why energy is released when electron is added to the atom?

Answer 1

Depending on what type of atom it is, for example is it a complete atom or lets say, 2,8,4?

For a complete one nothing as it cannot receive anymore electrons as it is a "safe" atom.

For the other i would say that energy is not released but the atom gains energy from the trasfer of electrons.

Answer 2

that is why e=mc2 is so great