Question

why fluorine has low eletron affnity unexpectedly than chlorine

Answer 1

Chlorine has a higher n value, there are more electrons that shield the negatively charged electrons from the positively charged protons in the nucleus. Recall that protons attract the electrons inward.

Because Fluorine has a lower n value, there are less electrons to shield, thus the electrons are more attracted to Fluorine's nucleus.

Electrons will have a higher affinity to Fluorine than Chlorine b/c there's a higher attraction.

Answer 2

The F- anion is about 30% smaller than the Cl- anion, because it has only 2 electron shells. Since the electrons are crammed into a smaller volume, the electron-electron repulsion in the 2p valence shell of F is significantly higher than that in the 3p valence shell of Cl. Hence when you add another electron, the enhanced electron-electron repulsion means the energy released is lower than you might think, based on Cl.

Most of the odd properties of F and F-, compared to the other halogens, can be attributed to the unusually small size of the F atom.