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Many first electron affinities are positive, indicating a favourable process, but the corresponding second electron affinities are negative. For example, the first and second electron affinities of oxygen are $+141$ and $-780~\mathrm{kJ~mol^{-1}}$ respectively. Why is this so?

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Many non-metals like those in Group 16 and 17 have first electron affinities that are negative. In other words, the following reaction is exothermic and energy is released.

$$\ce{X + e^{−} -> X^{−} + ~energy}$$

By gaining an electron the group 17 atoms attain a complete octet of electrons and become stabilized. By gaining an electron the Group 16 atoms are just one electron away from having a filled shell. Further in both the Group 16 and 17 cases, the protons in the nucleus are not fully screened by the outer electrons, so adding one more electron to form an anion is electrostatically favorable.

However, if we try to add a second electron the reaction becomes endothermic; adding a second electron is unfavorable and energy must be added (endothermic).

$$\ce{X^{-} + e^{−} + ~energy~ -> X^{2−}}$$

It is difficult to add a second electron primarily because of the increased electron-electron repulsion that occurs with the second electron.

Since adding the first electron is exothermic, while adding the second electron is endothermic, the sign reverses when we measure the energy added or released by these two processes.

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  • $\begingroup$ OK, but why basically the exothermic and endothermic reactions occur with different type of elements? In other words, how can we know,for example, that that elements of group 17 releases energy whereas others absorb??? I couldn't understand the relationship basically $\endgroup$ – user14840 Mar 16 '15 at 17:22
  • $\begingroup$ Elements that form stable negative ions (like oxygen, fluorine, etc.), and especially those that achieve a stable octet upon negative ion formation (fluorine, chlorine, etc.), will always have exothermic (negative) first electron affinities. Most other elements, especially those that can achieve an octet by losing one or two electrons, don't want to gain an electron (that moves them further away from the stable octet configuration), so when we "force" a metal to add an electron it is an endothermic reaction. $\endgroup$ – ron Mar 16 '15 at 17:40

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