Why is energy released when an electron is added to a neutral atom?

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You are probably referring to "electron affinity". Most nonmetal elements have large negative values of electron affinity, indicating that energy is released when an atom acquires an electron to make a negatively charged ion. Some elements (the alkaline earth metals) do not accept electrons and have zero or positive electron affinities. Metals in general have low values of electron affinity, indicating that a negative ion would be very unstable. It's also interesting to look at "second electron affinities" and "stability". It is often said that oxygen forms an O^2- "ion" in order to get more stable, (lower in energy), but that simply isn't the case. It is extremely difficult to get oxygen to gain a second electron to form the O^2- ion. Look at the numbers below. 1st electron affinity for oxygen = -142 kJ/mol ... gives O^- 2nd electron affinity for oxygen = +844 kJ/mol ... for O^2- The O^2- ion is extremely unstable, contrary to popular belief. That's because it requires a great deal ... more
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