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  4. For electron affinity of halogens which of the following is correct ?

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Q. For electron affinity of halogens which of the following is correct ?

AIIMSAIIMS 2004Classification of Elements and Periodicity in Properties

Solution:

Electron affinity is defined as the amount of energy released when an electron is added to the outermost shell of gaseous, neutral isolated atom. Electron affinity decrease in a group from top to bottom due to increase in size, so the expected order of EA for halogens is
$F > Cl > Br > I$
But it is not so. It is observed that $EA$ of elements of III period > II period, this is because elements of second period have small size and greater electron density so, the incoming electrons suffer a repulsive force.
Thus, the order of EA is
$Cl > F > Br > I$