The electron affinity of an element may be defined as amount of energy which is released when an extra electron enters the valence orbital of an isolated neutral atom to form a negative ion.
Atom(g) + Electron(g) → Negative ion(g) + Energy
The greater the energy released in the process of taking up the extra electron, the greater will be the electron affinity. Thus, ionization potential measures the tendency of an atom to change into a cation (M → M+ + le–) whereas the electron affinity measures the tendency of an atom to change into anion (X + e– → X–).
