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Homework Help: Electron Affinity Questions

  1. Jun 29, 2011 #1
    Hi guys, my first post/question. Excuse my stupidity if you think this is an easy question, I'm really having a difficult time with reviewing this part in general chemistry.

    1. The problem statement, all variables and given/known data
    Which has a more negative electron affinity, Na+ or Na? Na+ or Cl?

    2. The attempt at a solution
    My thinking was to write down the ground-state electron configurations.

    Na+: [Ne]
    Na: [Ne]3s^1
    Cl: [Ne]3s^2 3p^5

    Electron affinity is described as the energy change that occurs when an electron is added to an isolated atom in its gaseous state.

    I assumed for the first part of the question that Na had a greater negative electron affinity because it would become paired in the s orbital, and since Na+ already has a noble gas configuration, wouldn't adding an electron make the Electron affinity positive?

    For the second part I naturally assumed it was Cl for similar reasons, and since the textbook says halogens have the greatest Electron affinities.

    Answer for both questions: Na+

    I don't really understand how/why, so if someone could please explain in a manner that's simple yet concise, that would be awesome.

    Thanks Physics forum! I look forward to being an active member on here and learning a lot.
  2. jcsd
  3. Jul 1, 2011 #2


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    Staff: Mentor

    I think you should try to analyze it just taking simple electrostatic into account.

    What is the electron charge?

    What should be easier - attaching electron to neutral Na or to positively charged Na+?

    What should be easier - attaching electron to neutral Cl or to negatively charged Cl-?
  4. Jul 9, 2011 #3
    man WOW! it's much simpler
    electron affinity is inversly propertianal to atomic radius RIght? why?
    and atomic radius is directly proportianal to number of positive charges right? why?
  5. Jul 9, 2011 #4
    I think you misread the second question. It was "Na+ or Cl?", not "Cl- or Cl?".

    Na+ or Cl is pretty easy to answer by considering what happens if you have an Na+ ion and a Cl atom fight over an electron. If the Na+ ion gets it, you end up with an Na atom and a Cl atom. If the Cl atom gets it, you end up with an Na+ ion and a Cl- ion. If you look around you, you will see Na+ and Cl- ions all over the place, happily bumping into each other. I defy anyone to find elemental sodium metal and chlorine gas in peaceful coexistence.
  6. Jul 10, 2011 #5
    well, that in case they ARE in the same reaction medium
    but it's still right because any atom wants to get the electronic confugaration of the nearest noble gas so for Na it's Na+ Ne and for Cl its Cl- Ar
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