Periodicity in Electron Gain Enthalpy

Every atom has a tendency to gain or loose electrons in order to acquire a noble gas configuration. The atom which have five, six or seven electrons in their outer-most shell show tendency to accept electrons and attain the nearest noble gas configuration.

For example, halogens have seven electrons in their outermost orbit. They show a tendency to accept one more electron and attain the nearest noble gas configuration. The energy change (ΔE) for this process is called electron gain enthalpy of that atom.

Electron gain enthalpy is the energy released or adsorbed for one mole of neutral atoms in a gaseous state when electron is accepted by each atom.

X(g) + e^– → X^–(g)

The negative value shows release of energy and hence tendency to greater stabilization.

Period

The electron gain enthalpy becomes more in negative from left to right in a period. This is because it is easier to add an electron to a smaller atom since the added electron would be closer to the positively charged nucleus. Halogens release maximum energy when they accept an electron. On the other hand, metals do not accept electrons and show a high positive value for ΔE. Thus, electron gain enthalpy can be positive or negative.

Group

Electron gain enthalpy becomes less in negative as you go down the group showing that the electropositive character of the atoms increases. This is because the size of the atom increases down the group and the electron added goes to the higher shells.

The electron gain enthalpy of chlorine is more in negative value as compared to that of fluroine. This is due to the small size of the F atom. As the electron approaches the small F atom, it expriences a repulsion from other electrons.