The electron affinity of an element is defined as the energy liberated
when an electron is added to its neutral gaseous atom. Electron affinity of
an element is also called electron gain enthalpy of that element.
M(g) + e- J M-
(g) + E1 (M = Element, E1
= First electron affinity)
(g) + e- J M-2
(g) + E2 (E2
= Second electron affinity
The energy liberated when an electron is added to a uni-negative ion of
the element is called the 2nd electron affinity of that element. However,
practically no element shows liberation of energy when the 2nd electron is
added to its uni-negative ion. It does not mean that di-negative or trinegative ions do not form. They do form, but for adding 2nd electron, energy
in another way like bond formation must be given.
Groups
Electron affinity values(in KJ of mole-1)
VIIA (halogens)
F(-328); Cl(-349); Br (-325); I(-295) At(-270)
VIA (chalcogens)
O(-141); S(-200); Ge(-195) Te(-190) PO (-174)
Electron gain enthalpy values decrease as we go down in a group, but
increase along a period from left to right. Metals have very low electron
gain enthalpy values and alkaline earth metals have even positive values.
Note that the negative sign for energy value in table indicates that energy
is liberated or lost, and the positive sign tells that the energy is gained or
absorbed. All the factors which influence the ionization energy would also
influence the electron gain enthalpy
Electronagetivity
The ionization energy and the electron gain enthalpy are properties of
isolated atoms of elements. There has been a need to have a comparative
