Question

when do atoms release energy and when do they gain energy

Answer 1

Ionisation Enthalpy ....
The amount of energy released by an atom of element when the valence or the outermost electron is ejected out from the outer most shell...
here energyis radiated by the atom...

Electron gain Enthalpy ....
The amount of energy absorbed or gained by an isolated gaseous atom for addition of the electron in the outer most shell...
Here energy is absorbed by the atom.....

Answer 2

atoms release light : when an atom is struck by electricity (or heat) it gains some energy. this excites an electron and causes it to jump from its normal energy level to higher level.the electron then falls back down to its original state and releases some energy in form of light.