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Consider an nitrogen atom with 7 protons and 7 electrons. How can an nitrogen anion $\ce{N^-}$ exist? Shouldn't the 7 electrons in valence shell repel the extra one?

What force does hold the extra one electron in the valence shell? There isn't an extra proton in the $\ce{N^-}$ that would hold the extra electron.

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  • $\begingroup$ Note that nitrogen is, aside of noble gases, one of few elements ( also e.g. Be, Mg, Mn, Zn) with negative electron afinity, therefore energy is released by repulsion of the extra electron and such an anion can exist only for a minimal time period of the order of microseconds to milliseconds. $\endgroup$
    – Poutnik
    May 31 at 9:08

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One reason cations and anions exist is due to the stability of a full or half-full valence shell. The stability from those electronic configurations means that the atom or molecule does not require protons to "hold" the extra electron.

Recall also that nitrogen has three (or five) valence electrons, rather than seven. The 1s shell is full and is not considered part of its valency. The three 2p electrons are the valence electrons although they hybridize with the 2s electrons to produce the trigonal pyramidal structure of ammonia with its lone pair.

The single anion $\ce{N^-}$ could exist, but would not be stable because it puts four electrons in the p shell. The p shell would prefer to have three electrons as it does in the nitrogen atom or no electrons as it does in the $\ce{N^3+}$ cation.

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  • $\begingroup$ Thank you for your answer! Do I get your answer right when I suppose that the extra electron(s) in the valence shell of an anion are held there by the electrical field of the electrons in the electron shells and also thanks to "right" spatial formation of the electron shells? $\endgroup$
    – sarasvati
    Jan 12, 2016 at 16:03
  • $\begingroup$ An extra electron is held because it is attracted to the nucleus because the particular orbital is not shielded by inner electrons. A second electron is endothermic because the negative charges repel. Elements with s1 configurations [Group I] have exothermic electron affinities because the s orbitals penetrate closer to the nucleus overcoming electron repulsion. p and d orbitals penetrate less so electron repulsions have a greater effect, electrons are added to each orbital and the 4th electron as in nitrogen must share an orbital and repulsions are greater than the nuclear attraction. $\endgroup$
    – jimchmst
    May 31 at 23:10
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Something to consider: does N become an anion all by itself? No. It gets those electrons from a much less electronegative atom. When it does that it generates an anion and a cation. They attract each other. Potential energy decreases (energy is released) and that "released" energy drives the REACTION. As I like to say to students: It's not like a sodium atom is walking down the street and one of its electrons falls out of its pocket. Metals don't just LOSE electrons. That electron has to be TAKEN by a more electronegative atom.

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