(a) Halogens have maximum negative gain enthalpy in the respective periods of the periodic table. Why?
(b) Although electron gain enthalpy of fluorine is less negative as compared to chlorine, fluorine is a stronger oxidising agent than chlorine . Why?
Fluorine exhibits only -1 oxidation state, whereas other halogens exhibit `+1,+3,+5, and +7` oxidation states also. explain.

Explanation

Halogens have the smallest size in their respective periods and therefore high effective nuclear charge. As a consequence, they readily accept one electron to acquire noble gas configuration.
(b) Fluorine is a stronger oxidising agent than chlorine, due to :
(i) Low enthalpy of dissociation of `F-F` bond.
(ii) High hydration enthalpy of fluorine.
(iii) Fluorine is the most electronegative element and cannot exhibit positive oxidation state. Other halogens have vacant d-orbitals and therefore, can expand their octests and show `+1,+3,+5 and +7` oxidation states also.