why energy is released when an electron is added to valence shell of an isolated gaseous atom?

							
Dear Pooja
All electron shells represent an energy level - it doesn't matter if its the outermost shell or not. 
 In order for there to be a release of energy the electron has to be  coming 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 what-ever 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.
 
All the best.                                                           
AKASH GOYAL
AskiitiansExpert-IIT Delhi
 
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