The atomisation energy of fluorine is abnormally low. This reflects the low bond enthalpy of fluorine. The main reason for such a high oxidising power of fluorine is the very high hydration enthalpy of the fluoride ion. This is because the ion is very small. There is a very strong attraction between the fluoride ions and water molecules. The stronger the attraction, the more heat is evolved when the hydrated ions are formed. Hence, low heat of dissociation and high heat of hydration leads to high oxidising power of fluorine.