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From Wikipedia:

It is an extremely reactive element and a strong oxidising agent: among the elements, it has the highest electron affinity and the third-highest electronegativity on the Pauling scale, behind only oxygen and fluorine.

I am confused....

P.S.: I naively assumed that 'electronegativity' was the same thing as 'electron affinity'.... Perhaps that is why I got several downvotes....

The first answer, which I 'checked', explained the difference...

Apparently, the somewhat vague concept of Linus Pauling's 'electronegativity' includes two concepts: Both the relative, quantified tendency to grab an electron and the relative, quantified tendency not to lose one.... Combined into the somewhat-quantified, relative concept of electronegativity.....

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Electronegativity is a rather slippery thing, really. Roughly speaking, it depends on both electron affinity (the tendency to take on a negative charge) and ionization enthalpy (the tendency not to assume a positive charge). Chlorine has the highest electron affinity among halogens, true; but fluorine has a much higher ionization energy, a difference of about four electron volts according to typical ionization-energy tables. By comparison the electron affinity of chlorine exceeds that of fluorine by less than one electron volt. The way these factors balance out in the combined parameter called "electronegativity", the great difference in ionization energy dominates and puts fluorine at the top of the list.

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Fluorine is a rather small atom. The incoming electron is put into a region of space already crowded with electrons. So there is a significant amount of repulsion, which lessens the attraction the incoming electron feels, and so lessens the electron affinity. A similar reversal happens between Oxygen and Sulfur. The firs electron affinity of oxygen (-$142 kJ/mol$) is smaller that that of Sulfur (-$200 kJ/mol$), for the same reason.

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I will use electron gain enthalpy(ega) to explain this.

ega becomes less negative down a group in general. However, adding an electron to 2p orbital leads to greater repulsion than adding an electron to 3 p orbital and this, chlorine has the highest ega(the magnitude) and hence, the highest electron affinity.

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