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Question :

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

I read somewhere

“When electrons are added to an atom, the increased negative charge puts stress on the electrons already there, causing energy to be released.”

I didn't understand What is stress and how energy is released due to stress?

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  • $\begingroup$ Please do not personify atoms and sub-atomic particles. Electrons and atoms are not human beings who have to vent their energy in order to de-stress. Whoever wrote it just ignore this statement because the writer does not know what he/she is writing. Sub-atomic particles have a world of their own which is very different from what we know and understand. There is no such thing as stress or congestion of having too many electrons. $\endgroup$
    – M. Farooq
    Sep 6 '20 at 3:50
  • $\begingroup$ @M. Farooq Ok I ignore that statement. $\endgroup$
    – Wolgwang
    Sep 6 '20 at 4:37
  • $\begingroup$ Search about electron affinities "electron affinities can be negative (energy is released when an electron is added), positive (energy must be added to the system to produce an anion), or zero (the process is energetically neutral)". See google.com/… $\endgroup$
    – M. Farooq
    Sep 6 '20 at 5:06
  • $\begingroup$ The point is that energy is not always released. $\endgroup$
    – M. Farooq
    Sep 6 '20 at 5:06
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    $\begingroup$ @M. Farooq Note that stress has multiple meanings and only some of them are related to feelings as a high discomfort. Some others are objective, like the one used in mechanical engineering as a tension. It may be stretched being applied to quantum systems, but has nothing to do with human emotions. $\endgroup$
    – Poutnik
    Sep 6 '20 at 5:08
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If the electron statistical distribution around the atomic kernel had been perfectly spherically symmetric, and if the electron occurance distribution had not mutually overlapped, than by the Gauss law of electrostatics, the net force between a neutral atom and an electron would have been zero.

But as neither of above conditions is true, a kernel charge is not fully screened off by electrons, acting as having a residual, "effective charge", what allows releasing energy by bounding an extra electron. See Slater rules.

An extra electron puts among other electrons some extra stress=mutual repulsion, what somewhat decreases this released energy.

As effective kernel charge and electron mutual repulsion ( classical and Pauli ones ) are 2 major factors affecting energies of electron orbitals in multi-electron atoms.

When the former factor is stronger, energy is released by an extra electron bounding, like for fluorine. And vice versa, like for helium.

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  • $\begingroup$ Can you define energy? I thought heat energy changes. $\endgroup$
    – Wolgwang
    Sep 6 '20 at 5:37
  • $\begingroup$ Who am I to define energy, if it is already defined and definition available on many places ? :-) $\endgroup$
    – Poutnik
    Sep 6 '20 at 5:38
  • $\begingroup$ In my physics textbook, it is defined as "The capacity to do work". $\endgroup$
    – Wolgwang
    Sep 6 '20 at 5:41
  • $\begingroup$ See e.g.Wikipedia $\endgroup$
    – Poutnik
    Sep 6 '20 at 5:45
  • $\begingroup$ Ok Got it. Thanks $\endgroup$
    – Wolgwang
    Sep 6 '20 at 5:54
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Not every atom releases energy; it is all about the stability. For your understanding, stability is inversely proportional to energy, so as the stability increases atom try to de-excite itself. If you look at the periodic table, metals generally have no affinity towards the electron because they are e donor, so they release energy by losing an electron. In non-metals like chlorine, which has highest electron affinity, release much high energy and gain the stability. And why it happens and how it happens, you need something more than normal Bohr and other atomic model, which is not the real model of an atom.

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  • $\begingroup$ No element releases energy by losing an electron. $\endgroup$
    – user7951
    Sep 6 '20 at 11:27
  • $\begingroup$ @ Yuvraj. Sorry ! Your long comment is nearly unreadable, because it has no punctuation sign. No points, no commas ! $\endgroup$
    – Maurice
    Sep 6 '20 at 12:03

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