Answer :
As two atoms of fluorine combine to become a molecule of fluorine, a bond is formed as energy is released. A molecule is more stable at a lower eneergy level.
When two atoms of Florine combines, they share an electron pair to complete it's octet. This result is formation of single bond between two fluorine atoms. This process is exothermic in nature, and hence, heat is liberated during the process of bond formation.