Electron Affinity, Type of energy released?

[mishima]

This might be a dumb question, but for instance when oxygen gains an electron to become an anion, energy is released equal to its electron affinity. What exactly is the form of this energy? Is there a photon of a certain wavelength emitted? Does the anion slow/cool down because of the small increase in mass? Thanks for any insight.

 

[Simon Bridge]

It's a good question - you should be interested in the energy transformations.

Ultimately the energy is released as heat (and light) - hence the process is exothermal.
The details of the process depends on the specific reaction.
In chemistry you are not normally worried about these details.

Related concept: "free energy of binding".

Simplistically: you could imagine it as the O and e- are initially at rest close to each other.
Since conditions favor forming an anion, they initially accelerate towards each other - exchanging EM-PE for KE until the balance of PE and KE matches an available stationary state.

While the e- is accelerating, it radiates.
The end result is some, usually IR, photons from the radiation and an anion with some additional, random, KE.

 

[mishima]

While the e- is accelerating, it radiates.
The end result is some, usually IR, photons from the radiation and an anion with some additional, random, KE.

That makes sense. I wasn't sure if there was something like the Rydberg formula for hydrogen that applies to oxygen.

 

[Simon Bridge]

There is - with different energy levels.

You can think of it as the extra electron is initially in a far-away orbital and decays into a lower orbital.
At each step it releases a photon - depending on the transitions.

What I did before was the quasi-classical description of the same thing - electron radiates when it is not in a stationary state.

Note: The KE transferred to the oxygen is usually very very small since the mass of O is much bigger than the mass of a single e- so we would normally ignore it. I just thought it would be nice to have a tie into something less mysterious to you :)

You can also imagine shooting (low energy) electrons at oxygen - in which case you'll get many electrons in excited states of the anion due to recoil effects from the electrons initial KE.
Indeed do many things come to pass.