Question

what is electron affinity and what are its trend in modern table

Answer 1

Electron affinity (EA) is the energy or enthalpy given off when atom accepts an electron.
Cl(g) + e^- ----> Cl^- + (EA)
Along the period electron affinity increases and going down the group electron affinity decreases. The variation of EA is irregular, no particular trend can be found. In general, the electron affinity for an element in the fluorine group is usually large, indicating a very high tendency for it to acquire an electron.
The most electronegative element, F, releases -322 KJ/mol whereas chlorine, Cl, releases the most energy when it acquires an electron, -348.7 KJ/mol., the electron affinity for chlorine is larger than that of fluorine because fluorine is smaller in size and electron-electron repulsion is maximum.