Electron Gain Enthalpy [duplicate]








**This question already has answers here**:
 
 




[Why is electron gain generally exothermic?](/questions/32290/why-is-electron-gain-generally-exothermic)

 (3 answers)
 


[Does a positive electron affinity actually correspond to the release of energy?](/questions/78838/does-a-positive-electron-affinity-actually-correspond-to-the-release-of-energy)

 (3 answers)
 

Closed 3 years ago.



When an electron ($e^-$) is added to $\ce{Cl}$ ,i.e.,
$$\ce{Cl + e^- -> Cl^- + ∆\_{gh}H}$$ 
We can see that energy is released. My question is where is this energy coming from? What happens within the atom such that it loses energy and releases it to the surrounding?




The main question here is "why is it exothermic". The following diagram illustrates the conundrum:


[![enter image description here](https://i.stack.imgur.com/49GsN.png)](https://i.stack.imgur.com/49GsN.png)


This portrays a ball rolling over a track. Thanks to energy conservation, in frame 2 the ball has gained kinetic energy KE after descending into a well, KE diminishing as the ball reemerges again in frame 3. You can think of an electron as the ball and of the track as the potential generated by the atom. In this classical picture the electron will retain a fixed elliptical orbit or escape trajectory as it zooms past the nucleus. 


If there was no way for an electron (or ion) to shed energy then there would be no bound state. One way for the electron to shed energy is by emitting a photon, which causes it to relax into a lower energy bound state (in the cartoon this would be a tighter oscillation within the well). 


The cartoon above is admittedly a gross oversimplification, particularly inaccurate in suggesting that the system is classical and 1-dimensional. In reality binding of the electron to the atom with emission of a photon or some nonradiative process is best treated quantum-mechanically, but the essential principle of conservation of energy is retained.




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*Further reading*


A deeper explanation of electron affinity involves some heavy-duty physics, but [this](http://www-pnp.physics.ox.ac.uk/~libby/Teaching/2005/C4-JL-lecture2.pdf) presentation on the Breit-Wigner cross section might be useful. It explains: 



> 
> Broad states which can be formed by collisions between the particles into which they decay are referred to as resonances. They are described by the Breit-Wigner formula.
> 
> 
> 


Not the most recent reference, but the following article which mentions different ways of measuring and computing electron affinities may also be useful: Meunier et al, Molecular Simulation, 1999, Vol. 23. pp. 109-125


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