C,N,O,Fl and along the halogens group. It would make things alot easier.
     
  8. Aug 30, 2012 #7
    Charge dispersion is the key concept behind the acidity trend of the halogens. As the atomic radius increases down the row, the conjugate base stability increases due to the increased surface area of the ion.

    orgchem.chem.uconn.edu/2443s2012/2443-012312.pdf
     
  9. Aug 30, 2012 #8
  10. Aug 30, 2012 #9
    The trend of increasing reactivity down the alkali metal group is also due to the size trend. As the atomic radius increases, it's easier for the single valence electrons to leave. As the valence shell gets further away from the nucleus, the electrostatic attraction between the electron and the positively charged nucleus becomes weaker.
     
  11. Aug 30, 2012 #10
    maybe it is my vocabulary that is limited but i believe that has nothing to do with electron dispersion which is by the definition of electron cloud, essentially uniform, but rather the surface area of the ion which uniformly reduces the charge density. if it is elsewise then i apologize.
     
  12. Aug 30, 2012 #11

    chemisttree

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    You are asking about the stability of ions? How would you measure that? Is chloride more stable than sodium +1 ion? Is chloride more stable than oxide?

    You are asking us to explain nonsense to you.

    You are better served to forget you ever heard this since it isn't true.
     
  13. Aug 30, 2012 #12
    No worries, we're talking about the same thing.
     
  14. Aug 30, 2012 #13
    Actually my professor for organic chemistry said the same thing and I was confused, I asked, and his response was a pizza analogy..saying a more electronegative element will be more satisfied after eating his portion of pizza referring to the electronegative element as the pizza hogger.

    As to the atom size, he said a larger volume means the electrons are spread out further so more stability.

    Chemistree, you need to explain how both of our o-chem professors said the same thing and we need to forget about it. That is unless the OP is in my class or something!

    This lecture is being used in the case of explaining organic acid base reactions, where the F-, Cl-, etc are conjugate bases
     
  15. Aug 31, 2012 #14

    chemisttree

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    Perhaps you could expain just what the professor said and what context. Acid/base reactions where F- and Cl- are conjugate bases isn't detailed enough.
     
  16. Aug 31, 2012 #15
    Essentially the same thing as the OP
     
  17. Sep 1, 2012 #16

    chemisttree

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    My mistake... you should spend the next several months contemplating this unique concept and how it applies to everything you will ever find useful in organic chemistry.

    Good luck...
     
  18. Sep 1, 2012 #17
    People. Cool down. Let's not hurt each other's feeling here.

    Let me clarify what my professor said and what I'm confused about.

    He said that along the period anions become more stable due to increased electronegativity along the period.

    In addition he said that down the group anions become more stable because of increased charge size.

    ( He also mentioned that cations display the exact opposite behaviour but let's stick to anions for simplicity's sake).

    Now I accept these principles. But what I don't get is how increasing electronegativity increases stability of an anion along a period, and how increasing size increases stability of an anion down a group? If someone could help me out here it would be appreciated.
     
  19. Sep 1, 2012 #18
    Second one, If you have a larger size anion, you have a larger volume, and the electrons are spread over a larger space (less repulsion) so it is more stable. I can't give a clear answer for the first point, because my teacher as I said previously gave me a pizza analogy..
     
  20. Sep 1, 2012 #19
    i was explained it in terms of charge concentration. such as, a small, highly charged ion such as say, aluminum is reactive, while a big, less charged ion like iodide is less reactive.

    however when i think about it in this way there's alot of exceptions, such as the NH2- anion, which is big, but much more reactive than F-, which is smaller.
     
  21. Sep 2, 2012 #20
    Ok. Thank you.


    Any idea about the stability tendency along the period?
     
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