Electron affinity

The minimum amount of energy required to gain/acquire an extra electron by  an atom of element in its gaseous state is called  1st electron affinity.

The SI unit of electron affinity is kilojoules per mole/kJmole-1 . Electron affinity depend on atomic radius.

The higher the atomic radius, the less effective the nuclear attraction on outer energy level electrons and thus the lower the electron affinity.

For halogens the 1st electron affinity decrease down the group as the atomic radius increase and the effective nuclear attraction on outer energy level electrons decrease.

 Due to its small size/atomic radius Fluorine shows exceptionally low electron affinity.

This is because a lot of energy is required to overcome the high repulsion of the existing and incoming electrons.

Table showing the election affinity of halogens

The higher the electron affinity the more stable the ion. i.e

 Cl- is a more stable ion than Br- because it has a more negative / exothermic electron affinity than Br –

Electron affinity  is different from:

(i)Ionization energy.

  Ionization energy is the energy required to lose/donate an electron in an atom of an element in its gaseous state while electron affinity is the energy required to gain/acquire extra electron by an atom of an element in its gaseous state.

Electronegativity.

  -Electron affinity is the energy required to gain an electron in an atom of an element in gaseous state.

 It involves the process:

  X(g)  +   e   ->  X-(g)

  Electronegativity is the ease/tendency of gaining/ acquiring electrons by an element during chemical reactions.

       It does not involve use of energy but theoretical arbitrary Pauling’ scale of measurements

define the terms electron affinity, electronegativity and ionization energy