Electron gain Enthalpy
Electron gain enthalpy can be defined as the “amount of energy that is associated with the gain of electrons by the isolated gaseous atom. It is represented by Δeg H , measured in kJ/mol.
For example, X(g) + e– → X–(g)
Depending on the element, the process of adding an electron to the atom can be either endothermic or exothermic, though for many elements, energy is released when an electron is gained by an atom.
(Negative electron gain enthalpy)
The release of energy is represented by the negative values. The atom in order to gain stability tend to gain electron, as in the case of group 17 (halogens) elements. Halogens show a strong tendency to gain electrons to attain a stable noble gas electronic configuration. This is the reason that halogens have very high negative electron gain enthalpy (Δeg H ).
(Positive electron gain enthalpy)
It shows that the atom is reluctant for the addition of a new electron in its valence shell and this addition of electron is driving it towards unstability. For this addition of electron, energy needs to be spent making the positive. Let us take example of noble gases which have large positive electron gain enthalpies because the added electron has to enter the next higher principal quantum level leading to a very unstable electronic configuration.
In general , the electron gain enthalpy becomes more negative in going from left to right in a period while it becomes less negative going from top to bottom in a group. However, the variation in electron gain enthalpies does not show a perfectly regular trend along a period or a group with a number of exceptions.
Variation in electron gain enthalpies across the period (2nd period) and down the group (16th and 17th group)
Anomalous behaviour down the group:
Electron gain enthalpy of O (-141 kJ/mol) is less negative than that of S (-200 kJ/mol) and the electron gain enthalpy of F (-328 kJ/mol) is less negative than that of Cl (-349 kJ/mol). This occurs because when an electron is added to O or F, the added electron goes to the smaller n=2 quantum level. Due to the small size the interelectronic repulsion increases causing for the decrease in the electron gain enthalpy values. On the other hand, in S and Cl the electron enters the n =3 quantum level which occupies a larger region of space where the electron-electron repulsion is much less hence, there is an increase in the values of electron gain enthalpy.
Anomalous behaviour across the period:
Be, N and Ne show positive electron gain enthalpy values while moving across the period. This happens because of the extra stability of the atoms due to the half – filled degenerate orbitals. The addition of electrons to the atoms of these element requires energy. Here Be and Ne have fully-filled orbitals where N has the half-filled p-orbital.