Electron affinity is the amount of energy change during the addition of an electron to an isolated gaseous atom.
It can be positive as well as negative.
Negative electron affinity means that the energy is released during the addition of electron. Which is a favorable condition for gaining an electron.
But positive value means that we have to supply the energy along with addition of an electron,which does not favor the process or makes it difficult.
Now let’s see why the second electron affinity (to add 2nd electron) of oxygen is positive.
There are two major reasons for this:-
- The interelectronic repulsions are the major reason for this phenomenon. The inner electrons repell upcoming electrons due to high electron density.
- The size of oxygen is small due to this the atom becomes less stable on addition of 2nd electron.