Fluorine is a highly reactive gas, so is chlorine for that matter. Fluorine is also capable of supporting combustion, much like oxygen. Fluorine is a small atom with a tremendous ability to attract and share pairs of electrons, thus forming bonds. 

The electron affinity is the energy given off when one electron is added to a neutral, gaseous atom. Fluorine is such a small atom, with a high electron density in its outer-most level, so much so that there is more repulsion in forming a -1 charged atom than there is for chlorine.
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Flourine is highly reactive element.italso help in combustion. Chlorine is Less reactive because the outer shell electrons are further than compared to fluorine