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Homework Help: Electron Affinity

  1. Apr 27, 2007 #1
    Why is electron affinity negative?

    In my book its written, that on moving down a group, tit decreases. But why is it that period 3 elements have greater E.A. than period 2 elements?
  2. jcsd
  3. Apr 27, 2007 #2
    you should precise which E.A.!! The first E.A. is the amount of energy released when 1 mol of a gaseous atom accepts 1 mol electrons. the first E.A. is always negative. E.A. is in fact an energy change; when an energy change is -ve, it implies that the reaction is exothermic, i.e energy is liberated.....and when energy change is +ve, it implies that the reaction is endothermic, i.e energy is absorbed. right

    the attraction between the nucleus and the first electron to be accepted releases energy. the second E.A. and succeeding ones are all endothermic. this is because energy is needed to force the electron into the atom. energy is now required as the second incoming electron is being at the same time attracted by the nucleus and repelled(to a larger extent) by the other electrons of the atom. right

    E.A. decreases down a group. this is because, down a group the atoms become shielded by the larger numbers of electrons. (e.g. Li has 3 e-, Na has 11e-, K has 19e-,....) the shielding decreases the attractive force of the nucleus on the incoming electron and furthermore, the incoming electron is being repelled at a greater extent by the larger number of electrons. i.e more energy is required to force that electron into the atom.

    period 3 elements have SMALLER electron affinities than period 2 elements for the reasons above. But an exception is F(period 2) and Cl(period 3). This is due to the fact that F has a very small size.....

    Hope i helped you
  4. Apr 27, 2007 #3
    how is that an exception?...doesnt F have a greater E.A than Cl?

    did u switch period and group by accident? The rule of thumb is that the closer something is to Flourine the larger its E.A is...
  5. Apr 27, 2007 #4

    Yes understood it quite well now. But please do tell me that while comparing electron affinities of 2 elements, do we consider the -ve sign?

    Thanks a lot for the valuable help.
  6. Apr 27, 2007 #5
  7. Apr 28, 2007 #6
    the sign is just to distinguish between a reaction being exothermic or endothermic. -ve means exothermic and +ve means endothermic.

    for example... first EA of Cl is - 364, Br is -342, I is -295. It just means that, when 1 mol Cl accepts 1 mol e-, 364kJ of energy is liberated. For Br it's 342 kJ and for I it's 295 kJ. The amount of energy liberated decreases from Cl to Br...and therefore e- affinity decreases.

    So.... the answer is no.
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