electron affinity: Electron affinity is the measure of change in energy when a neutral atom gains an electron in the gaseous state.It is defined as exothermic (-) but can be endothermic (ex. some alkali earth metals and all noble gasses). This is because noble gasses already have a full shell, therefore, adding an electron would make the atom more unstable, similar to alkali earth metals, who already have a full s orbital, and would need to extend into p or d orbitals with one extra electron. If more energy is released, the electron affinity is higher.