Question

why energy is released when an electron is added to a isolated gaseous atom ?

Answer 1

All electron shells represent an energy level. In order for there to be a release of energy the electron needs to come from a higher energy state. The only energy state higher than the outer-most electron shell would be a free electron. The only way an electron becomes a free electron is that sufficient was provided to lift it from lower electron shell (energy level) it was previously in to escape velocity. The energy that it then releases in returning is then this exact same amount of energy.