It is the amount of energy given out when an electron is added to a neutral gaseous atom. This results in the formation of an anion.
For example Cl + e– → Cl + energy. The electron affinity of chlorine is 348 kJ mol-1.
Electron affinity changes across a period. The electron affinity of the atoms increases from left to right along a period because atomic size decreases and so the force of attraction exerted by the nucleus on the electrons increases. Thus atom will have greater tendency to attract the incoming electron .
The electron affinity changes down a group. The electron affinity decreases down a group because there is increase of atomic size with the increase in atomic number. Therefore the electron being added goes to the higher shell, thus less energy released.
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