Electron affinity of an element is equal to the energy released when an electron is added to valence shell of an isolated gaseous atoms. For example, when an electron is added to oxygen atom to form O−ion, energy is released, i.e., EA1, value is exoergic or negative. O(g)+e−(g)⟶O−(g) ∆H(EA1)=−141‌kJ‌mol−1 (Energy is released) But the addition of second electron to O−ion to form O2− ion is more difficult and energy is needed to overpower the repulsion forces between negatively charged atomic sphere and test electrons. Thus, the EA2 value of oxygen atom is positive or endoergic. ‌O−(g)+e−(g)⟶O2−(g) ‌∆H(EA2)=+780‌kJ‌mol−1 (Energy is absorbed.) Thus, the EA1 values for all elements are exoergic (however for some elements endoergic, e.g., noble gases and alkali earth metals) while rest all (EA2,EA3...) endoergic.