The halogens have the smallest atomic radii in their respective periods. The atomic radius of fluorine is extremely small. All halogens exhibit -1 oxidation state. They are strong oxidising agents and have maximum negative electron gain enthalpy. Among halogens, fluorine shows anomalous behaviour in many properties: For example electronegativity and ionisation enthalpy are higher for fluorine than expected whereas bond dissociation enthalpy, m.p. and b.p.and electron gain enthalpy are quite lower than expected. Halogens react with hydrogen to give hydrogen halides (HX) and combine amongst themselves to form a number of compounds of the type `XX', XX'_(3), XX'_(5)` and `XX'_(7)` called inter halogens.
Why halogens have maximum negative electron gain enthalpy?

Explanation

Halogens have only seven electrons in their valence sheil. So they require only one electron to attain a noble gas configuration. Hence they have maximum electron gain enthalpy.