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

Question

I've been reading Chemguide, and came across the definition of "first electron affinity":

The first electron affinity is the energy released when 1 mole of gaseous atoms each acquire an electron to form 1 mole of gaseous 1- ions.

But why does this release of energy happen? Surely we'd have to expend some energy to install an electron where it does not belong, i.e. inside a neutral atom?

Natalie Anton · Accepted Answer

Here's my explanation for this:

The force between two charges #q_1# and #q_2# separated by a distance #r# is given by: #F=(q_1q_2)/(4πepsilon_@r^2# Where #epsilon_@# is the permittivity of free space and is a constant. If we separate one of the charges from #r# to infinity the work done is given by: #W=int_r^ooF.dr# Thus: #W=int_r^oo(q_1q_2)/(4πepsilon_@r^2).dr# # W=-(q_1q_2)/(4piepsilon_0r)# Similarly if a charge #q_1# (in this case the electron) is brought in from infinity to an energy level in an atom at a distance #r# from the nucleus then this energy would be released, assuming #q_2# is equal to the effective nuclear charge. When the electron collides with the atom, its kinetic energy is transferred, resulting in the appearance of heat. Since this is also an electron transition from #n=oo#, photons would also be given off. For those whose affinities have been measured, all of them are exothermic. You can see that even sodium has an exothermic value in the table below. Electron affinity (data page): https://tutor.hix.ai The reaction between iron and fluorine is depicted in the video below, but keep in mind that the electron affinity is only one phase in the entire energy cycle that forms an ionic compound from its constituent elements. 009.htm on https://tutor.hix.ai An electron "doesn't belong in a neutral atom" for no reason at all; in fact, this is the typical state for many elements (chlorine, for example, only occurs naturally as a negative ion).

Avery Adams · Answer

undefined Energy is released when an electron is added to a neutral atom because the electron is attracted to the positively charged nucleus of the atom, thus lowering the overall energy of the system. This release of energy is due to the formation of a more stable configuration with a complete or partially complete electron shell.