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    From what i understood, the first electron affinities are generally negative and the second electron affinities are always positive or endothermic due to spin pair repulsion. But in the case of oxygen, the first electron is added to a 2p subshell where one electron is already present. Therefore spin pair repulsion exists and energy is needed to overcome it ...however the 1st electron affinity of oxygen is still negative. Why is that?
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    Sorry you've not had any responses about this. Are you sure you’ve posted in the right place? Posting in the specific Study Help forum should help get responses.

    I'm going to quote in Tank Girl now so she can move your thread to the right place if it's needed. :yy:

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    Hi I've moved this to chemistry for you.
    (I was torn between chemistry and physics though so apologies if it's physics and let me know and I'll move it )
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    (Original post by areej218)
    From what i understood, the first electron affinities are generally negative and the second electron affinities are always positive or endothermic due to spin pair repulsion. But in the case of oxygen, the first electron is added to a 2p subshell where one electron is already present. Therefore spin pair repulsion exists and energy is needed to overcome it ...however the 1st electron affinity of oxygen is still negative. Why is that?
    The attraction of an 8+ nucleus coupled with the small size of the atom means that there is a considerable thermodynamic advantage to the formation of a single charge ion.

    This makes the first electron affinity negative (exothermic).
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    For the second one, remember that you're adding a -ve e- to an already -ve ion.
 
 
 
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